AC Light Circuit Voltage Drop

Overview

Calculates voltage drop for single-phase parallel-connected AC lighting circuits with sequential cable runs per IEC 60364, BS 7671, and AS/NZS 3000. Each light fixture taps off a shared cable that runs from the supply through each light in series. The cable section between any two lights carries the cumulative current of all downstream lights, producing a different voltage drop at each fixture location.

This calculator (c0018) shares the same electrical methodology as the Lighting Circuit Voltage Drop calculator (c0015) but is registered under a separate calc_type (ac_light_circuit) and uses its own lookup tables for conductor resistance at 20 °C. The resistance data in c0018 covers CSA values from 0.75 mm² to 25 mm², which is the typical range for lighting sub-circuits. Where c0015 calculates resistance from a broader table set, c0018 also provides a calculate_recommended_cable_size() helper that iterates through standard sizes to find the minimum compliant cable.

Input Parameters

Circuit Parameters

Parameter	Type	Default	Range	Description
`supply_voltage`	Number	230 V	100 – 400 V	Nominal single-phase supply voltage
`power_factor`	Number	1.0	0.5 – 1.0	Load power factor (typically 0.95–1.0 for LED)
`frequency`	Enum: 50 / 60 Hz	50	—	System frequency
`temperature`	Number	30 °C	−20 – 90 °C	Cable operating temperature for resistance adjustment

Conductor Parameters

Parameter	Type	Default	Range	Description
`conductor_material`	Enum: copper / aluminium	copper	—	Conductor material
`cable_csa`	Enum	1.5 mm²	0.75 – 25 mm²	Conductor cross-sectional area (standard sizes: 0.75, 1.0, 1.5, 2.5, 4.0, 6.0, 10, 16, 25)
`voltage_drop_limit`	Number	3%	0.5 – 10%	Maximum voltage drop at any light, % of supply

Light Configuration

Parameter	Type	Default	Range	Description
`num_lights`	Integer	4	1 – 20	Total quantity of lights in the circuit
`lights`	Array of objects	—	1 – 20 items	Sequential light fixtures with cable lengths and power ratings
`lights[n].power`	Number	—	1 – 1000 W	Light fixture power consumption
`lights[n].cable_length`	Number	—	0.1 – 500 m	Cable length from previous light (or from supply for first light)

Output Results

Summary Outputs

Output	Unit	Description
`total_power`	W	Sum of all light fixture powers
`cable_resistance`	Ω/km	Conductor resistance at operating temperature
`max_current`	A	Total current drawn at supply point (first section current)
`max_cable_current`	A	Maximum current rating for the selected cable size
`current_ok`	Boolean	Whether supply current is within cable current rating
`max_voltage_drop`	V	Maximum cumulative voltage drop at any light
`max_voltage_drop_percent`	%	Maximum voltage drop as percentage of supply
`min_voltage`	V	Minimum voltage at any light fixture
`is_compliant`	Boolean	Whether \(V_{d\%,max} \leq 3\%\) and current is within cable rating
`compliance_status`	Text	“PASS” or “FAIL”
`warnings`	List	Warning messages (over-current, excessive VD, low PF)

Per-Light Outputs (`light_results` array)

Output	Unit	Description
`current_in`	A	Current through the cable section feeding this light
`voltage_drop_section`	V	Voltage drop across this cable section only
`cumulative_voltage_drop`	V	Total voltage drop from supply to this light
`voltage_at_light`	V	Actual voltage at this light fixture
`voltage_drop_percent`	%	Cumulative voltage drop as percentage of supply

Cable Section Outputs (`cable_sections` array)

Output	Unit	Description
`section_num`	Integer	Section identifier (1-based)
`from` / `to`	Text	Start and end points (e.g. “Supply” to “Light 1”)
`current`	A	Current through this section
`downstream_power`	W	Cumulative power of all downstream loads
`voltage_drop`	V	Voltage drop in this section

Formulas

Conductor Resistance at Temperature

\[ R(T) = R_{20} \times \bigl(1 + \alpha \times (T - 20)\bigr) \quad (\Omega/\text{km}) \]

Where \(R_{20}\) is the conductor resistance at 20 °C (from lookup table), \(\alpha\) is the temperature coefficient, and \(T\) is the operating temperature in °C.

Material	\(\alpha\) (/°C)
Copper	0.00393
Aluminium	0.00403

Section Current

\[ I_{\text{section},i} = \frac{\displaystyle\sum_{k=i}^{n} P_k}{V_{\text{supply}} \times \cos\phi} \]

Each cable section carries the cumulative current of all downstream lights. Section 1 (supply to first light) carries the total circuit current. As each light taps off, the remaining downstream power decreases and so does the section current.

Section Voltage Drop

\[ \Delta V_i = \frac{2 \times I_{\text{section},i} \times L_i \times R(T)}{1000} \]

Where the factor of 2 accounts for active and neutral conductors (go-and-return path), \(L_i\) is the cable section length in meters, and \(R(T)\) is in Ω/km. Division by 1000 converts the length–resistance product from m·Ω/km to Ω.

Cumulative Voltage Drop

\[ \Delta V_{\text{cum},n} = \sum_{i=1}^{n} \Delta V_i \]

The total voltage drop at light \(n\) is the sum of all section voltage drops from the supply to that light.

Voltage at Light

\[ V_{\text{light},n} = V_{\text{supply}} - \Delta V_{\text{cum},n} \]

Voltage Drop Percentage

\[ V_{d\%,n} = \frac{\Delta V_{\text{cum},n}}{V_{\text{supply}}} \times 100 \]

Compliance Check

\[ \max(V_{d\%,n}) \leq V_{d,\text{limit}\%} \;\;\text{AND}\;\; I_{\text{max}} \leq I_{\text{cable rating}} \implies \text{PASS} \]

Both the voltage drop limit (voltage_drop_limit, default 3% — the IEC 60364-5-52 lighting value; AS/NZS 3000 allows 5% from the point of supply) and the indicative cable current rating must be satisfied for an overall PASS.

Key Lookup Tables

Copper Resistance at 20 °C (Ω/km)

CSA (mm²)	R₂₀ (Ω/km)	Max Current (A)
0.75	24.5	6
1.0	18.1	10
1.5	12.1	16
2.5	7.41	25
4.0	4.61	32
6.0	3.08	40
10	1.83	63
16	1.15	85
25	0.727	110

Aluminium Resistance at 20 °C (Ω/km)

IEC 60228 lists aluminium conductors from 10 mm²; the 2.5, 4 and 6 mm² aluminium values are the component's own (about 1.64× the copper value).

CSA (mm²)	R₂₀ (Ω/km)	Max Current (A)
2.5	12.2	20
4.0	7.56	27
6.0	5.08	35
10	3.08	50
16	1.91	68
25	1.20	88

Worked Examples

The 7 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_voltage_drop; worked examples for every calculation).

#	Case	Cable	Lights / load	R(T) (Ω/km)	Worst light	Max VD	Total vs limit	Result
1	Four LED downlights on 1.5 mm² Cu (c0018)	1.5 mm² Cu, 230 V	4 / 95 W	12.5755	4 (229.7 V)	0.297 V (0.129 %)	0.129 / 3 %	PASS
2	Three 18 W downlights on 1.5 mm² Cu	1.5 mm² Cu, 230 V	3 / 54 W	12.5755	3 (229.9 V)	0.114 V (0.050 %)	0.050 / 3 %	PASS
3	Three 200 W lights 50 m apart on 2.5 mm²	2.5 mm² Cu, 230 V	3 / 600 W	7.7012	3 (226.0 V)	4.018 V (1.747 %)	1.747 / 3 %	PASS
4	Warehouse high-bays on 2.5 mm² (c0018)	2.5 mm² Cu, 230 V	5 / 1,250 W	8.1380	5 (219.8 V)	10.242 V (4.453 %)	4.453 / 3 %	FAIL
5	Aluminium 4 mm² at 50 °C (c0018)	4 mm² Al, 230 V	3 / 300 W	8.4740	3 (229.2 V)	0.774 V (0.336 %)	0.336 / 3 %	PASS
6	18 × 150 W on 1 mm²: current screen fails	1 mm² Cu, 240 V	18 / 2,700 W	20.9453	18 (230.6 V)	9.425 V (3.927 %)	3.927 / 5 %	FAIL
7	110 V site lighting, 8 × 60 W on 2.5 mm²	2.5 mm² Cu, 110 V	8 / 480 W	8.2836	8 (105.7 V)	4.338 V (3.943 %)	3.943 / 5 %	PASS

Example 1 — Four LED downlights on 1.5 mm² Cu (c0018)

The c0018 specification example: the same string as c0015 plus the indicative 16 A current screen.

Given: 4 lights fed in sequence on 1.5 mm² Cu (active + neutral), 230 V single-phase, cos φ = 1, conductor at 30 °C; (section length, light power) from the supply: (15 m, 20 W), (8 m, 30 W), (10 m, 25 W), (12 m, 20 W); voltage-drop limit 3 % from the point of supply (tool default). Engine c0018 (with indicative current screen).

Resistance R(T) = R₂₀ × (1 + α(T − 20)) = 12.1 × (1 + 0.00393 × (30 − 20)) = 12.57553 Ω/km

Load ΣP = 95 W; supply current I₁ = ΣP / (V × cos φ) = 95 / (230 × 1) = 0.4130 A

Sections

I_k = ΣP_downstream / (V × cos φ); ΔV_k = 2 × I_k × L_k × R / 1000; cumulative from the supply:

k	L (m)	ΣP (W)	I_k (A)	ΔV_k (V)	ΣΔV (V)	V (V)	VD %
1	15	95	0.4130	2 × 0.4130 × 15 × 12.5755/1000 = 0.1558	0.1558	229.84	0.068
2	8	75	0.3261	2 × 0.3261 × 8 × 12.5755/1000 = 0.0656	0.2214	229.78	0.096
3	10	45	0.1957	2 × 0.1957 × 10 × 12.5755/1000 = 0.0492	0.2706	229.73	0.118
4	12	20	0.0870	2 × 0.0870 × 12 × 12.5755/1000 = 0.0262	0.2969	229.70	0.129

Worst light Light 4: ΣΔV = 0.2969 V → 0.2969 / 230 × 100 = 0.129 %; minimum voltage 229.70 V

Voltage drop check 0.129 % ≤ 3 %? → 0.129 vs limit 3.000 % PASS

Current screen supply current 0.413 A vs the component's indicative 16 A for 1.5 mm² Cu → 0.41 vs limit 16.00 A PASS

Headroom Load at the limit = (limit − upstream) / max VD = 3.00 / 0.129 = 23.24 × the present load

Result 2/2 checks pass → PASS; worst light 4 at 0.129 % total; governing: voltage drop

Reference c0018 CLAUDE.md 'Expected Results for Test Case' (95 W, ~0.413 A) refined by hand: R(30 °C) = 12.5755 Ω/km; ΣΔV = 0.2969 V = 0.129 %, V = 229.70 V; 0.413 A ≤ 16 A (validation case c0018-4-lights-1mm5-230v)

Example 2 — Three 18 W downlights on 1.5 mm² Cu

The help page's first example: a very light load, drop a twentieth of a percent.

Given: 3 lights fed in sequence on 1.5 mm² Cu (active + neutral), 230 V single-phase, cos φ = 1, conductor at 30 °C; (section length, light power) from the supply: (10 m, 18 W), (8 m, 18 W), (12 m, 18 W); voltage-drop limit 3 % from the point of supply (tool default). Engine c0018 (with indicative current screen).

Resistance R(T) = R₂₀ × (1 + α(T − 20)) = 12.1 × (1 + 0.00393 × (30 − 20)) = 12.57553 Ω/km

Load ΣP = 54 W; supply current I₁ = ΣP / (V × cos φ) = 54 / (230 × 1) = 0.2348 A

Sections

I_k = ΣP_downstream / (V × cos φ); ΔV_k = 2 × I_k × L_k × R / 1000; cumulative from the supply:

k	L (m)	ΣP (W)	I_k (A)	ΔV_k (V)	ΣΔV (V)	V (V)	VD %
1	10	54	0.2348	2 × 0.2348 × 10 × 12.5755/1000 = 0.0591	0.0591	229.94	0.026
2	8	36	0.1565	2 × 0.1565 × 8 × 12.5755/1000 = 0.0315	0.0905	229.91	0.039
3	12	18	0.0783	2 × 0.0783 × 12 × 12.5755/1000 = 0.0236	0.1142	229.89	0.050

Worst light Light 3: ΣΔV = 0.1142 V → 0.1142 / 230 × 100 = 0.050 %; minimum voltage 229.89 V

Voltage drop check 0.050 % ≤ 3 %? → 0.050 vs limit 3.000 % PASS

Current screen supply current 0.235 A vs the component's indicative 16 A for 1.5 mm² Cu → 0.23 vs limit 16.00 A PASS

Headroom Load at the limit = (limit − upstream) / max VD = 3.00 / 0.050 = 60.44 × the present load

Result 2/2 checks pass → PASS; worst light 3 at 0.050 % total; governing: voltage drop

Reference Hand calculation: R(30 °C) = 12.5755 Ω/km; I = 54/36/18 W ÷ 230 V = 0.23478/0.15652/0.07826 A; ΔV = 2 × 12.5755/1000 × (0.23478 × 10 + 0.15652 × 8 + 0.07826 × 12) = 0.0590 + 0.0315 + 0.0236 = 0.1142 V = 0.050 % (validation case c0018-3x18w-1mm5)

Example 3 — Three 200 W lights 50 m apart on 2.5 mm²

Long equal sections: the first section carries the whole load and contributes half the drop.

Given: 3 lights fed in sequence on 2.5 mm² Cu (active + neutral), 230 V single-phase, cos φ = 1, conductor at 30 °C; (section length, light power) from the supply: 3 × (50 m, 200 W); voltage-drop limit 3 % from the point of supply (tool default). Engine c0018 (with indicative current screen).

Resistance R(T) = R₂₀ × (1 + α(T − 20)) = 7.41 × (1 + 0.00393 × (30 − 20)) = 7.70121 Ω/km

Load ΣP = 600 W; supply current I₁ = ΣP / (V × cos φ) = 600 / (230 × 1) = 2.6087 A

Sections

I_k = ΣP_downstream / (V × cos φ); ΔV_k = 2 × I_k × L_k × R / 1000; cumulative from the supply:

k	L (m)	ΣP (W)	I_k (A)	ΔV_k (V)	ΣΔV (V)	V (V)	VD %
1	50	600	2.6087	2 × 2.6087 × 50 × 7.7012/1000 = 2.0090	2.0090	227.99	0.873
2	50	400	1.7391	2 × 1.7391 × 50 × 7.7012/1000 = 1.3393	3.3484	226.65	1.456
3	50	200	0.8696	2 × 0.8696 × 50 × 7.7012/1000 = 0.6697	4.0180	225.98	1.747

Worst light Light 3: ΣΔV = 4.0180 V → 4.0180 / 230 × 100 = 1.747 %; minimum voltage 225.98 V

Voltage drop check 1.747 % ≤ 3 %? → 1.747 vs limit 3.000 % PASS

Current screen supply current 2.609 A vs the component's indicative 25 A for 2.5 mm² Cu → 2.61 vs limit 25.00 A PASS

Headroom Load at the limit = (limit − upstream) / max VD = 3.00 / 1.747 = 1.72 × the present load

Result 2/2 checks pass → PASS; worst light 3 at 1.747 % total; governing: voltage drop

Reference Hand calculation: R = 7.41 × 1.0393 = 7.70121 Ω/km; I = 600/400/200 W ÷ 230 V, ΣI = 5.21739 A; ΔV = 2 × 50 × 7.70121/1000 × 5.21739 = 4.0180 V = 1.747 % (validation case c0018-3-lights-2mm5-200w-50m)

Example 4 — Warehouse high-bays on 2.5 mm² (c0018)

The help page's failing run on c0018: voltage drop fails while the 25 A current screen passes.

Given: 5 lights fed in sequence on 2.5 mm² Cu (active + neutral), 230 V single-phase, cos φ = 0.95, conductor at 45 °C; (section length, light power) from the supply: (50 m, 250 W), 4 × (30 m, 250 W); voltage-drop limit 3 % from the point of supply (tool default). Engine c0018 (with indicative current screen).

Resistance R(T) = R₂₀ × (1 + α(T − 20)) = 7.41 × (1 + 0.00393 × (45 − 20)) = 8.13803 Ω/km

Load ΣP = 1,250 W; supply current I₁ = ΣP / (V × cos φ) = 1,250 / (230 × 0.95) = 5.7208 A

Sections

I_k = ΣP_downstream / (V × cos φ); ΔV_k = 2 × I_k × L_k × R / 1000; cumulative from the supply:

k	L (m)	ΣP (W)	I_k (A)	ΔV_k (V)	ΣΔV (V)	V (V)	VD %
1	50	1,250	5.7208	2 × 5.7208 × 50 × 8.1380/1000 = 4.6556	4.6556	225.34	2.024
2	30	1,000	4.5767	2 × 4.5767 × 30 × 8.1380/1000 = 2.2347	6.8903	223.11	2.996
3	30	750	3.4325	2 × 3.4325 × 30 × 8.1380/1000 = 1.6760	8.5663	221.43	3.724
4	30	500	2.2883	2 × 2.2883 × 30 × 8.1380/1000 = 1.1174	9.6837	220.32	4.210
5	30	250	1.1442	2 × 1.1442 × 30 × 8.1380/1000 = 0.5587	10.2424	219.76	4.453

Worst light Light 5: ΣΔV = 10.2424 V → 10.2424 / 230 × 100 = 4.453 %; minimum voltage 219.76 V

Voltage drop check 4.453 % ≤ 3 %? → 4.453 vs limit 3.000 % FAIL

Current screen supply current 5.721 A vs the component's indicative 25 A for 2.5 mm² Cu → 5.72 vs limit 25.00 A PASS

Headroom Load at the limit = (limit − upstream) / max VD = 3.00 / 4.453 = 0.67 × the present load

Result 1/2 checks pass → FAIL; worst light 5 at 4.453 % total; governing: voltage drop. Voltage drop of 4.45% exceeds the limit of 3%

Reference Hand calculation: R(45 °C) = 8.13803 Ω/km; ΔV = 4.6556 + 2.2347 + 1.6760 + 1.1173 + 0.5587 = 10.2424 V = 4.453 % > 3 %; supply current 1250 ÷ 218.5 = 5.721 A ≤ 25 A (validation case c0018-warehouse-5x250w-2mm5-fail)

Example 5 — Aluminium 4 mm² at 50 °C (c0018)

The help page's aluminium example on c0018, with the 27 A aluminium current screen.

Given: 3 lights fed in sequence on 4 mm² Al (active + neutral), 230 V single-phase, cos φ = 1, conductor at 50 °C; (section length, light power) from the supply: (20 m, 100 W), 2 × (15 m, 100 W); voltage-drop limit 3 % from the point of supply (tool default). Engine c0018 (with indicative current screen).

Resistance R(T) = R₂₀ × (1 + α(T − 20)) = 7.56 × (1 + 0.00403 × (50 − 20)) = 8.47400 Ω/km

Load ΣP = 300 W; supply current I₁ = ΣP / (V × cos φ) = 300 / (230 × 1) = 1.3043 A

Sections

I_k = ΣP_downstream / (V × cos φ); ΔV_k = 2 × I_k × L_k × R / 1000; cumulative from the supply:

k	L (m)	ΣP (W)	I_k (A)	ΔV_k (V)	ΣΔV (V)	V (V)	VD %
1	20	300	1.3043	2 × 1.3043 × 20 × 8.4740/1000 = 0.4421	0.4421	229.56	0.192
2	15	200	0.8696	2 × 0.8696 × 15 × 8.4740/1000 = 0.2211	0.6632	229.34	0.288
3	15	100	0.4348	2 × 0.4348 × 15 × 8.4740/1000 = 0.1105	0.7737	229.23	0.336

Worst light Light 3: ΣΔV = 0.7737 V → 0.7737 / 230 × 100 = 0.336 %; minimum voltage 229.23 V

Voltage drop check 0.336 % ≤ 3 %? → 0.336 vs limit 3.000 % PASS

Current screen supply current 1.304 A vs the component's indicative 27 A for 4 mm² Al → 1.30 vs limit 27.00 A PASS

Headroom Load at the limit = (limit − upstream) / max VD = 3.00 / 0.336 = 8.92 × the present load

Result 2/2 checks pass → PASS; worst light 3 at 0.336 % total; governing: voltage drop

Reference Hand calculation: R(50 °C) = 7.56 × 1.1209 = 8.4740 Ω/km; ΔV = 0.4421 + 0.2211 + 0.1105 = 0.7737 V = 0.336 %; 1.304 A ≤ 27 A (validation case c0018-al-4mm2-50c-3x100w)

Example 6 — 18 × 150 W on 1 mm²: current screen fails

Closely spaced heavy fittings: the drop is within 5 % but 11.8 A exceeds the 10 A indicative rating, which governs.

Given: 18 lights fed in sequence on 1 mm² Cu (active + neutral), 240 V single-phase, cos φ = 0.95, conductor at 60 °C; (section length, light power) from the supply: 18 × (2 m, 150 W); voltage-drop limit 5 % from the point of supply. Engine c0018 (with indicative current screen).

Resistance R(T) = R₂₀ × (1 + α(T − 20)) = 18.1 × (1 + 0.00393 × (60 − 20)) = 20.94532 Ω/km

Load ΣP = 2,700 W; supply current I₁ = ΣP / (V × cos φ) = 2,700 / (240 × 0.95) = 11.8421 A

Sections

I_k = ΣP_downstream / (V × cos φ); ΔV_k = 2 × I_k × L_k × R / 1000; cumulative from the supply:

k	L (m)	ΣP (W)	I_k (A)	ΔV_k (V)	ΣΔV (V)	V (V)	VD %
1	2	2,700	11.8421	2 × 11.8421 × 2 × 20.9453/1000 = 0.9921	0.9921	239.01	0.413
2	2	2,550	11.1842	2 × 11.1842 × 2 × 20.9453/1000 = 0.9370	1.9292	238.07	0.804
3	2	2,400	10.5263	2 × 10.5263 × 2 × 20.9453/1000 = 0.8819	2.8111	237.19	1.171
4	2	2,250	9.8684	2 × 9.8684 × 2 × 20.9453/1000 = 0.8268	3.6379	236.36	1.516
5	2	2,100	9.2105	2 × 9.2105 × 2 × 20.9453/1000 = 0.7717	4.4095	235.59	1.837
6	2	1,950	8.5526	2 × 8.5526 × 2 × 20.9453/1000 = 0.7166	5.1261	234.87	2.136
7	2	1,800	7.8947	2 × 7.8947 × 2 × 20.9453/1000 = 0.6614	5.7875	234.21	2.411
8	2	1,650	7.2368	2 × 7.2368 × 2 × 20.9453/1000 = 0.6063	6.3938	233.61	2.664
9	2	1,500	6.5789	2 × 6.5789 × 2 × 20.9453/1000 = 0.5512	6.9450	233.05	2.894
10	2	1,350	5.9211	2 × 5.9211 × 2 × 20.9453/1000 = 0.4961	7.4411	232.56	3.100
11	2	1,200	5.2632	2 × 5.2632 × 2 × 20.9453/1000 = 0.4410	7.8821	232.12	3.284
12	2	1,050	4.6053	2 × 4.6053 × 2 × 20.9453/1000 = 0.3858	8.2679	231.73	3.445
13	2	900	3.9474	2 × 3.9474 × 2 × 20.9453/1000 = 0.3307	8.5986	231.40	3.583
14	2	750	3.2895	2 × 3.2895 × 2 × 20.9453/1000 = 0.2756	8.8742	231.13	3.698
15	2	600	2.6316	2 × 2.6316 × 2 × 20.9453/1000 = 0.2205	9.0947	230.91	3.789
16	2	450	1.9737	2 × 1.9737 × 2 × 20.9453/1000 = 0.1654	9.2600	230.74	3.858
17	2	300	1.3158	2 × 1.3158 × 2 × 20.9453/1000 = 0.1102	9.3703	230.63	3.904
18	2	150	0.6579	2 × 0.6579 × 2 × 20.9453/1000 = 0.0551	9.4254	230.57	3.927

Worst light Light 18: ΣΔV = 9.4254 V → 9.4254 / 240 × 100 = 3.927 %; minimum voltage 230.57 V

Voltage drop check 3.927 % ≤ 5 %? → 3.927 vs limit 5.000 % PASS

Current screen supply current 11.842 A vs the component's indicative 10 A for 1 mm² Cu → 11.84 vs limit 10.00 A FAIL

Headroom Load at the limit = (limit − upstream) / max VD = 5.00 / 3.927 = 1.27 × the present load

Result 1/2 checks pass → FAIL; worst light 18 at 3.927 % total; governing: indicative current capacity. Current of 11.84A exceeds recommended capacity of 10A for 1.0mm² cable

Reference Hand calculation: R(60 °C) = 18.1 × (1 + 0.00393 × 40) = 20.94532 Ω/km; I_k = (19 − k) × 150 ÷ (240 × 0.95); ΣI = 171 × 150 ÷ 228 = 112.5 A; ΔV = 2 × 2 × 20.94532/1000 × 112.5 = 9.4254 V = 3.927 % ≤ 5 %; supply current 2700 ÷ 228 = 11.842 A > 10 A (validation case c0018-1mm2-18x150w-capacity-fail)

Example 7 — 110 V site lighting, 8 × 60 W on 2.5 mm²

A reduced-voltage supply: the same watts draw twice the current, so the percentage drop is far higher.

Given: 8 lights fed in sequence on 2.5 mm² Cu (active + neutral), 110 V single-phase, cos φ = 0.9, conductor at 50 °C; (section length, light power) from the supply: 8 × (12 m, 60 W); voltage-drop limit 5 % from the point of supply. Engine c0018 (with indicative current screen).

Resistance R(T) = R₂₀ × (1 + α(T − 20)) = 7.41 × (1 + 0.00393 × (50 − 20)) = 8.28364 Ω/km

Load ΣP = 480 W; supply current I₁ = ΣP / (V × cos φ) = 480 / (110 × 0.9) = 4.8485 A

Sections

I_k = ΣP_downstream / (V × cos φ); ΔV_k = 2 × I_k × L_k × R / 1000; cumulative from the supply:

k	L (m)	ΣP (W)	I_k (A)	ΔV_k (V)	ΣΔV (V)	V (V)	VD %
1	12	480	4.8485	2 × 4.8485 × 12 × 8.2836/1000 = 0.9639	0.9639	109.04	0.876
2	12	420	4.2424	2 × 4.2424 × 12 × 8.2836/1000 = 0.8434	1.8073	108.19	1.643
3	12	360	3.6364	2 × 3.6364 × 12 × 8.2836/1000 = 0.7229	2.5303	107.47	2.300
4	12	300	3.0303	2 × 3.0303 × 12 × 8.2836/1000 = 0.6024	3.1327	106.87	2.848
5	12	240	2.4242	2 × 2.4242 × 12 × 8.2836/1000 = 0.4820	3.6147	106.39	3.286
6	12	180	1.8182	2 × 1.8182 × 12 × 8.2836/1000 = 0.3615	3.9761	106.02	3.615
7	12	120	1.2121	2 × 1.2121 × 12 × 8.2836/1000 = 0.2410	4.2171	105.78	3.834
8	12	60	0.6061	2 × 0.6061 × 12 × 8.2836/1000 = 0.1205	4.3376	105.66	3.943

Worst light Light 8: ΣΔV = 4.3376 V → 4.3376 / 110 × 100 = 3.943 %; minimum voltage 105.66 V

Voltage drop check 3.943 % ≤ 5 %? → 3.943 vs limit 5.000 % PASS

Current screen supply current 4.848 A vs the component's indicative 25 A for 2.5 mm² Cu → 4.85 vs limit 25.00 A PASS

Headroom Load at the limit = (limit − upstream) / max VD = 5.00 / 3.943 = 1.27 × the present load

Result 2/2 checks pass → PASS; worst light 8 at 3.943 % total; governing: voltage drop

Reference Hand calculation: R(50 °C) = 7.41 × (1 + 0.00393 × 30) = 8.28364 Ω/km; I_k = (9 − k) × 60 ÷ (110 × 0.9); ΣI = 36 × 60 ÷ 99 = 21.8182 A; ΔV = 2 × 12 × 8.28364/1000 × 21.8182 = 4.3376 V = 3.943 % of 110 V; V = 105.66 V (validation case c0018-110v-site-lighting-2mm5)

Standards & References

IEC 60364-5-52 — Low-voltage electrical installations — Selection and erection of electrical equipment — Wiring systems
BS 7671 (IET Wiring Regulations) — Requirements for electrical installations, Appendix 12 (voltage drop)
AS/NZS 3000:2018 — Electrical installations (Wiring Rules), Clause 3.6 (voltage drop)
NEC (NFPA 70) — National Electrical Code, Articles 210.19(A) and 215.2(A) (voltage drop recommendations)
IEC 60228 — Conductors of insulated cables (resistance values at 20 °C)
Standard voltage drop limit for lighting sub-circuits: typically 3% (configurable per local regulations)

Tips & Limitations

Tips

The default 3% limit is typical for lighting sub-circuits. Some standards allow up to 5% total (main + sub-circuit combined).
The worst voltage drop always occurs at the last (most distant) light because cumulative drops accumulate along the cable run.
LED drivers with wide input voltage ranges (e.g., 180–265 V) tolerate higher voltage drops than fluorescent or incandescent loads.
For long runs, increasing cable CSA by one standard size often eliminates voltage drop issues cost-effectively.
Power factor for modern LED luminaires is typically 0.95–1.0. Older magnetic ballast fluorescents may be 0.5–0.6 without correction.
Higher ambient temperatures increase cable resistance — account for ceiling void, roof space, or enclosed luminaire temperatures.
Use the calculate_recommended_cable_size() helper to automatically find the minimum compliant cable size when a circuit fails.

Limitations

Single-phase only — three-phase lighting circuits are not supported by this calculator.
Resistive model only — cable reactance is not included. For small lighting conductors (≤ 16 mm²) reactance is negligible; for larger conductors this is slightly non-conservative.
No inrush current analysis — LED driver inrush currents at switch-on are not modelled. This calculator is for steady-state voltage drop only.
Uniform cable — the same conductor material and CSA is assumed for all sections. Mixed cable sizes are not supported.
Simplified current capacity check — the built-in over-current warning uses an indicative rating table. Always verify current capacity separately using the full cable sizing calculator (c0030 / c0031).
Balanced loading assumed — each light draws its rated power at the supply voltage. Voltage-dependent load variation is not modelled.
CSA range limited to 25 mm² — the lookup tables cover 0.75–25 mm² for copper and 2.5–25 mm² for aluminium. For sizes outside this range, resistance is calculated from resistivity (\(\rho \times 1000 / A\)).

Developer Guide

Architecture

File layout:

components/c0018_ac_light_circuit/component.py — Component registration, parameter schema, summary cards builder, QR code generator
components/c0018_ac_light_circuit/calculations.py — Pure math functions, resistance lookup tables, and current rating tables (no Django dependencies)
components/c0018_ac_light_circuit/equations.py — MathJax equation builder for report generation (uses calcs.core.mathjax_utils)
components/c0018_ac_light_circuit/excel_generator.py — Excel workbook generation with per-light breakdown

The calculator class is ACLightCircuitCalculator, extending ElectricalCalculator. The lights array input is passed directly to the pure calculation function calculate_ac_light_circuit().

Lookup tables are defined as Python dicts in calculations.py:

COPPER_RESISTANCE_20C — Cu R at 20 °C (Ω/km), keyed by CSA string (0.75–25 mm²)
ALUMINIUM_RESISTANCE_20C — Al R at 20 °C (Ω/km), keyed by CSA string (2.5–25 mm²)
MAX_CURRENT_COPPER — Indicative current ratings for copper conductors
MAX_CURRENT_ALUMINIUM — Indicative current ratings for aluminium conductors
RESISTIVITY_20C — Resistivity values (Cu: 0.01724, Al: 0.0283 Ω·mm²/m) used as fallback when CSA is not in the lookup table

Temperature constants in TEMP_COEFFICIENT:

Copper: 0.00393 /°C
Aluminium: 0.00403 /°C

Code Paths

Key functions in calculations.py:

Function	Purpose
`get_cable_resistance(material, csa, temperature)`	Lookup R₂₀ from table (or calculate from resistivity), apply temperature correction, return Ω/km
`calculate_current(power, voltage, power_factor)`	Compute \(I = P / (V \times \cos\phi)\); returns 0 if voltage or PF is ≤ 0
`calculate_voltage_drop_section(current, length, resistance)`	Compute \(\Delta V = 2 \times I \times L \times R / 1000\) for a single cable section
`calculate_ac_light_circuit(data)`	Main entry point — iterates through lights, computes section currents (from cumulative downstream power), voltage drops, cumulative drops, compliance, and warnings. Returns complete results dict.
`calculate_recommended_cable_size(data)`	Iterates standard cable sizes from current upward, returns the first size that achieves compliance (or None)

Key functions in component.py:

Function / Class	Purpose
`ACLightCircuitCalculator.validate_parameters()`	Validates supply voltage, num_lights, lights array length match, individual light power > 0 and cable_length > 0
`ACLightCircuitCalculator.calculate()`	Extracts parameters, calls `calculate_ac_light_circuit()`, attaches parameters to results for equation builder
`equation_builder(calculation_results)`	Ensures `inputs` structure exists, delegates to `equations.build_equations()` for report articles
`summary_cards_builder(results)`	Returns 3 summary cards: max VD% (red/green), min voltage, total power
`register_component(registry)`	Registers calc type `'ac_light_circuit'` with schema, builders, QR code generator, and sidebar template

Calculation flow:

Calculate R(T) from lookup table + temperature correction via get_cable_resistance()
Sum total power from all lights
Iterate through lights: for each section, compute current from remaining downstream power divided by \(V \times \cos\phi\)
Compute \(\Delta V = 2 \times I \times L \times R / 1000\) per section
Accumulate voltage drops, derive voltage at each light
Find maximum VD%, check against voltage_drop_limit
Check maximum current against cable rating table
Generate warnings for over-current, excessive VD, or low PF

Testing

Running tests:

pytest calcs/components/c0018_ac_light_circuit/ -v
pytest calcs/test_c0018* -v

How to extend:

To add new conductor materials, define a new resistance table dict and temperature coefficient in calculations.py, then update TEMP_COEFFICIENT, RESISTIVITY_20C, and the conductor_material enum in PARAMETER_SCHEMA.
To support mixed cable sizes per section, refactor calculate_ac_light_circuit() to accept a per-section CSA value in the lights array and call get_cable_resistance() once per section.
To add reactance for larger conductors, introduce a reactance lookup table and modify the section voltage drop formula to use impedance \(Z = \sqrt{R^2 + X^2}\) instead of resistance alone.
The worked examples above are the fe_calc_voltage_drop validation cases (mcp_public/validation/cases/fe_calc_voltage_drop/c0018-*.json); regenerate with python manage.py generate_help_examples --tool fe_calc_voltage_drop.
Write tests covering edge cases: single light, 20 lights (max), very long runs, aluminium with high temperature, PF < 1, CSA values not in lookup table (fallback to resistivity).

Overview

Input Parameters

Circuit Parameters

Conductor Parameters

Light Configuration

Output Results

Summary Outputs

Per-Light Outputs (light_results array)

Cable Section Outputs (cable_sections array)

Formulas

Conductor Resistance at Temperature

Section Current

Section Voltage Drop

Cumulative Voltage Drop

Voltage at Light

Voltage Drop Percentage

Compliance Check

Key Lookup Tables

Copper Resistance at 20 °C (Ω/km)

Aluminium Resistance at 20 °C (Ω/km)

Worked Examples

Example 1 — Four LED downlights on 1.5 mm² Cu (c0018)

Example 2 — Three 18 W downlights on 1.5 mm² Cu

Example 3 — Three 200 W lights 50 m apart on 2.5 mm²

Example 4 — Warehouse high-bays on 2.5 mm² (c0018)

Example 5 — Aluminium 4 mm² at 50 °C (c0018)

Example 6 — 18 × 150 W on 1 mm²: current screen fails

Example 7 — 110 V site lighting, 8 × 60 W on 2.5 mm²

Standards & References

Tips & Limitations

Tips

Limitations

Developer Guide

Architecture

Code Paths

Testing

Per-Light Outputs (`light_results` array)

Cable Section Outputs (`cable_sections` array)