What Is the Voltage Drop for 4/0 AWG at 20A and 125 Feet?

Running 20A through 4/0 AWG copper for 125 feet on a single-phase / DC circuit produces a 0.304-volt drop. On a 120V source that is 0.2533%; on 240V it is 0.1267%. 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.

4/0 AWG, 20A, 125ft · single-phase / DC

0.304 V drop (0.2533% on 120V)

On 120V circuit0.2533%

On 240V circuit0.1267%

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

4/0 AWG

Amps

Feet

Drop @ 120V

0.30V (0.25%)

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 × 20 × 0.0608) ÷ 1000 = 0.304 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: (0.304 ÷ 120) × 100 = 0.2533%

On 240V: (0.304 ÷ 240) × 100 = 0.1267%

How This Estimate Changes with Run Length and Gauge

Gauge Check

4/0 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 4/0 AWG at 20A at different distances:

Distance	Drop (V)	% on 120V	% on 240V	NEC (120V)
25ft	0.0608V	0.0507%	0.0253%	OK
50ft	0.1216V	0.1013%	0.0507%	OK
75ft	0.1824V	0.152%	0.076%	OK
100ft	0.2432V	0.2027%	0.1013%	OK
150ft	0.3648V	0.304%	0.152%	OK
200ft	0.4864V	0.4053%	0.2027%	OK
300ft	0.7296V	0.608%	0.304%	OK

Same Run, Different Wire Gauges

How does wire gauge affect voltage drop for 20A at 125 feet on 120V single-phase / DC? Only gauges whose branch-circuit OCP cap is at or above the 20A 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)
4/0 AWG	0.304V	0.2533%	0.1267%	OK
250 kcmil	0.2575V	0.2146%	0.1073%	OK
300 kcmil	0.2145V	0.1788%	0.0894%	OK
350 kcmil	0.1835V	0.1529%	0.0765%	OK
500 kcmil	0.129V	0.1075%	0.0538%	OK
750 kcmil	0.0855V	0.0713%	0.0356%	OK

Related Calculations

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Frequently Asked Questions

What is the voltage drop for 4/0 AWG at 20A and 125ft?expand_more

4/0 AWG carrying 20A over 125ft has a 0.304V drop (0.2533% on 120V). Reference: 0.1267% 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 0.1267% on 240V versus 0.2533% on 120V.

Does 4/0 AWG meet the 3% drop target at 20A and 125ft on 120V?expand_more

On 120V, this run sits at 0.2533%, 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 4/0 AWG for this run?expand_more

4/0 AWG already sits within the 3% branch-circuit drop target at these inputs (0.2533% 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.