What Is the Voltage Drop for 4 AWG at 21A and 250 Feet?

Running 21A through 4 AWG copper for 250 feet on a single-phase / DC circuit produces a 3.23-volt drop. On a 120V source that is 2.7%; on 240V it is 1.35%. 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 AWG, 21A, 250ft · single-phase / DC

3.23 V drop (2.7% on 120V)

On 120V circuit2.7%

On 240V circuit1.35%

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

3.23V (2.70%)

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 × 250 × 21 × 0.308) ÷ 1000 = 3.23 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: (3.23 ÷ 120) × 100 = 2.7%

On 240V: (3.23 ÷ 240) × 100 = 1.35%

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 21A at different distances:

Distance	Drop (V)	% on 120V	% on 240V	NEC (120V)
25ft	0.3234V	0.2695%	0.1348%	OK
50ft	0.6468V	0.539%	0.2695%	OK
75ft	0.9702V	0.8085%	0.4043%	OK
100ft	1.29V	1.08%	0.539%	OK
150ft	1.94V	1.62%	0.8085%	OK
200ft	2.59V	2.16%	1.08%	OK
300ft	3.88V	3.23%	1.62%	Caution

Same Run, Different Wire Gauges

How does wire gauge affect voltage drop for 21A at 250 feet on 120V single-phase / DC? Only gauges whose branch-circuit OCP cap is at or above the 21A 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	3.23V	2.7%	1.35%	OK
3 AWG	2.57V	2.14%	1.07%	OK
2 AWG	2.04V	1.7%	0.8488%	OK
1 AWG	1.62V	1.35%	0.6738%	OK
1/0 AWG	1.28V	1.07%	0.5338%	OK
2/0 AWG	1.02V	0.8461%	0.4231%	OK

Related Calculations

cable Wire size: 21A, 250ft bolt Watts to amps calculator functions Voltage drop formula

Frequently Asked Questions

What is the voltage drop for 4 AWG at 21A and 250ft?expand_more

4 AWG carrying 21A over 250ft has a 3.23V drop (2.7% on 120V). Reference: 1.35% on 240V.

Does wire material affect voltage drop?expand_more

Yes. Aluminum has roughly 1.3 to 1.4 times the resistance of copper at the NEC Chapter 9 Table 8 75°C reference temperature, so for the same voltage drop an aluminum conductor is typically one to two gauges larger than copper. The exact gap depends on whether ampacity or voltage drop is binding, and the install still needs anti-oxidant compound and aluminum-rated lugs.

Does 4 AWG meet the 3% drop target at 21A and 250ft on 120V?expand_more

On 120V, this run sits at 2.7%, 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 (2.7% 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.

How do I reduce voltage drop?expand_more

Use a larger wire gauge (lower AWG number), shorten the run, or increase the source voltage. Each option reduces the percentage drop, and higher source voltage is usually the most effective change for long runs because the drop is a smaller fraction of a larger reference.

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.