WireWorks

Voltage Drop Calculator

Longer wire runs and thinner gauges lose more voltage — this quantifies exactly how much.

Current (A)

One-way wire length (ft)

Wire resistance (Ω per 1000 ft)

Voltage drop

9.21 V

12 AWG copper is about 1.98 Ω/1000ft, 14 AWG is about 3.07 Ω/1000ft — look up your specific gauge's resistance.

What you entered

Current (A): 15One-way wire length (ft): 100Wire resistance (Ω per 1000 ft): 3.07

1
Apply the voltage drop formula (round-trip distance)
(2 × 15 × 100 × 3.07) ÷ 1000= 9.21 V

Result

Voltage drop: 9.21 V

15 A over a 100 ft one-way run drops about 9.21 volts — keep this under 3% of the circuit voltage for good practice.

Voltage drop over a run of copper wire

Drop is current times total circuit resistance, and the current travels both ways, so the run length is doubled. Resistances shown are per 1,000 feet of copper.

Current	One-way run	Wire (ohms/1000 ft)	Voltage drop	% of 120 V
15 A	50 ft	14 AWG (1.588)	2.382 V	2.0%
20 A	50 ft	14 AWG (1.588)	3.176 V	2.6%
15 A	100 ft	16 AWG (2.525)	7.575 V	6.3%
20 A	100 ft	14 AWG (1.588)	6.352 V	5.3%
20 A	100 ft	12 AWG (0.999)	3.996 V	3.3%
30 A	150 ft	10 AWG (0.628)	5.652 V	4.7%

The usual guidance is to keep drop under 3% on a branch circuit and under 5% overall. Rows three and four exceed that, which is why long runs get upsized: moving from 14 to 12 AWG on the 100-foot run cuts the drop from 5.3% to 3.3%. Electrical work is life-safety work and is subject to local code and inspection. Use these figures for planning and have a licensed electrician verify anything you intend to build.

Why round-trip distance is used

Current flows out to the load and back through the circuit, so the actual resistance encountered is based on double the one-way distance — this calculator already accounts for that in its formula.

The 3% rule of thumb

A voltage drop under about 3% of the circuit's nominal voltage is generally considered acceptable for branch circuits — bigger drops waste energy as heat in the wire and can cause dim lights or underperforming motors at the far end of a long run.

Frequently asked questions

I'm running a 15-amp circuit 100 feet to my detached garage — what's the voltage drop?

On 14 AWG wire at 120V, you'd lose about 7.7V (6.4%) over 100 feet — well above the 3% recommendation. You'd need to upgrade to 10 AWG wire to keep the drop under 3% at that distance.

What voltage drop percentage is acceptable?

3% is the standard recommendation for branch circuits (the NEC suggests 3% for branches and 5% total for feeder + branch combined). Above 3%, lights dim noticeably and motors may overheat or underperform.

Does the calculator account for the return wire?

Yes — the formula uses round-trip distance (double the one-way run length) because current flows out through the hot wire and back through the neutral. The 100-foot run you measure is actually 200 feet of total wire resistance.

How do I fix excessive voltage drop?

Three options: use thicker wire (lower AWG gauge), shorten the run distance, or reduce the load on the circuit. For long runs to outbuildings, thicker wire is usually the only practical solution.