Size wire for voltage drop: a five-minute method

Ampacity keeps wire from overheating. Voltage drop keeps your system from wasting energy. Here’s the quick math for both.

My Own Energy GridSep 1, 20265 min read

Code tells you the smallest wire that won’t overheat. It doesn’t tell you how much energy you’ll lose on a long run. For solar, aim to keep voltage drop under about 2%.

The formula

Voltage drop (V) = 2 × one-way length (ft) × current (A) × resistance (Ω per 1,000 ft) ÷ 1,000

Divide by the circuit voltage and multiply by 100 for percent.

Copper resistance (approx., per 1,000 ft)

AWG	Ω / 1,000 ft
10	1.24
8	0.778
6	0.491
4	0.308
2	0.194
1/0	0.122
2/0	0.0967
4/0	0.0608

Example 1: a PV string

An 80 ft run, 10.6 A, 333 V string, 10 AWG: 2 × 80 × 10.6 × 1.24 ÷ 1,000 = 2.1 V, or 0.6%. Fine.

Example 2: a 48 V battery cable

A 10 ft run carrying 167 A on 4/0: 2 × 10 × 167 × 0.0608 ÷ 1,000 = 0.20 V, or 0.4% of 48 V. Fine. Try the same run on 2 AWG: 0.65 V, or 1.4%, plus far more heat. Low-voltage, high-current wiring is where people get this wrong.

RememberPick the larger of the two answers: the size code requires for ampacity, and the size that keeps voltage drop under your target.

Quick answers

What voltage drop should I aim for?

Keep solar and battery circuits under about 2% for efficiency; code minimums are about safety, not efficiency.

Next step

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