What Is the Voltage Drop for 4 AWG at 8A and 400 Feet?

4 AWG copper carrying 8 amps over 400 feet on a single-phase / DC circuit drops 1.97 volts (1.64% 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, 8A, 400ft · single-phase / DC

1.97 V drop (1.64% on 120V)

On 120V circuit1.64%

On 240V circuit0.8213%

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

1.97V (1.64%)

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 × 400 × 8 × 0.308) ÷ 1000 = 1.97 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: (1.97 ÷ 120) × 100 = 1.64%

On 240V: (1.97 ÷ 240) × 100 = 0.8213%

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 8A at different distances:

Distance	Drop (V)	% on 120V	% on 240V	NEC (120V)
25ft	0.1232V	0.1027%	0.0513%	OK
50ft	0.2464V	0.2053%	0.1027%	OK
75ft	0.3696V	0.308%	0.154%	OK
100ft	0.4928V	0.4107%	0.2053%	OK
150ft	0.7392V	0.616%	0.308%	OK
200ft	0.9856V	0.8213%	0.4107%	OK
300ft	1.48V	1.23%	0.616%	OK

Same Run, Different Wire Gauges

How does wire gauge affect voltage drop for 8A at 400 feet on 120V single-phase / DC? Only gauges whose branch-circuit OCP cap is at or above the 8A 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	1.97V	1.64%	0.8213%	OK
3 AWG	1.57V	1.31%	0.6533%	OK
2 AWG	1.24V	1.03%	0.5173%	OK
1 AWG	0.9856V	0.8213%	0.4107%	OK
1/0 AWG	0.7808V	0.6507%	0.3253%	OK
2/0 AWG	0.6189V	0.5157%	0.2579%	OK

Related Calculations

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Frequently Asked Questions

What is the voltage drop for 4 AWG at 8A and 400ft?expand_more

4 AWG carrying 8A over 400ft has a 1.97V drop (1.64% on 120V). Reference: 0.8213% on 240V.

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.

Does 4 AWG meet the 3% drop target at 8A and 400ft on 120V?expand_more

On 120V, this run sits at 1.64%, 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.

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.

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 (1.64% 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.

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.