DC Cable Sizing Calculator — help

Overview

Calculates conductor size and voltage drop for DC cable circuits per IEC 60364, IEC 60287, and AS/NZS 3008. Designed for solar PV string-to-inverter, battery-to-inverter, general DC distribution, critical loads, and motor circuit applications. The calculator determines the minimum conductor cross-sectional area based on both voltage drop and current-carrying capacity (ampacity), applying temperature, grouping, and installation method derating factors, then selects the larger of the two results as the final conductor size.

Input Parameters

System Parameters

Parameter	Type	Default	Range	Description
`system_voltage`	Number	48 V	1 – 2000 V	DC system nominal voltage
`continuous_current`	Number	100 A	min 0.1 A	Maximum continuous operating current
`cable_length`	Number	3 m	min 0.1 m	One-way cable length (source to load)
`max_voltage_drop_percent`	Number	3%	0.1 – 10%	Maximum acceptable voltage drop percentage
`safety_factor`	Number	1.25	1.0 – 2.0	Multiplier applied to continuous current to obtain design current
`application_type`	Enum	Battery to Inverter	—	Solar PV String to Inverter / Battery to Inverter / DC Distribution (General) / Critical Loads / Motor Circuits

Cable & Installation Parameters

Parameter	Type	Default	Range	Description
`installation_method`	Enum	Open Air (Clipped Direct)	—	Open Air (Clipped Direct) / Enclosed in Conduit / Buried Direct in Ground / Cable Tray (Perforated) / Enclosed Trunking
`insulation_type`	Enum	XLPE	—	PVC / XLPE / EPR / Silicone
`conductor_temp_rating`	Enum	75 °C	—	Maximum conductor temperature rating: 75 °C or 90 °C
`conductor_operating_temp`	Number	70 °C	20 – 90 °C	Assumed conductor operating temperature for resistance calculation
`ambient_temperature`	Number	30 °C	−20 – 80 °C	Ambient temperature at installation location
`num_grouped_circuits`	Integer	1	1 – 20	Number of circuits bundled together (affects grouping derating)

Output Results

Output	Unit	Description
`design_current_a`	A	Design current after applying safety factor \(I_{design} = I \times k_s\)
`min_size_voltage_drop_mm2`	mm²	Minimum conductor CSA required to meet voltage drop limit
`selected_conductor_size_mm2`	mm²	Final selected standard conductor size (larger of VD and ampacity requirements)
`actual_voltage_drop_volts`	V	Actual voltage drop with selected conductor
`actual_voltage_drop_percent`	%	Actual voltage drop as percentage of system voltage
`voltage_drop_margin_percent`	%	Margin between actual and maximum allowable voltage drop
`temperature_correction_factor`	—	Temperature derating factor \(C_t\)
`grouping_correction_factor`	—	Grouping derating factor \(C_g\)
`installation_method_factor`	—	Installation method correction factor \(C_c\)
`combined_derating_factor`	—	Combined derating \(C_t \times C_g \times C_c\)
`tabulated_ampacity_a`	A	Base tabulated ampacity for selected conductor
`adjusted_ampacity_a`	A	Ampacity after applying all derating factors
`ampacity_margin_percent`	%	Percentage margin between adjusted ampacity and design current
`current_loading_percent`	%	Design current as percentage of adjusted ampacity
`conductor_resistance_ohm_per_km`	Ω/km	Conductor resistance at operating temperature
`copper_resistivity_operating`	Ω·mm²/m	Copper resistivity at operating temperature
`power_loss_w`	W	Power dissipated in the cable
`power_loss_percent`	%	Power loss as percentage of delivered power
`voltage_drop_compliant`	Boolean	Whether voltage drop is within the limit
`ampacity_compliant`	Boolean	Whether adjusted ampacity exceeds design current
`overall_compliant`	Boolean	Both voltage drop and ampacity requirements met

Formulas

Design Current

\[ I_{design} = I_{continuous} \times k_s \]

Where \(I_{continuous}\) is the maximum continuous operating current (A) and \(k_s\) is the safety factor (default 1.25).

Resistivity at Operating Temperature

\[ \rho(T) = \rho_{20}\bigl[1 + \alpha\,(T - 20)\bigr] \]

Where \(\rho_{20} = 0.017241\) Ω·mm²/m (copper at 20 °C) and \(\alpha = 0.00393\) /°C. This adjusts resistivity from the reference temperature of 20 °C to the actual conductor operating temperature.

Minimum Conductor CSA from Voltage Drop

\[ A_{min} = \frac{2\,\rho(T)\,I_{design}\,L}{V_{drop,max}} \]

Where the factor of 2 accounts for both positive and negative conductors in a DC circuit, \(L\) is one-way cable length in metres, and \(V_{drop,max} = V_{system} \times V_{d\%,max} / 100\).

Conductor Resistance per Kilometre

\[ R_{c} = \frac{\rho(T) \times 1000}{A} \quad (\Omega/\text{km}) \]

Where \(A\) is the conductor cross-sectional area in mm².

Actual Voltage Drop

\[ V_d = \frac{2 \times I_{design} \times R_c \times L}{1000} \]

Where \(L\) is one-way cable length in metres. Division by 1000 converts the length–resistance product from m·Ω/km to Ω.

Voltage Drop Percentage

\[ V_{d\%} = \frac{V_d}{V_{system}} \times 100 \]

Temperature Correction Factor

\[ C_t = \sqrt{\frac{T_{max} - T_{ambient}}{T_{max} - T_{ref}}} \]

Where \(T_{max}\) is the conductor temperature rating (75 or 90 °C), \(T_{ref} = 30\) °C is the reference ambient temperature, and \(T_{ambient}\) is the actual ambient temperature. Per IEC 60287.

Grouping Correction Factor

\(C_g\) is read from a lookup table based on the number of grouped circuits and whether the installation is enclosed or open air:

Circuits	Enclosed	Open Air
1	1.00	1.00
2	0.80	0.88
3	0.70	0.82
4 – 6	0.65	0.77
7 – 9	0.60	0.73
10 – 20	0.50	0.68

Installation Method Correction Factor

Method	\(C_c\)
Open Air (Clipped Direct)	1.00
Cable Tray (Perforated)	0.95
Buried Direct in Ground	0.90
Enclosed in Conduit	0.80
Enclosed Trunking	0.75

Adjusted Ampacity

\[ I_{adjusted} = I_{tabulated} \times C_t \times C_g \times C_c \]

The design current must not exceed the adjusted ampacity: \(I_{design} \leq I_{adjusted}\).

Final Conductor Selection

\[ A_{selected} = \max\bigl(A_{vd},\; A_{ampacity}\bigr) \]

The selected conductor is the larger of the size required for voltage drop compliance and the size required for ampacity compliance, rounded up to the next standard size.

Power Loss

\[ P_{loss} = V_d \times I_{design} \]

Power dissipated as heat in the cable conductors (watts).

Ampacity Table — Copper 75 °C (Reference: 30 °C Ambient, Single Circuit)

CSA (mm²)	Enclosed (A)	Open Air (A)
1.0	13.5	17
1.5	17.5	22
2.5	24	30
4	32	40
6	41	51
10	57	70
16	76	94
25	101	119
35	125	148
50	151	180
70	192	232
95	232	282
120	269	328
150	309	379
185	353	434
240	415	514
300	477	593
400	542	674
500	621	772
630	716	891

Worked Examples

The 3 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_cable_size; worked examples for every calculation).

#	Case	Cable	I_design (A)	I_adjusted (A)	V_d (%)	Governing	Result
1	48 V battery to inverter, 100 A over 3 m, PVC	35 mm² Cu PVC	125.0	140.6	0.92	current-carrying capacity	PASS
2	600 V DC 30 kW in conduit, three grouped circuits at 40 °C	35 mm² Cu XLPE	62.5	73.5	0.61	current-carrying capacity	PASS
3	24 V solar array, 20 A over 15 m: voltage drop governs	25 mm² Cu XLPE	25.0	136.8	2.58	voltage drop	PASS

Example 1 — 48 V battery to inverter, 100 A over 3 m, PVC

Short, heavy DC run: voltage drop needs only 10.7 mm² but the derated ampacity for the 125 A design current needs 35 mm².

Given: 48 V DC, continuous current 100 A, route 3 m one way; copper PVC (75 °C) installed unenclosed (engine method “Cable Tray (Perforated)”), 1 circuit, ambient 30 °C (default), safety factor 1.25 (default), voltage-drop limit 3 % (default), conductor resistance at 70 °C (engine default).

Design current	I = 100.00 A (given); I_design = 100.00 × 1.25 = 125.00 A
Voltage-drop minimum	V_drop,max = 3 / 100 × 48 = 1.440 V; ρ(70 °C) = 0.017241 × (1 + 0.00393 × 50) = 0.020629 Ω·mm²/m; A_min = 2 × 0.020629 × 125.00 × 3 / 1.440 = 10.74 mm² → next standard size 16 mm²
Derating	C_t = √((75 − 30) / (75 − 30)) = 1.000; C_g = 1.00 (1 circuit, open air); C_c = 0.95 (Cable Tray (Perforated)); F = 1.000 × 1.00 × 0.95 = 0.9500
Ampacity	I_tab,req = 125.00 / 0.9500 = 131.6 A; first size ≥ 16 mm² with a tabulated rating above it: 35 mm² (open-air column 148.0 A = 148.00 A). I_adjusted = 148.00 × 0.9500 = 140.60 A vs I_design 125.00 A (loading 88.9 %) PASS
Voltage drop	R_c = 0.020629 × 1000 / 35 = 0.5894 Ω/km; V_d = 2 × 125.00 × 0.5894 × 3 / 1000 = 0.442 V = 0.921 % of 48 V, limit 3 % PASS
Power loss	P_loss = 0.442 × 125.00 = 55.3 W
Result	2/2 checks pass → PASS; 35 mm² copper; power loss 55.3 W; governing: current carrying capacity. DC engine: voltage drop is calculated at the design current (load × safety factor), which is conservative; no short-circuit or earth-fault checks are made.
Reference	calcs/test_c0019 TestDefaultParameters (48 V, 100 A, 3 m, 75 °C → 35 mm²); hand: design 1.25 × 100 = 125 A; ρ70 = 0.017241 × (1 + 0.00393 × 50) = 0.020629 Ω·mm²/m; ΔV = 2 × 125 × 0.020629 × 3 / 35 = 0.442 V = 0.921 %. (validation case `c0019-48v-100a-3m-pvc`)

Example 2 — 600 V DC 30 kW in conduit, three grouped circuits at 40 °C

Temperature, grouping and conduit factors multiply to 0.51, so ampacity sets 35 mm² although voltage drop would allow 10 mm².

Given: 600 V DC, load 30 kW, route 50 m one way; copper XLPE (90 °C) installed conduit (engine method “Enclosed in Conduit”), 3 circuits, ambient 40 °C, safety factor 1.25 (default), voltage-drop limit 3 % (default), conductor resistance at 70 °C (engine default).

Design current	I = 30 kW × 1000 / 600 V = 50.00 A; I_design = 50.00 × 1.25 = 62.50 A
Voltage-drop minimum	V_drop,max = 3 / 100 × 600 = 18.000 V; ρ(70 °C) = 0.017241 × (1 + 0.00393 × 50) = 0.020629 Ω·mm²/m; A_min = 2 × 0.020629 × 62.50 × 50 / 18.000 = 7.16 mm² → next standard size 10 mm²
Derating	C_t = √((90 − 40) / (90 − 30)) = 0.9129; C_g = 0.70 (3 circuits, enclosed); C_c = 0.80 (Enclosed in Conduit); F = 0.9129 × 0.70 × 0.80 = 0.5112
Ampacity	I_tab,req = 62.50 / 0.5112 = 122.3 A; first size ≥ 10 mm² with a tabulated rating above it: 35 mm² (enclosed column 125.0 A × 1.15 (90 °C) = 143.75 A). I_adjusted = 143.75 × 0.5112 = 73.49 A vs I_design 62.50 A (loading 85.1 %) PASS
Voltage drop	R_c = 0.020629 × 1000 / 35 = 0.5894 Ω/km; V_d = 2 × 62.50 × 0.5894 × 50 / 1000 = 3.684 V = 0.614 % of 600 V, limit 3 % PASS
Power loss	P_loss = 3.684 × 62.50 = 230.2 W
Result	2/2 checks pass → PASS; 35 mm² copper; power loss 230.2 W; governing: current carrying capacity. DC engine: voltage drop is calculated at the design current (load × safety factor), which is conservative; no short-circuit or earth-fault checks are made.
Reference	Hand calculation on the DC engine's method (calcs/test_c0019 factor tests): I = 30 kW / 600 V = 50 A, design 62.5 A; Ct = √((90−40)/(90−30)) = 0.9129, Cg = 0.70 (3 enclosed circuits), Cc = 0.80 (conduit); 25 mm² gives 101 × 1.15 × 0.5112 = 59.4 A < 62.5 A, 35 mm² gives 125 × 1.15 × 0.5112 = 73.49 A; ΔV = 2 × 62.5 × 0.589 Ω/km × 0.05 km = 3.68 V = 0.614 %. (validation case `c0019-600v-30kw-conduit-grouped`)

Example 3 — 24 V solar array, 20 A over 15 m: voltage drop governs

Low-voltage DC: the 3 % limit is only 0.72 V, so 25 mm² is set by voltage drop while the open-air ampacity is barely used.

Given: 24 V DC, continuous current 20 A, route 15 m one way; copper XLPE (90 °C) installed unenclosed (engine method “Open Air (Clipped Direct)”), 1 circuit, ambient 30 °C (default), safety factor 1.25 (default), voltage-drop limit 3 % (default), conductor resistance at 70 °C (engine default).

Design current	I = 20.00 A (given); I_design = 20.00 × 1.25 = 25.00 A
Voltage-drop minimum	V_drop,max = 3 / 100 × 24 = 0.720 V; ρ(70 °C) = 0.017241 × (1 + 0.00393 × 50) = 0.020629 Ω·mm²/m; A_min = 2 × 0.020629 × 25.00 × 15 / 0.720 = 21.49 mm² → next standard size 25 mm²
Derating	C_t = √((90 − 30) / (90 − 30)) = 1.000; C_g = 1.00 (1 circuit, open air); C_c = 1.00 (Open Air (Clipped Direct)); F = 1.000 × 1.00 × 1.00 = 1.000
Ampacity	I_tab,req = 25.00 / 1.000 = 25.0 A; first size ≥ 25 mm² with a tabulated rating above it: 25 mm² (open-air column 119.0 A × 1.15 (90 °C) = 136.85 A). I_adjusted = 136.85 × 1.000 = 136.85 A vs I_design 25.00 A (loading 18.3 %) PASS
Voltage drop	R_c = 0.020629 × 1000 / 25 = 0.8252 Ω/km; V_d = 2 × 25.00 × 0.8252 × 15 / 1000 = 0.619 V = 2.579 % of 24 V, limit 3 % PASS
Power loss	P_loss = 0.619 × 25.00 = 15.5 W
Result	2/2 checks pass → PASS; 25 mm² copper; power loss 15.5 W; governing: voltage drop. DC engine: voltage drop is calculated at the design current (load × safety factor), which is conservative; no short-circuit or earth-fault checks are made.
Reference	Hand calculation on the DC engine's method (help page Formulas): design 1.25 × 20 = 25 A; ρ70 = 0.017241 × (1 + 0.00393 × 50) = 0.020629 Ω·mm²/m; Vmax = 0.03 × 24 = 0.72 V; Amin = 2 × 0.020629 × 25 × 15 / 0.72 = 21.49 mm² → 25 mm²; Rc = 0.020629 × 1000 / 25 = 0.8252 Ω/km; ΔV = 2 × 25 × 0.8252 × 15 / 1000 = 0.619 V = 2.579 %; ampacity 119 A (open air, 25 mm²) × 1.15 (90 °C) × Ct 1.00 × Cg 1.00 × Cc 1.00 = 136.85 A ≥ 25 A. (validation case `c0019-24v-solar-20a-15m-vd`)

Standards & References

AS/NZS 3008.1.1:2017 — Electrical installations — Selection of cables — Cables for alternating voltages up to and including 0.6/1 kV (ampacity tables and derating factors)
IEC 60364-5-52 — Low-voltage electrical installations — Selection and erection of electrical equipment — Wiring systems
IEC 60287 — Electric cables — Calculation of the current rating (temperature correction factor formula)
AS/NZS 5033 — Installation and safety requirements for photovoltaic (PV) arrays (solar-specific voltage drop limits)
IEC 62548 — Photovoltaic (PV) arrays — Design requirements
Standard conductor sizes per IEC 60228: 1.0, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630 mm²

Tips & Limitations

Tips

Low-voltage DC systems (12 V, 24 V, 48 V) are highly sensitive to voltage drop — even short runs may require large conductors.
The default safety factor of 1.25 is standard practice for continuous DC loads per NEC 210.19 and AS/NZS 3000.
For solar PV, a 2% voltage drop limit from string to inverter is common best practice; for battery systems, 3% is typical.
DC circuits have no reactance component, so voltage drop depends only on resistance — this makes temperature a significant factor.
When ampacity governs the selection (common at low voltages), the actual voltage drop will be well below the limit.
For currents exceeding the largest single conductor (630 mm²), consider parallel conductors — the calculator provides recommendations in the warnings.
A 90 °C conductor rating provides approximately 15% more ampacity than 75 °C for the same conductor size.

Limitations

Copper only — aluminium conductors are not currently supported. For aluminium, resistivity is approximately 1.6× that of copper.
Steady-state only — fault current withstand and short-circuit sizing are not considered. Use a separate short-circuit calculator for protective device coordination.
No harmonic derating — DC circuits do not have harmonics, but if the DC source contains ripple (e.g., rectified AC), additional derating may be required.
Single cable pair — parallel conductor sizing is calculated as a recommendation only, not a full parallel circuit analysis.
Temperature correction factor is clamped between 0.1 and 1.5 — extreme ambient temperatures outside −20 to 80 °C are not supported.
Operating temperature must not exceed the conductor rating, and ambient temperature must not exceed the operating temperature. The calculator will reject invalid combinations.

Developer Guide

Architecture

File layout:

components/c0019_dc_cable_sizing/component.py — Component registration, parameter schema, SVG diagram generator, QR code generator
components/c0019_dc_cable_sizing/calculations.py — Pure calculation functions, constants, ampacity lookup tables (no Django dependencies)
components/c0019_dc_cable_sizing/equations.py — MathJax equation builders for PDF/HTML rendering (unified articles pattern)
components/c0019_dc_cable_sizing/excel_generator.py — Excel report generator (openpyxl workbook)

The calculator class is DCCableSizingCalculator, extending ElectricalCalculator. It validates parameters then delegates to the pure function calculate_dc_cable_sizing(data).

Constants:

COPPER_RESISTIVITY_20C = 0.017241 Ω·mm²/m
COPPER_TEMP_COEFFICIENT = 0.00393 /°C
STANDARD_CONDUCTOR_SIZES — 20 sizes from 1.0 to 630 mm²
AMPACITY_TABLE_75C — enclosed and open-air ratings for each standard size

Registered as: calc_type='dc_cable_sizing'

Code Paths

Key functions in calculations.py:

Function	Purpose
`calculate_dc_cable_sizing(data)`	Main entry point — orchestrates the full sizing workflow and returns results dict
`calculate_resistivity_at_temp(temperature)`	Copper resistivity adjusted to operating temperature
`calculate_resistance_per_km(conductor_size_mm2, temperature)`	Conductor resistance in Ω/km
`calculate_voltage_drop(current, resistance_per_km, length_m)`	DC voltage drop for two-conductor circuit
`get_temperature_correction_factor(ambient_temp, conductor_rating)`	Temperature derating factor \(C_t\) per IEC 60287
`get_grouping_correction_factor(num_circuits, installation_method)`	Grouping derating factor \(C_g\)
`get_installation_method_factor(installation_method)`	Installation method correction factor \(C_c\)
`get_tabulated_ampacity(conductor_size_mm2, installation_method, conductor_rating)`	Lookup base ampacity from table (with 90 °C adjustment)
`find_next_standard_size(calculated_size_mm2)`	Round up to next standard conductor size
`calculate_parallel_conductors(design_current, max_single_conductor_size)`	Parallel conductor recommendation for high currents

Key functions in component.py:

Function	Purpose
`DCCableSizingCalculator.validate_parameters()`	Validates voltage, current, length, VD%, and temperature constraints
`DCCableSizingCalculator.calculate()`	Prepares data dict and delegates to pure calculation function
`generate_cable_sizing_diagram_svg(parameters, results)`	SVG visualisation of voltage drop and current capacity bars

Calculation flow (10 steps):

Design current = continuous current × safety factor
Max voltage drop in volts = VD% × system voltage / 100
Resistivity at operating temperature
Minimum CSA from voltage drop formula
Round up to next standard size
Calculate derating factors: \(C_t\), \(C_g\), \(C_c\)
Required tabulated ampacity = design current / combined derating
Find smallest standard size meeting ampacity requirement
Final size = max(VD-sized, ampacity-sized)
Calculate actual performance, power loss, and compliance status

Testing

Running tests:

pytest calcs/test_c0019_dc_cable_sizing.py -v

Worked examples: the examples above are generated from the MCP validation corpus (mcp_public/validation/cases/fe_calc_cable_size/) by python manage.py generate_help_examples --tool fe_calc_cable_size, which writes help/_dc_cable_sizing_examples.html; calcs/test_help_examples.py fails if the committed fragment is stale.

Key test scenarios:

Low-voltage high-current (48 V, 100 A) — ampacity-governed
High-voltage low-current (600 V, 10 A) — voltage-drop-governed
Elevated ambient temperature derating
Multiple grouped circuits derating
Enclosed installation method derating
90 °C rated conductor vs 75 °C
Edge cases: very short cable, very long cable, maximum current
Validation: operating temp exceeding rating, ambient exceeding operating temp

How to extend:

To add aluminium support, create a parallel ampacity table (AMPACITY_TABLE_AL_75C) and add a conductor_material parameter to the schema.
To add new insulation types, extend the insulation_type enum and add corresponding ampacity adjustments.
To support parallel conductor sizing as a first-class output, integrate calculate_parallel_conductors() into the main calculation flow.
Write tests covering new lookup values and edge cases.