What Is the Voltage Drop for 4 AWG at 38A and 100 Feet?

4 AWG copper carrying 38 amps over 100 feet on a single-phase / DC circuit drops 2.34 volts (1.95% 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, 38A, 100ft · single-phase / DC

2.34 V drop (1.95% on 120V)

On 120V circuit1.95%

On 240V circuit0.9753%

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

2.34V (1.95%)

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 × 38 × 0.308) ÷ 1000 = 2.34 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: (2.34 ÷ 120) × 100 = 1.95%

On 240V: (2.34 ÷ 240) × 100 = 0.9753%

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 38A at different distances:

Distance	Drop (V)	% on 120V	% on 240V	NEC (120V)
25ft	0.5852V	0.4877%	0.2438%	OK
50ft	1.17V	0.9753%	0.4877%	OK
75ft	1.76V	1.46%	0.7315%	OK
100ft	2.34V	1.95%	0.9753%	OK
150ft	3.51V	2.93%	1.46%	OK
200ft	4.68V	3.9%	1.95%	Caution
300ft	7.02V	5.85%	2.93%	Past 5%

Same Run, Different Wire Gauges

How does wire gauge affect voltage drop for 38A at 100 feet on 120V single-phase / DC? Only gauges whose branch-circuit OCP cap is at or above the 38A 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	2.34V	1.95%	0.9753%	OK
3 AWG	1.86V	1.55%	0.7758%	OK
2 AWG	1.47V	1.23%	0.6143%	OK
1 AWG	1.17V	0.9753%	0.4877%	OK
1/0 AWG	0.9272V	0.7727%	0.3863%	OK
2/0 AWG	0.7349V	0.6124%	0.3062%	OK

Related Calculations

cable Wire size: 38A, 100ft bolt Watts to amps calculator functions Voltage drop formula

Frequently Asked Questions

What is the voltage drop for 4 AWG at 38A and 100ft?expand_more

4 AWG carrying 38A over 100ft has a 2.34V drop (1.95% on 120V). Reference: 0.9753% 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 0.9753% on 240V versus 1.95% on 120V.

Does 4 AWG meet the 3% drop target at 38A and 100ft on 120V?expand_more

On 120V, this run sits at 1.95%, 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 (1.95% 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.