What Is the Voltage Drop for 4 AWG at 13A and 125 Feet?

4 AWG copper carrying 13 amps over 125 feet on a single-phase / DC circuit drops 1 volts (0.8342% on a 120V source). This sits within the 3% branch target and the 5% feeder+branch total target that NEC 210.19(A) Informational Note 4 cites. Both are planning targets, not code requirements.

4 AWG, 13A, 125ft · single-phase / DC

1 V drop (0.8342% on 120V)

On 120V circuit0.8342%

On 240V circuit0.4171%

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

4 AWG

Amps

Feet

Drop @ 120V

1.00V (0.83%)

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 × 125 × 13 × 0.308) ÷ 1000 = 1 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 ÷ 120) × 100 = 0.8342%

On 240V: (1 ÷ 240) × 100 = 0.4171%

How This Estimate Changes with Run Length and Gauge

Gauge Check

4 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 4 AWG at 13A at different distances:

Distance	Drop (V)	% on 120V	% on 240V	NEC (120V)
25ft	0.2002V	0.1668%	0.0834%	OK
50ft	0.4004V	0.3337%	0.1668%	OK
75ft	0.6006V	0.5005%	0.2503%	OK
100ft	0.8008V	0.6673%	0.3337%	OK
150ft	1.2V	1%	0.5005%	OK
200ft	1.6V	1.33%	0.6673%	OK
300ft	2.4V	2%	1%	OK

Same Run, Different Wire Gauges

How does wire gauge affect voltage drop for 13A at 125 feet on 120V single-phase / DC? Only gauges whose branch-circuit OCP cap is at or above the 13A 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)
4 AWG	1V	0.8342%	0.4171%	OK
3 AWG	0.7963V	0.6635%	0.3318%	OK
2 AWG	0.6305V	0.5254%	0.2627%	OK
1 AWG	0.5005V	0.4171%	0.2085%	OK
1/0 AWG	0.3965V	0.3304%	0.1652%	OK
2/0 AWG	0.3143V	0.2619%	0.1309%	OK

Related Calculations

cable Wire size: 13A, 125ft bolt Watts to amps calculator functions Voltage drop formula

Frequently Asked Questions

What is the voltage drop for 4 AWG at 13A and 125ft?expand_more

4 AWG carrying 13A over 125ft has a 1V drop (0.8342% on 120V). Reference: 0.4171% 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 0.4171% on 240V versus 0.8342% on 120V.

Does 4 AWG meet the 3% drop target at 13A and 125ft on 120V?expand_more

On 120V, this run sits at 0.8342%, 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 4 AWG for this run?expand_more

4 AWG already sits within the 3% branch-circuit drop target at these inputs (0.8342% 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.