What Is the Voltage Drop for 3 AWG at 39A and 100 Feet?

Running 39A through 3 AWG copper for 100 feet on a single-phase / DC circuit produces a 1.91-volt drop. On a 120V source that is 1.59%; on 240V it is 0.7963%. NEC 210.19(A) Informational Note 4 recommends keeping branch-circuit drop at or below 3% and total feeder+branch drop at or below 5%, these are performance recommendations, not code requirements.

3 AWG, 39A, 100ft · single-phase / DC

1.91 V drop (1.59% on 120V)

On 120V circuit1.59%

On 240V circuit0.7963%

Circuit basis: This uses the single-phase / DC round-trip formula (factor of 2) for the voltage drop across the two circuit conductors. For a three-phase line-to-line run use the three-phase version of the page (append ?type=3ph). Switch to the three-phase version →

Gauge

3 AWG

Amps

Feet

Drop @ 120V

1.91V (1.59%)

Assumes a 120V source on a single-phase / DC circuit. Use the circuit-basis link above to switch between single-phase/DC and three-phase.

Voltage Drop Formula (single-phase / DC)

V_drop = (2 × L × I × R) ÷ 1000

(2 × 100 × 39 × 0.245) ÷ 1000 = 1.91 V

DC and single-phase AC use the round-trip factor of 2. Current travels out to the load on one conductor and returns on another.

For a three-phase circuit at the same amps and distance, see the three-phase version (uses √3 instead of 2, so the drop is about 13.4% lower).

Percentage

%VD = (V_drop ÷ V_source) × 100

On 120V: (1.91 ÷ 120) × 100 = 1.59%

On 240V: (1.91 ÷ 240) × 100 = 0.7963%

How This Estimate Changes with Run Length and Gauge

Gauge Check

3 AWG clears the 3% drop target at these inputs. A smaller conductor may also meet it with less margin. See the minimum gauge for this load and distance.

Impact of Distance

Voltage drop is proportional to distance. Here is 3 AWG at 39A at different distances:

Distance	Drop (V)	% on 120V	% on 240V	NEC (120V)
25ft	0.4778V	0.3981%	0.1991%	OK
50ft	0.9555V	0.7963%	0.3981%	OK
75ft	1.43V	1.19%	0.5972%	OK
100ft	1.91V	1.59%	0.7963%	OK
150ft	2.87V	2.39%	1.19%	OK
200ft	3.82V	3.19%	1.59%	Caution
300ft	5.73V	4.78%	2.39%	Caution

Same Run, Different Wire Gauges

How does wire gauge affect voltage drop for 39A at 100 feet on 120V single-phase / DC? Only gauges whose branch-circuit OCP cap is at or above the 39A load are listed, since thinner gauges would fail the ampacity check before drop even matters.

Gauge	Drop (V)	% on 120V	% on 240V	3% Target (120V)
3 AWG	1.91V	1.59%	0.7963%	OK
2 AWG	1.51V	1.26%	0.6305%	OK
1 AWG	1.2V	1%	0.5005%	OK
1/0 AWG	0.9516V	0.793%	0.3965%	OK
2/0 AWG	0.7543V	0.6286%	0.3143%	OK
3/0 AWG	0.5975V	0.4979%	0.249%	OK

Related Calculations

cable Wire size: 39A, 100ft bolt Watts to amps calculator functions Voltage drop formula

Frequently Asked Questions

What is the voltage drop for 3 AWG at 39A and 100ft?expand_more

3 AWG carrying 39A over 100ft has a 1.91V drop (1.59% on 120V). Reference: 0.7963% on 240V.

Is voltage drop worse on 120V or 240V?expand_more

Same wire, same amps, same distance: the volts dropped are identical. But the percentage is worse on 120V because the drop is a larger fraction of the source voltage. This run would be 0.7963% on 240V versus 1.59% on 120V.

Does 3 AWG meet the 3% drop target at 39A and 100ft on 120V?expand_more

On 120V, this run sits at 1.59%, which is within the 3% branch and 5% feeder+branch total drop targets. NEC 210.19(A) Informational Note 4 cites 3% for branch circuits and 5% for total feeder+branch drop as performance recommendations, not hard code requirements.

What wire gauge should I use instead of 3 AWG for this run?expand_more

3 AWG already sits within the 3% branch-circuit drop target at these inputs (1.59% on 120V). Going to a larger gauge is only useful if you want more headroom for future load growth, longer runs, or tighter drop targets like the 5% feeder+branch total recommendation used in sensitive or motor-heavy installations.

Why does distance affect voltage drop?expand_more

Voltage drop is proportional to distance. The formula multiplies by 2 × the distance (out and back). Doubling the run doubles the drop.

This calculator provides estimates for reference purposes only. Always consult a licensed electrician and verify compliance with the National Electrical Code (NEC) and local electrical codes before performing any electrical work.