What is an acceptable voltage drop?

The NEC's informational notes suggest 3% or less for a branch circuit and 5% or less for feeder plus branch together. Sensitive or motor loads benefit from staying lower.

Does aluminum wire have more voltage drop than copper?

Yes, about 64% more at the same size, because aluminum's resistivity is higher. That's why aluminum runs are usually one or two sizes larger.

How do I reduce voltage drop?

Use a larger wire, shorten the run, run conductors in parallel, or use a higher supply voltage so the same power needs less current.

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Voltage Drop Calculator

Voltage lost along a copper or aluminum wire run, and which wire size to use.

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Voltage Drop Calculator

Voltage lost along a wire run, AWG or mm²

Wire material

Wire sizes

Wire size

Circuit

Source voltage

Load current

One-way distance

Distance unit

Conductors in parallel per phase

Conductor temperature

°C

NEC tables use 75 °C.

Voltage drop

6.34 V

5.28% of 120 V, leaving 113.66 V at the load

Wire: 12 AWG copper
Cross-section: 3.309 mm²
Resistance1.5844 Ω per 1,000 ft: 5.1983 Ω/km
Voltage at the load: 113.66 V
Smallest size under 3% dropCommon target for branch circuits: 9 AWG

Uses DC resistance (skin effect and reactance ignored, which is fine for small and mid-size wire at 50/60 Hz). The NEC recommends at most 3% drop on a branch circuit and 5% in total.

How it was calculated

ρ = 1.720e-8 × (1 + 0.00393 × (20 - 20)) = 1.7200e-8 Ω·m
R = ρ ÷ A = 1.7200e-8 ÷ 3.309e-6 m² = 0.005198 Ω/m
Vd = 2 × 20 A × 0.005198 Ω/m × 30.48 m = 6.338 V

Size	mm²	Drop (V)	Drop (%)
1000 kcmil	506.707	0.04	0.03%
750 kcmil	380.03	0.06	0.05%
600 kcmil	304.024	0.07	0.06%
500 kcmil	253.354	0.08	0.07%
400 kcmil	202.683	0.1	0.09%
350 kcmil	177.347	0.12	0.1%
300 kcmil	152.012	0.14	0.11%
250 kcmil	126.677	0.17	0.14%
4/0 AWG	107.219	0.2	0.16%
3/0 AWG	85.029	0.25	0.21%
2/0 AWG	67.431	0.31	0.26%
1/0 AWG	53.475	0.39	0.33%
1 AWG	42.408	0.49	0.41%
2 AWG	33.631	0.62	0.52%
3 AWG	26.67	0.79	0.66%
4 AWG	21.151	0.99	0.83%
5 AWG	16.773	1.25	1.04%
6 AWG	13.302	1.58	1.31%
7 AWG	10.549	1.99	1.66%
8 AWG	8.366	2.51	2.09%
9 AWG	6.634	3.16	2.63%
10 AWG	5.261	3.99	3.32%
11 AWG	4.172	5.03	4.19%
12 AWG (selected)	3.309	6.34	5.28%
13 AWG	2.624	7.99	6.66%
14 AWG	2.081	10.08	8.4%
15 AWG	1.65	12.71	10.59%
16 AWG	1.309	16.02	13.35%
17 AWG	1.038	20.21	16.84%
18 AWG	0.823	25.48	21.23%
19 AWG	0.653	32.13	26.77%
20 AWG	0.518	40.51	33.76%
21 AWG	0.41	51.09	42.57%
22 AWG	0.326	64.42	53.68%
23 AWG	0.258	81.23	67.69%
24 AWG	0.205	102.43	85.36%
25 AWG	0.162	129.16	107.63%
26 AWG	0.129	162.87	135.72%
27 AWG	0.102	205.37	171.14%
28 AWG	0.081	258.97	215.81%
29 AWG	0.064	326.56	272.13%
30 AWG	0.051	411.78	343.15%

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How the voltage drop calculator works

Voltage drop is current × the resistance of the wire. For DC and single phase AC the current travels out and back, so Vd = 2 × I × R × L, where L is the one-way length and R the resistance per unit length. Three phase circuits use √3 instead of 2.

Resistance per length = resistivity ÷ cross-sectional area. Copper is about 1.72 × 10⁻⁸ Ω·m at 20 °C and aluminum about 2.82 × 10⁻⁸ Ω·m; both rise roughly 0.4% per degree. AWG diameters follow d = 0.127 mm × 92^((36 - gauge) ÷ 39).

Running conductors in parallel divides the resistance by the number of sets. Always check the ampacity tables and local code; this calculator only covers voltage drop.

Using and checking your result

Published by JustYourCalculator. Check the units and assumptions above, and compare a known example before relying on the output. Calculations use browser arithmetic and may round displayed values.

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