Overview
Sizes three-phase distribution panels per the National Electrical Code (NEC, NFPA 70). The calculator follows Article 220 (Branch-Circuit, Feeder, and Service Load Calculations), Article 430 (Motors), and Article 310 (Conductor Ampacities) to determine total connected load, apply demand factors, compute the design current including the 125% continuous-load adjustment, select a standard panel/OCPD rating, and size feeder conductors from Table 310.15(B)(16) with ambient temperature and conductor-count correction factors.
Supports Wye and Delta systems at 208 V, 240 V, 480 V, and 600 V. Load categories include general lighting, receptacles, fixed appliances, HVAC, up to three motors (entered as FLA), transformers, and miscellaneous equipment. Optional NEC demand factor reductions can be applied to lighting and appliance loads.
Input Parameters
System / Panel Settings
| Parameter | Type | Default | Range | Description |
|---|---|---|---|---|
voltage_ll |
Enum: 208 / 240 / 480 / 600 | 208 | — | Nominal three-phase line-to-line voltage (V) |
system_type |
Enum: Wye / Delta | Wye | — | System configuration |
Building Characteristics
| Parameter | Type | Default | Range | Description |
|---|---|---|---|---|
floor_area |
Number | 0 | ≥ 0 | Total floor area (ft²). Used for lighting if lighting_va is 0, at the occupancy unit load (NEC 2020 Table 220.12 / 220.14(J)) |
occupancy_type |
Enum: Dwelling / Office / Retail / Industrial / Warehouse / Hospital / Hotel / Restaurant / School / Other | Office | — | Building occupancy classification |
General Lighting & Receptacles
| Parameter | Type | Default | Range | Description |
|---|---|---|---|---|
lighting_va |
Number | 0 | ≥ 0 | Connected general lighting load (VA). If zero, computed as floor_area × the occupancy unit load (see Formulas) |
lighting_continuous |
Boolean | True | — | Whether lighting operates 3+ hours (applies 125% factor per NEC 215.2(A)(1)) |
receptacle_va |
Number | 0 | ≥ 0 | Small power, computers, general receptacle load (VA) |
Fixed Appliances
| Parameter | Type | Default | Range | Description |
|---|---|---|---|---|
appliance_va |
Number | 0 | ≥ 0 | Total fixed appliance load — kitchen equipment, water heaters, etc. (VA) |
appliance_count |
Integer | 0 | ≥ 0 | Number of fixed appliances (used for demand factor reference) |
HVAC & Transformer Loads
| Parameter | Type | Default | Range | Description |
|---|---|---|---|---|
hvac_va |
Number | 0 | ≥ 0 | Heating, ventilation, air conditioning load (VA) |
transformer_va |
Number | 0 | ≥ 0 | Transformer rated VA |
misc_va |
Number | 0 | ≥ 0 | Other equipment — UPS, copiers, etc. (VA) |
Motor Loads (up to 3)
| Parameter | Type | Default | Range | Description |
|---|---|---|---|---|
motor1_fla |
Number | 0 | ≥ 0 | Motor 1 nameplate full-load amps (A) |
motor1_hp |
Number | 0 | ≥ 0 | Motor 1 horsepower rating |
motor2_fla |
Number | 0 | ≥ 0 | Motor 2 nameplate full-load amps (A) |
motor2_hp |
Number | 0 | ≥ 0 | Motor 2 horsepower rating |
motor3_fla |
Number | 0 | ≥ 0 | Motor 3 nameplate full-load amps (A) |
motor3_hp |
Number | 0 | ≥ 0 | Motor 3 horsepower rating |
Calculation Settings
| Parameter | Type | Default | Range | Description |
|---|---|---|---|---|
apply_demand_factors |
Boolean | False | — | Apply NEC Table 220.42 lighting demand factors for occupancy_type and appliance_demand_percent to the fixed appliances |
appliance_demand_percent |
Number | 100 | 0 – 100 | Demand factor (%) applied to fixed appliance load (hidden when demand factors disabled) |
Conductor Sizing Settings
| Parameter | Type | Default | Range | Description |
|---|---|---|---|---|
ambient_temp |
Number | 30 | -40 – 80 °C | Ambient temperature for ampacity correction per NEC 310.15(C) |
conductor_count |
Integer | 3 | 1 – 40 | Number of current-carrying conductors in raceway (for adjustment factor per NEC 310.15(B)(3)(a)) |
conductor_insulation |
Enum: 60C / 75C / 90C | 75C | — | Conductor insulation temperature rating for Table 310.15(B)(16) |
conductor_material |
Enum: Copper / Aluminum | Copper | — | Conductor material |
Output Results
| Output | Unit | Description |
|---|---|---|
total_connected_va |
VA | Sum of all connected loads (lighting + receptacle + appliance + HVAC + motors + transformer + misc) |
total_demand_va |
VA | Total demand load after NEC demand factors are applied (or 100% if disabled) |
continuous_va |
VA | Portion of demand classified as continuous (lighting when lighting_continuous is true) |
adjusted_continuous_va |
VA | Continuous load × 1.25 per NEC 215.2(A)(1) |
noncontinuous_va |
VA | Portion of demand classified as non-continuous |
total_design_va |
VA | Adjusted continuous + non-continuous load, used for conductor/OCPD sizing |
design_current |
A | Three-phase design current \( I = VA_{design} / (\sqrt{3} \times V_{LL}) \) |
recommended_panel_rating |
A | Next standard panel/OCPD rating at or above the design current |
recommended_conductor |
AWG/kcmil | Feeder conductor size from Table 310.15(B)(16) after correction factors |
conductor_table_ampacity |
A | Base ampacity from Table 310.15(B)(16) before corrections |
conductor_effective_ampacity |
A | Ampacity after ambient temperature and conductor-count corrections |
temp_correction_factor |
— | Ambient temperature correction factor from NEC 310.15(C)(1) |
count_adjustment_factor |
— | Conductor count adjustment factor from NEC 310.15(B)(3)(a) |
combined_correction_factor |
— | \( K_{temp} \times K_{count} \) — combined correction applied to table ampacity |
motor_conductor_details |
— | Per-motor branch circuit conductor sizing (FLA × 1.25 per NEC 430.22) |
Formulas
Floor-Area Lighting Load (NEC 2020 220.12 / 220.14(J))
Applied when lighting_va is zero and floor_area is specified. The unit load
\(u\) is taken from occupancy_type (NEC 2020; NEC 2023 moved the same values to
Table 220.42(A) and 220.41). Occupancy Other has no unit load, so give
lighting_va. Where the actual lighting exceeds the unit-load minimum, enter the actual load
as lighting_va.
occupancy_type | NEC 2020 occupancy | Unit load (VA/ft²) |
|---|---|---|
| Dwelling | Dwelling units, 220.14(J) | 3 |
| Office | Office, Table 220.12 | 1.3 |
| Retail | Retail | 1.9 |
| Industrial | Manufacturing facility | 2.2 |
| Warehouse | Warehouse | 1.2 |
| Hospital | Hospital | 1.6 |
| Hotel | Hotels and motels, incl. apartment houses without cooking by tenants | 1.7 |
| Restaurant | Restaurant | 1.5 |
| School | School/university | 1.5 |
| Other | — | enter lighting_va |
Motor VA Conversion
Converts each motor’s nameplate full-load amps to VA for inclusion in the total connected load.
Total Connected Load
Where \(L\) = lighting, \(R\) = receptacles, \(A\) = fixed appliances, \(HVAC\) = heating/ventilation/AC, \(M\) = total motor VA, \(T\) = transformer, \(Misc\) = miscellaneous.
Lighting Demand Factors (NEC 2020 Table 220.42, optional)
The general lighting load is split into the bands of NEC Table 220.42 for occupancy_type
and each band \(L_k\) is multiplied by its factor. Only applied when apply_demand_factors
is true (NEC 2023 Table 220.45 has the same values).
| Occupancy | Portion of lighting load (VA) | Demand factor |
|---|---|---|
| Dwelling units | First 3,000 | 100 % |
| 3,001 to 120,000 | 35 % | |
| Over 120,000 | 25 % | |
| Hospitals* | First 50,000 | 40 % |
| Over 50,000 | 20 % | |
| Hotels and motels* | First 20,000 | 50 % |
| 20,001 to 100,000 | 40 % | |
| Over 100,000 | 30 % | |
| Warehouses (storage) | First 12,500 | 100 % |
| Over 12,500 | 50 % | |
| All others | Total | 100 % |
* Not for areas of hospitals and hotels where the whole lighting is likely to be on at once (operating
rooms, ballrooms, dining rooms): enter that lighting separately as misc_va.
Appliance Demand Factor (optional)
Reduces the fixed appliance load by the specified demand factor.
Only applied when apply_demand_factors is true. Typical
NEC values: 75 % for four or more fastened-in-place appliances in
dwellings (220.53); Table 220.56 for commercial kitchen equipment
(3 units 90 %, 4 units 80 %, 5 units 70 %, 6 or more 65 %).
Continuous Load and Largest Motor (NEC 215.2(A)(1), 430.24)
Continuous loads (those expected to operate for 3 hours or more) must
be multiplied by 125% for conductor and OCPD sizing. When
lighting_continuous is true, the lighting demand is
classified as continuous; all other loads are non-continuous.
Motors are included at 100 % of FLA in \(VA_{noncontinuous}\); NEC 220.50 and 430.24 then add 25 % of the full-load current of the largest motor (\(VA_{largest\,motor} = \sqrt{3} \times V_{LL} \times I_{FLA,max}\)), so the feeder carries 125 % of the largest motor plus the others at 100 %.
Three-Phase Design Current
Standard three-phase power-to-current conversion.
Conductor Ampacity Correction
Where \(I_{table}\) is the base ampacity from NEC Table 310.15(B)(16), \(K_{temp}\) is the ambient temperature correction factor from NEC 310.15(C)(1), and \(K_{count}\) is the conductor count adjustment factor from NEC 310.15(B)(3)(a).
Required Table Ampacity
The minimum table ampacity needed before corrections. The smallest conductor whose table ampacity meets or exceeds this value is selected.
Motor Branch Circuit Conductor (NEC 430.22)
Each motor branch circuit conductor must be sized at 125% of the motor full-load current.
Key Lookup Tables
Standard Panel Ratings (A)
| Available Standard Ratings | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 30 | 60 | 100 | 125 | 150 | 175 | 200 | 225 | 400 | 600 | 800 | 1000 | 1200 | 1600 | 2000 |
Conductor Ampacities — Table 310.15(B)(16) (excerpt, Copper 75°C)
| Size (AWG/kcmil) | Ampacity (A) |
|---|---|
| 14 | 20 |
| 12 | 25 |
| 10 | 35 |
| 8 | 50 |
| 6 | 65 |
| 4 | 85 |
| 3 | 100 |
| 2 | 115 |
| 1 | 130 |
| 1/0 | 150 |
| 2/0 | 175 |
| 3/0 | 200 |
| 4/0 | 230 |
| 250 | 255 |
| 300 | 285 |
| 350 | 310 |
| 400 | 335 |
| 500 | 380 |
Ambient Temperature Correction Factors — NEC 310.15(C)(1) (excerpt)
| Ambient (°C) | 60°C | 75°C | 90°C |
|---|---|---|---|
| 20 | 1.15 | 1.11 | 1.08 |
| 25 | 1.08 | 1.05 | 1.04 |
| 30 | 1.00 | 1.00 | 1.00 |
| 35 | 0.91 | 0.94 | 0.96 |
| 40 | 0.82 | 0.88 | 0.91 |
| 45 | 0.71 | 0.82 | 0.87 |
| 50 | 0.58 | 0.75 | 0.82 |
Conductor Count Adjustment Factors — NEC 310.15(B)(3)(a)
| Conductors | Adjustment Factor |
|---|---|
| 1 – 3 | 1.00 |
| 4 – 6 | 0.80 |
| 7 – 9 | 0.70 |
| 10 – 20 | 0.50 |
| 21 – 30 | 0.45 |
| 31 – 40 | 0.40 |
| 41+ | 0.35 |
Worked Examples
The 10 examples below are the validation cases for this calculation. Each one was produced by the calculation engine from the inputs shown, every figure was checked against the independent reference quoted at the end of the example, and the same cases are run against every engine build (see validation scorecard, tool fe_calc_max_demand; worked examples for every calculation).
| # | Case | Method | Demand (kVA) | Current (A) | Main switch / rating (A) | Result |
|---|---|---|---|---|---|---|
| 1 | Dwelling floor area at 3 VA/ft² | NEC 220, 208 V | 40.36 | 112.04 | 125 panel; 2 AWG | PASS |
| 2 | 480 V commercial feeder with a motor | NEC 220, 480 V | 149.07 | 179.30 | 200 panel; 3/0 AWG | PASS |
| 3 | Small office panel at 208Y/120 V | NEC 220, 208 V | 36.75 | 102.01 | 125 panel; 2 AWG | PASS |
| 4 | 12-unit apartment feeder with demand factors | NEC 220, 208 V | 110.19 | 305.86 | 400 panel; 350 kcmil | PASS |
| 5 | Restaurant kitchen, derated aluminium feeder | NEC 220, 480 V | 123.50 | 148.55 | 150 panel; 300 kcmil | PASS |
| 6 | 800 A panel feeder beyond one conductor | NEC 220, 480 V | 550.00 | 661.55 | 800 panel; EXCEEDS TABLE | FAIL |
| 7 | 600 V feeder on 60 °C terminations | NEC 220, 600 V | 100.00 | 96.23 | 100 panel; 1 AWG | PASS |
| 8 | Light industrial panel with two motors | NEC 220, 480 V | 67.59 | 81.30 | 100 panel; 4 AWG | PASS |
| 9 | Retail store lighting from floor area | NEC 220, 208 V | 37.78 | 104.87 | 125 panel; 2 AWG | PASS |
| 10 | Hotel feeder with Table 220.42 lighting tiers | NEC 220, 480 V | 201.25 | 242.07 | 400 panel; 250 kcmil | PASS |
Example 1 — Dwelling floor area at 3 VA/ft²
With no lighting VA given, the general lighting load comes from floor area at the dwelling unit load of 3 VA/ft², counted as continuous.
Given: 208 V three-phase wye, occupancy dwelling: floor area 1,000 m² (no lighting VA given). Lighting continuous. No demand factors. Feeder: copper 75 °C, 30 °C ambient, 3 current-carrying conductors.
| Lighting load | No lighting VA given: 1,000 m² × 10.764 = 10,764 ft² × 3 VA/ft² = 32,291.7 VA (NEC 220.14(J) dwelling units, NEC 2020) |
|---|---|
| Connected | VAconn = 32,292 (L) = 32,291.7 VA |
| Demand factors | Not applied: every load at 100 % of connected |
| Demand | VAdemand = 32,291.7 VA |
| Design load | VAdesign = 1.25 × 32,291.7 (continuous lighting) + 0.0 (non-continuous) = 40,364.7 VA (NEC 215.2(A)(1)) |
| Design current | I = 40,364.7 / (√3 × 208) = 40,364.7 / 360.27 = 112.04 A |
| Panel rating | next standard rating ≥ 112.04 A → 125 A → 112.04 vs limit 125.00 A PASS |
| Feeder | K = 1 (30 °C) × 1 (3 conductors) = 1; required table ampacity 112.04 / 1 = 112.0 A → Table 310.16 copper 75 °C: 2 AWG (115 A) × 1 = 115.0 A → 112.04 vs limit 115.00 A PASS |
| Result | 2/2 checks pass → PASS; 125 A panel, feeder 2 AWG; governing: feeder ampacity |
| Reference | Hand calculation: 1 000 m² = 10 764 ft² × 3 VA/ft² (NEC 2020 220.14(J) / NEC 2023 220.41 dwelling unit load) = 32 291.7 VA, continuous × 125 % (NEC 215.2(A)(1)) = 40 364.7 VA; I = 40 364.7 / (√3 × 208) = 112.04 A → 125 A panel; NEC Table 310.16 75 °C copper ≥ 112.04 A → 2 AWG (115 A), 97 % utilised. (validation case c0028-nec220-208v-floor-area) |
Example 2 — 480 V commercial feeder with a motor
Continuous lighting at 125 %, receptacles and HVAC at 100 %, and the 40 A motor at 125 % of its FLC (NEC 430.24) push the feeder to 3/0 AWG.
Given: 480 V three-phase wye, occupancy office: lighting 30,000 VA, receptacles 20,000 VA, HVAC 50,000 VA; motors 40 A FLA. Lighting continuous. No demand factors. Feeder: copper 75 °C, 30 °C ambient, 3 current-carrying conductors.
| Lighting load | Given: 30,000 VA (continuous) |
|---|---|
| Motor 1 | √3 × 480 × 40 A = 33,255.4 VA |
| Connected | VAconn = 30,000 (L) + 20,000 (R) + 50,000 (HVAC) + 33,255 (M) = 133,255.4 VA |
| Demand factors | Not applied: every load at 100 % of connected |
| Demand | VAdemand = 133,255.4 VA |
| Design load | VAdesign = 1.25 × 30,000.0 (continuous lighting) + 103,255.4 (non-continuous) + 0.25 × 33,255.4 (Motor 1, largest motor, NEC 430.24) = 149,069.2 VA (NEC 215.2(A)(1), 430.24) |
| Design current | I = 149,069.2 / (√3 × 480) = 149,069.2 / 831.38 = 179.30 A |
| Panel rating | next standard rating ≥ 179.30 A → 200 A → 179.30 vs limit 200.00 A PASS |
| Feeder | K = 1 (30 °C) × 1 (3 conductors) = 1; required table ampacity 179.30 / 1 = 179.3 A → Table 310.16 copper 75 °C: 3/0 AWG (200 A) × 1 = 200.0 A → 179.30 vs limit 200.00 A PASS |
| Motor 1 branch | NEC 430.22: 1.25 × 40 = 50.00 A → 8 AWG |
| Result | 2/2 checks pass → PASS; 200 A panel, feeder 3/0 AWG; governing: panel rating |
| Reference | Hand calculation: continuous lighting 30 000 VA × 125 % (NEC 215.2(A)(1)) = 37 500 VA; receptacles 20 000 VA at 100 % (NEC 220.44 demand factor permitted, not applied) + HVAC 50 000 VA (non-continuous) = 107 500 VA; motor √3 × 480 × 40 = 33 255.4 VA × 125 % (NEC 430.24 via 220.50) = 41 569.2 VA; design 149 069.2 VA; I = 149 069.2 / (√3 × 480) = 129.30 + 1.25 × 40 = 179.30 A → 200 A panel; NEC Table 310.16 75 °C copper ≥ 179.30 A → 3/0 AWG (200 A; 2/0 = 175 A is short). (validation case c0028-nec220-480v-mixed) |
Example 3 — Small office panel at 208Y/120 V
Given lighting VA overrides the floor-area unit load; continuous lighting at 125 % plus the other loads at 100 %.
Given: 208 V three-phase wye, occupancy office: lighting 15,000 VA, receptacles 6,000 VA, HVAC 9,000 VA, miscellaneous 3,000 VA. Lighting continuous. No demand factors. Feeder: copper 75 °C, 30 °C ambient, 3 current-carrying conductors.
| Lighting load | Given: 15,000 VA (continuous) |
|---|---|
| Connected | VAconn = 15,000 (L) + 6,000 (R) + 9,000 (HVAC) + 3,000 (Misc) = 33,000.0 VA |
| Demand factors | Not applied: every load at 100 % of connected |
| Demand | VAdemand = 33,000.0 VA |
| Design load | VAdesign = 1.25 × 15,000.0 (continuous lighting) + 18,000.0 (non-continuous) = 36,750.0 VA (NEC 215.2(A)(1)) |
| Design current | I = 36,750.0 / (√3 × 208) = 36,750.0 / 360.27 = 102.01 A |
| Panel rating | next standard rating ≥ 102.01 A → 125 A → 102.01 vs limit 125.00 A PASS |
| Feeder | K = 1 (30 °C) × 1 (3 conductors) = 1; required table ampacity 102.01 / 1 = 102.0 A → Table 310.16 copper 75 °C: 2 AWG (115 A) × 1 = 115.0 A → 102.01 vs limit 115.00 A PASS |
| Result | 2/2 checks pass → PASS; 125 A panel, feeder 2 AWG; governing: feeder ampacity |
| Reference | Hand calculation: connected 15 000 + 6 000 + 9 000 + 3 000 = 33 000 VA; design 1.25 × 15 000 + 18 000 = 36 750 VA (NEC 215.2(A)(1)); I = 36 750 / (√3 × 208 = 360.27) = 102.01 A → 125 A panel; Table 310.16 75 °C Cu → 2 AWG (115 A). Lighting 15 000 VA exceeds the NEC 2020 Table 220.12 office minimum 1.3 VA/ft² × 5 000 ft² = 6 500 VA. (Former hand-written help-page Example 1.) (validation case c0028-nec220-208v-small-office) |
Example 4 — 12-unit apartment feeder with demand factors
Dwelling general lighting/small-appliance load takes Table 220.42 (first 3000 VA at 100 %, rest at 35 %) and four or more fastened-in-place appliances 75 % (220.53).
Given: 208 V three-phase wye, occupancy dwelling: lighting 86,400 VA, fixed appliances 24,000 VA, HVAC 60,000 VA. Lighting non-continuous. Demand factors on: lighting per NEC Table 220.42 for the occupancy, appliances at 75 %. Feeder: copper 75 °C, 30 °C ambient, 3 current-carrying conductors.
| Lighting load | Given: 86,400 VA (non-continuous) |
|---|---|
| Connected | VAconn = 86,400 (L) + 24,000 (A) + 60,000 (HVAC) = 170,400.0 VA |
| Demand factors | Lighting (NEC Table 220.42, dwelling) 3,000 × 100 % + 83,400 × 35 % = 32,190.0 VA; appliances 24,000 × 75 % = 18,000.0 VA; other loads at 100 % |
| Demand | VAdemand = 110,190.0 VA |
| Design load | No continuous load: VAdesign = 110,190.0 VA |
| Design current | I = 110,190.0 / (√3 × 208) = 110,190.0 / 360.27 = 305.86 A |
| Panel rating | next standard rating ≥ 305.86 A → 400 A → 305.86 vs limit 400.00 A PASS |
| Feeder | K = 1 (30 °C) × 1 (3 conductors) = 1; required table ampacity 305.86 / 1 = 305.9 A → Table 310.16 copper 75 °C: 350 kcmil (310 A) × 1 = 310.0 A → 305.86 vs limit 310.00 A PASS |
| Result | 2/2 checks pass → PASS; 400 A panel, feeder 350 kcmil; governing: feeder ampacity |
| Reference | Hand calculation, NEC standard method: 12 units × (900 ft² × 3 VA/ft² + 2 × 1 500 small-appliance + 1 500 laundry) = 86 400 VA; Table 220.42 dwelling: 3 000 + (86 400 − 3 000) × 35 % = 32 190 VA; fastened-in-place appliances 24 000 VA × 75 % (220.53) = 18 000 VA; HVAC 60 000 VA (larger of heating/cooling, 220.60); design 110 190 VA (no continuous loads) ÷ (√3 × 208) = 305.86 A → 400 A panel; Table 310.16 75 °C Cu ≥ 305.86 A → 350 kcmil (310 A). (Replaces help-page Example 3, which applied the dwelling 35 % factor to a retail store.) (validation case c0028-nec220-multifamily-demand-factors) |
Example 5 — Restaurant kitchen, derated aluminium feeder
Six commercial kitchen units take 65 % (Table 220.56); 40 °C ambient and six current-carrying conductors derate an aluminium feeder to 0.704.
Given: 480 V three-phase wye, occupancy retail: lighting 20,000 VA, fixed appliances 90,000 VA, HVAC 40,000 VA. Lighting continuous. Demand factors on: lighting per NEC Table 220.42 for the occupancy, appliances at 65 %. Feeder: aluminium 75 °C, 40 °C ambient, 6 current-carrying conductors.
| Lighting load | Given: 20,000 VA (continuous) |
|---|---|
| Connected | VAconn = 20,000 (L) + 90,000 (A) + 40,000 (HVAC) = 150,000.0 VA |
| Demand factors | Lighting (NEC Table 220.42, retail) 20,000 × 100 % = 20,000.0 VA; appliances 90,000 × 65 % = 58,500.0 VA; other loads at 100 % |
| Demand | VAdemand = 118,500.0 VA |
| Design load | VAdesign = 1.25 × 20,000.0 (continuous lighting) + 98,500.0 (non-continuous) = 123,500.0 VA (NEC 215.2(A)(1)) |
| Design current | I = 123,500.0 / (√3 × 480) = 123,500.0 / 831.38 = 148.55 A |
| Panel rating | next standard rating ≥ 148.55 A → 150 A → 148.55 vs limit 150.00 A PASS |
| Feeder | K = 0.88 (40 °C) × 0.8 (6 conductors) = 0.704; required table ampacity 148.55 / 0.704 = 211.0 A → Table 310.16 aluminium 75 °C: 300 kcmil (230 A) × 0.704 = 161.9 A → 148.55 vs limit 161.90 A PASS |
| Result | 2/2 checks pass → PASS; 150 A panel, feeder 300 kcmil; governing: panel rating |
| Reference | Hand calculation: lighting 20 000 VA (first 3 000 at 100 %, remainder at 100 %) continuous → 25 000 VA; 6 kitchen units 90 000 VA × 65 % (NEC Table 220.56) = 58 500 VA; HVAC 40 000 VA; design 123 500 VA; I = 123 500 / (√3 × 480) = 148.55 A → 150 A panel (99.0 %); correction 0.88 (40 °C, 75 °C column, 310.15(B)(1)) × 0.80 (4–6 conductors, 310.15(C)(1)) = 0.704; required table ampacity 148.55 / 0.704 = 211.00 A → aluminium 75 °C 300 kcmil (230 A; 250 kcmil 205 A is short) → 161.9 A corrected. (validation case c0028-nec220-kitchen-aluminium-derated) |
Example 6 — 800 A panel feeder beyond one conductor
When the design current exceeds the largest single conductor in Table 310.16 (1000 kcmil, 545 A) the feeder check fails: parallel sets are needed.
Given: 480 V three-phase wye, occupancy office: lighting 200,000 VA, HVAC 300,000 VA. Lighting continuous. No demand factors. Feeder: copper 75 °C, 30 °C ambient, 3 current-carrying conductors. Supply rating 800 A.
| Lighting load | Given: 200,000 VA (continuous) |
|---|---|
| Connected | VAconn = 200,000 (L) + 300,000 (HVAC) = 500,000.0 VA |
| Demand factors | Not applied: every load at 100 % of connected |
| Demand | VAdemand = 500,000.0 VA |
| Design load | VAdesign = 1.25 × 200,000.0 (continuous lighting) + 300,000.0 (non-continuous) = 550,000.0 VA (NEC 215.2(A)(1)) |
| Design current | I = 550,000.0 / (√3 × 480) = 550,000.0 / 831.38 = 661.55 A |
| Supply rating | I = 661.55 A against the given supply rating → 661.55 vs limit 800.00 A PASS |
| Panel rating | next standard rating ≥ 661.55 A → 800 A → 661.55 vs limit 800.00 A PASS |
| Feeder | K = 1 (30 °C) × 1 (3 conductors) = 1; required table ampacity 661.55 / 1 = 661.5 A exceeds the largest single copper 75 °C conductor in Table 310.16 (1000 kcmil) → parallel conductors needed → 661.55 vs limit 0.00 A FAIL |
| Result | 2/3 checks pass → FAIL; 800 A panel, feeder EXCEEDS TABLE; governing: feeder ampacity |
| Reference | Hand calculation: 200 000 VA continuous lighting × 125 % = 250 000 VA + HVAC 300 000 VA = 550 000 VA; I = 550 000 / (√3 × 480) = 661.55 A → 800 A panel and supply (82.7 %); NEC Table 310.16 75 °C copper tops out at 1000 kcmil = 545 A < 661.5 A → no single conductor (FAIL; NEC 310.10(G) parallel conductors required). (validation case c0028-nec220-feeder-exceeds-table) |
Example 7 — 600 V feeder on 60 °C terminations
A 100 A feeder sized from the 60 °C column (NEC 110.14(C)(1)(a)) with the 35 °C ambient factor 0.91, including a 75 kVA transformer load.
Given: 600 V three-phase wye, occupancy office: lighting 12,000 VA, transformer 75,000 VA, miscellaneous 10,000 VA. Lighting continuous. No demand factors. Feeder: copper 60 °C, 35 °C ambient, 3 current-carrying conductors. Supply rating 100 A.
| Lighting load | Given: 12,000 VA (continuous) |
|---|---|
| Connected | VAconn = 12,000 (L) + 75,000 (T) + 10,000 (Misc) = 97,000.0 VA |
| Demand factors | Not applied: every load at 100 % of connected |
| Demand | VAdemand = 97,000.0 VA |
| Design load | VAdesign = 1.25 × 12,000.0 (continuous lighting) + 85,000.0 (non-continuous) = 100,000.0 VA (NEC 215.2(A)(1)) |
| Design current | I = 100,000.0 / (√3 × 600) = 100,000.0 / 1,039.23 = 96.23 A |
| Supply rating | I = 96.23 A against the given supply rating → 96.23 vs limit 100.00 A PASS |
| Panel rating | next standard rating ≥ 96.23 A → 100 A → 96.23 vs limit 100.00 A PASS |
| Feeder | K = 0.91 (35 °C) × 1 (3 conductors) = 0.91; required table ampacity 96.23 / 0.91 = 105.7 A → Table 310.16 copper 60 °C: 1 AWG (110 A) × 0.91 = 100.1 A → 96.23 vs limit 100.10 A PASS |
| Result | 3/3 checks pass → PASS; 100 A panel, feeder 1 AWG; governing: supply rating |
| Reference | Hand calculation: lighting 12 000 VA × 125 % = 15 000 VA + transformer 75 000 VA + misc 10 000 VA = 100 000 VA; I = 100 000 / (√3 × 600) = 96.23 A → 100 A panel and supply; 60 °C copper at 35 °C: factor 0.91 (310.15(B)(1)); required 96.23 / 0.91 = 105.74 A → 1 AWG (110 A at 60 °C) → 110 × 0.91 = 100.1 A corrected. (validation case c0028-nec220-600v-60c-feeder) |
Example 8 — Light industrial panel with two motors
Motors enter at √3·V·FLA and the feeder adds 25 % of the largest motor (NEC 430.24); each motor branch conductor is sized at 125 % of its FLA (430.22).
Given: 480 V three-phase wye, occupancy industrial: lighting 6,000 VA, transformer 30,000 VA; motors 20 A FLA, 13 A FLA. Lighting non-continuous. No demand factors. Feeder: copper 75 °C, 30 °C ambient, 3 current-carrying conductors.
| Lighting load | Given: 6,000 VA (non-continuous) |
|---|---|
| Motor 1 | √3 × 480 × 20 A = 16,627.7 VA |
| Motor 2 | √3 × 480 × 13 A = 10,808.0 VA |
| Connected | VAconn = 6,000 (L) + 30,000 (T) + 27,436 (M) = 63,435.7 VA |
| Demand factors | Not applied: every load at 100 % of connected |
| Demand | VAdemand = 63,435.7 VA |
| Design load | No continuous load: VAdesign = 63,435.7 + 0.25 × 16,627.7 (Motor 1, largest motor, NEC 430.24) = 67,592.6 VA |
| Design current | I = 67,592.6 / (√3 × 480) = 67,592.6 / 831.38 = 81.30 A |
| Panel rating | next standard rating ≥ 81.30 A → 100 A → 81.30 vs limit 100.00 A PASS |
| Feeder | K = 1 (30 °C) × 1 (3 conductors) = 1; required table ampacity 81.30 / 1 = 81.3 A → Table 310.16 copper 75 °C: 4 AWG (85 A) × 1 = 85.0 A → 81.30 vs limit 85.00 A PASS |
| Motor 1 branch | NEC 430.22: 1.25 × 20 = 25.00 A → 12 AWG |
| Motor 2 branch | NEC 430.22: 1.25 × 13 = 16.25 A → 14 AWG |
| Result | 2/2 checks pass → PASS; 100 A panel, feeder 4 AWG; governing: feeder ampacity |
| Reference | Hand calculation: motors √3 × 480 × 20 = 16 627.7 VA and √3 × 480 × 13 = 10 808.0 VA; lighting 6 000 VA (non-continuous) + transformer 30 000 VA; NEC 430.24: + 25 % × 16 627.7 = 4 156.9 VA; design 67 592.6 VA; I = 67 592.6 / (√3 × 480 = 831.38) = 43.30 + 1.25 × 20 + 13 = 81.30 A → 100 A panel; Table 310.16 75 °C Cu ≥ 81.30 A → 4 AWG (85 A). Branch conductors (430.22): 1.25 × 20 = 25 A → 12 AWG (25 A); 1.25 × 13 = 16.25 A → 14 AWG (20 A). (Former hand-written help-page Example 2.) (validation case c0028-nec220-480v-motors-transformer) |
Example 9 — Retail store lighting from floor area
With no lighting VA the occupancy sets the unit load: retail is 1.9 VA/ft² in NEC 2020 Table 220.12, not the dwelling 3 VA/ft².
Given: 208 V three-phase wye, occupancy retail: floor area 500 m² (no lighting VA given), receptacles 10,000 VA, HVAC 15,000 VA. Lighting continuous. No demand factors. Feeder: copper 75 °C, 30 °C ambient, 3 current-carrying conductors.
| Lighting load | No lighting VA given: 500 m² × 10.764 = 5,382 ft² × 1.9 VA/ft² = 10,225.7 VA (NEC Table 220.12 retail, NEC 2020) |
|---|---|
| Connected | VAconn = 10,226 (L) + 10,000 (R) + 15,000 (HVAC) = 35,225.7 VA |
| Demand factors | Not applied: every load at 100 % of connected |
| Demand | VAdemand = 35,225.7 VA |
| Design load | VAdesign = 1.25 × 10,225.7 (continuous lighting) + 25,000.0 (non-continuous) = 37,782.1 VA (NEC 215.2(A)(1)) |
| Design current | I = 37,782.1 / (√3 × 208) = 37,782.1 / 360.27 = 104.87 A |
| Panel rating | next standard rating ≥ 104.87 A → 125 A → 104.87 vs limit 125.00 A PASS |
| Feeder | K = 1 (30 °C) × 1 (3 conductors) = 1; required table ampacity 104.87 / 1 = 104.9 A → Table 310.16 copper 75 °C: 2 AWG (115 A) × 1 = 115.0 A → 104.87 vs limit 115.00 A PASS |
| Result | 2/2 checks pass → PASS; 125 A panel, feeder 2 AWG; governing: feeder ampacity |
| Reference | Hand calculation: 500 m² × 10.764 = 5 382.0 ft² × 1.9 VA/ft² (NEC 2020 Table 220.12, retail) = 10 225.7 VA, continuous × 125 % = 12 782.1 VA; + receptacles 10 000 + HVAC 15 000 VA = 37 782.1 VA; I = 37 782.1 / (√3 × 208 = 360.27) = 104.87 A → 125 A panel; Table 310.16 75 °C Cu ≥ 104.87 A → 2 AWG (115 A). At the dwelling 3 VA/ft² the panel would be 150 A. (validation case c0028-nec220-retail-floor-area) |
Example 10 — Hotel feeder with Table 220.42 lighting tiers
Hotel general lighting takes 50 % of the first 20 000 VA, 40 % of the next 80 000 VA and 30 % of the rest (NEC 2020 Table 220.42); the demand is then continuous at 125 %.
Given: 480 V three-phase wye, occupancy hotel: lighting 150,000 VA, receptacles 40,000 VA, HVAC 90,000 VA. Lighting continuous. Demand factors on: lighting per NEC Table 220.42 for the occupancy, appliances at 100 %. Feeder: copper 75 °C, 30 °C ambient, 3 current-carrying conductors.
| Lighting load | Given: 150,000 VA (continuous) |
|---|---|
| Connected | VAconn = 150,000 (L) + 40,000 (R) + 90,000 (HVAC) = 280,000.0 VA |
| Demand factors | Lighting (NEC Table 220.42, hotel) 20,000 × 50 % + 80,000 × 40 % + 50,000 × 30 % = 57,000.0 VA; other loads at 100 % |
| Demand | VAdemand = 187,000.0 VA |
| Design load | VAdesign = 1.25 × 57,000.0 (continuous lighting) + 130,000.0 (non-continuous) = 201,250.0 VA (NEC 215.2(A)(1)) |
| Design current | I = 201,250.0 / (√3 × 480) = 201,250.0 / 831.38 = 242.07 A |
| Panel rating | next standard rating ≥ 242.07 A → 400 A → 242.07 vs limit 400.00 A PASS |
| Feeder | K = 1 (30 °C) × 1 (3 conductors) = 1; required table ampacity 242.07 / 1 = 242.1 A → Table 310.16 copper 75 °C: 250 kcmil (255 A) × 1 = 255.0 A → 242.07 vs limit 255.00 A PASS |
| Result | 2/2 checks pass → PASS; 400 A panel, feeder 250 kcmil; governing: feeder ampacity |
| Reference | Hand calculation, NEC 2020 Table 220.42 (hotels and motels): 20 000 × 50 % + 80 000 × 40 % + 50 000 × 30 % = 10 000 + 32 000 + 15 000 = 57 000 VA; demand 57 000 + receptacles 40 000 + HVAC 90 000 = 187 000 VA; continuous lighting × 125 % → design 71 250 + 130 000 = 201 250 VA; I = 201 250 / (√3 × 480) = 242.07 A → 400 A panel; Table 310.16 75 °C Cu ≥ 242.07 A → 250 kcmil (255 A). (validation case c0028-nec220-hotel-demand-tiers) |
Standards & References
- NFPA 70 (NEC) Article 220 — Branch-Circuit, Feeder, and Service Load Calculations
- NEC Table 220.12 — General Lighting Loads by Occupancy (unit lighting load per ft²)
- NEC 215.2(A)(1) / 230.42 — Continuous load 125% multiplier for conductor and OCPD sizing
- NEC Article 310 — Conductors for General Wiring; Table 310.15(B)(16) conductor ampacities
- NEC 310.15(C)(1) — Ambient temperature correction factors for conductor ampacity
- NEC 310.15(B)(3)(a) — Adjustment factors for more than 3 current-carrying conductors in a raceway
- NEC Article 430 — Motors, Motor Circuits, and Controllers; Section 430.22 (motor branch circuit conductors at 125% FLA)
Tips & Limitations
Tips
- Use the conservative approach (demand factors disabled) for initial design or when occupancy loads are uncertain. Enable demand factors only when the building type and load profile are well established.
- Motor FLA should come from the motor nameplate or NEC Table 430.250, not from calculated values. The calculator converts FLA to VA automatically.
- At 30°C ambient with 3 or fewer conductors in a raceway, both correction factors are 1.00 and conductor sizing is straightforward from the table.
- For elevated ambient temperatures (e.g., rooftop installations at 45–50°C), the correction factor can substantially reduce effective ampacity — size up accordingly.
- When more than 3 current-carrying conductors share a raceway, the adjustment factor can be as low as 0.35 (41+ conductors), requiring significantly larger conductor sizes.
- The 125% continuous load factor and the NEC demand factors are independent adjustments. The demand factor reduces the load to a realistic estimate; the 125% factor adds a safety margin for continuous operation.
Limitations
- Three-phase only — this calculator assumes a balanced three-phase system. Single-phase panels and unbalanced loading are not modelled.
- Up to 3 motors — the calculator supports a maximum of 3 motor loads. For panels with more motors, combine additional motors into the miscellaneous VA field.
- Dwelling optional methods not implemented — general lighting follows NEC Table 220.42 for the occupancy; the dwelling-unit optional calculations (NEC 220.82/220.83/220.84) and per-category demand factors other than the appliance percentage are not implemented.
- No voltage drop calculation — conductor sizing is based on ampacity only. Use the Cable Voltage Drop calculator (c0005) to verify feeder voltage drop for long runs.
- No short-circuit analysis — conductor withstand during fault conditions is not evaluated. Use the Short Circuit calculator for that assessment.
- Ambient temperature interpolation — correction factors snap to the closest standard temperature in the NEC table. Linear interpolation between table values is not performed.
Developer Guide
Architecture
File layout:
components/c0028_nec_panel_sizing/component.py— Component registration, parameter schema (PARAMETER_SCHEMA), calculator class (NECPanelSizingCalculator), summary cards builder, equation builder adapter, QR code generatorcomponents/c0028_nec_panel_sizing/calculations.py— Pure calculation functions and NEC lookup tables (no Django dependencies). Contains conductor ampacity table, ambient temp correction factors, conductor count adjustments, and the maincalculate_nec_panel_sizing()functioncomponents/c0028_nec_panel_sizing/equations.py— Equation/report builder generating LaTeX-formatted articles for PDF/HTML output. Builds input/output tables and step-by-step calculation displaycomponents/c0028_nec_panel_sizing/test_examples.py— Three worked example test cases runnable via Django shell
The calculator class NECPanelSizingCalculator extends
ElectricalCalculator (from calcs.core.base_calculator).
Validation requires a positive voltage_ll and at least one
non-zero load input or a non-zero floor_area.
Lookup tables in calculations.py:
CONDUCTOR_AMPACITY_TABLE— NEC Table 310.15(B)(16), keyed by conductor size → material → insulation temp ratingAMBIENT_TEMP_CORRECTION— NEC 310.15(C)(1), keyed by temperature → insulation ratingCONDUCTOR_COUNT_ADJUSTMENT— NEC 310.15(B)(3)(a), keyed by (min, max) conductor count rangesSTANDARD_PANEL_RATINGS— list of standard OCPD ratings from 30 A to 2,000 ACONDUCTOR_SIZES— ordered list of AWG/kcmil sizes from 14 to 1000
Code Paths
Key functions in calculations.py:
| Function | Purpose |
|---|---|
calculate_nec_panel_sizing(data) |
Main entry point. Accepts input dict, returns full results dict. Steps: sum connected loads, apply demand factors, adjust for continuous loads, compute design current, select panel rating, select conductor size |
select_conductor_size(required_ampacity, material, insulation, ambient_temp, conductor_count) |
Selects the smallest conductor from Table 310.15(B)(16) that meets the required ampacity after applying temperature and conductor-count correction factors |
get_ambient_correction_factor(ambient_temp_c, conductor_insulation) |
Returns the temperature correction factor from NEC 310.15(C)(1) by snapping to the closest standard temperature |
get_conductor_count_adjustment(conductor_count) |
Returns the conductor count adjustment factor from NEC 310.15(B)(3)(a) based on range lookup |
Key functions in equations.py:
| Function | Purpose |
|---|---|
build_equations(result) |
Generates the full report article list (title, equation, table, and signature articles) with LaTeX substitutions |
build_table_data(result) |
Builds row dicts for input parameter and output result
tables, flagged with input_table /
results_table booleans |
Key functions in component.py:
| Function | Purpose |
|---|---|
equation_builder(calculation_results) |
Adapter that converts raw equation articles from
build_equations() into the unified article
format expected by the core report renderer |
summary_cards_builder(results) |
Returns 4 summary cards: Design Current, Total Demand, Panel Rating, Conductor Size |
register_component(registry) |
Registers as nec_panel_sizing calc type with
schema, calculator class, and all builder functions |
Testing
Running tests:
pytest calcs/components/c0028_nec_panel_sizing/ -v
python manage.py shell < calcs/components/c0028_nec_panel_sizing/test_examples.pyTest examples cover:
- Example 1 — Small commercial office at 208 V. Conservative (no demand factors), continuous lighting. Expected: ~102 A design current, 125 A panel.
- Example 2 — Light industrial at 480 V with two motors (20 A and 13 A FLA) and a transformer. Non-continuous lighting; 125 % of the largest motor (430.24). Expected: 81.3 A design current, 100 A panel.
- Example 3 — Multifamily dwelling feeder at 208 V with NEC Table 220.42 dwelling demand factors (first 3,000 VA at 100 %, rest at 35 %). Continuous lighting. Expected: 158.7 A design current, 175 A panel.
Worked examples: the examples above are generated from the validation corpus
(mcp_public/validation/cases/fe_calc_max_demand/) by
python manage.py generate_help_examples --tool fe_calc_max_demand, which writes
_nec_panel_sizing_examples.html. Add or change a case there, then regenerate; the
fragment is never edited by hand.
How to extend:
- To add an occupancy, add its NEC Table 220.12 unit load to
GENERAL_LIGHTING_UNIT_LOADS(and a Table 220.42 band list toLIGHTING_DEMAND_FACTORSif it has one) incalculations.py; the schema enum, sidebar options and MCP adapter literal must list it too. - To support more motors, add
motor4_fla/motor4_hpetc. toPARAMETER_SCHEMAand replicate the motor VA conversion and conductor sizing blocks. - Write pytest tests covering new parameters, boundary conditions (zero loads, maximum conductor count, extreme ambient temps), and edge cases (all motors at zero, demand factors at 0%).