What Is the Voltage Drop for 12 AWG at 3A and 400 Feet?

Running 3A through 12 AWG copper for 400 feet on a single-phase / DC circuit produces a 4.75-volt drop. On a 120V source that is 3.96%; on 240V it is 1.98%. 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.

12 AWG, 3A, 400ft · single-phase / DC

4.75 V drop (3.96% on 120V)

On 120V circuit3.96%

On 240V circuit1.98%

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

12 AWG

Amps

Feet

Drop @ 120V

4.75V (3.96%)

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 × 3 × 1.98) ÷ 1000 = 4.75 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: (4.75 ÷ 120) × 100 = 3.96%

On 240V: (4.75 ÷ 240) × 100 = 1.98%

How This Estimate Changes with Run Length and Gauge

Gauge That Meets the 3% Target

The smallest gauge in our table that clears the 3% drop target at 3A over 400ft on 120V is 10 AWG. Shorter runs, higher source voltage, or a higher drop tolerance (feeder-only applications often accept up to 5%) can change the pick. Run the full wire-size calculator with your actual variables.

Impact of Distance

Voltage drop is proportional to distance. Here is 12 AWG at 3A at different distances:

Distance	Drop (V)	% on 120V	% on 240V	NEC (120V)
25ft	0.297V	0.2475%	0.1237%	OK
50ft	0.594V	0.495%	0.2475%	OK
75ft	0.891V	0.7425%	0.3713%	OK
100ft	1.19V	0.99%	0.495%	OK
150ft	1.78V	1.49%	0.7425%	OK
200ft	2.38V	1.98%	0.99%	OK
300ft	3.56V	2.97%	1.49%	OK

Same Run, Different Wire Gauges

How does wire gauge affect voltage drop for 3A at 400 feet on 120V single-phase / DC? Only gauges whose branch-circuit OCP cap is at or above the 3A 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)
12 AWG	4.75V	3.96%	1.98%	Caution
10 AWG	2.98V	2.48%	1.24%	OK
8 AWG	1.87V	1.56%	0.778%	OK
6 AWG	1.18V	0.982%	0.491%	OK
4 AWG	0.7392V	0.616%	0.308%	OK
3 AWG	0.588V	0.49%	0.245%	OK

Related Calculations

cable Wire size: 3A, 400ft bolt Watts to amps calculator functions Voltage drop formula

Frequently Asked Questions

What is the voltage drop for 12 AWG at 3A and 400ft?expand_more

12 AWG carrying 3A over 400ft has a 4.75V drop (3.96% on 120V). Reference: 1.98% on 240V.

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.

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 1.98% on 240V versus 3.96% on 120V.

What wire gauge should I use instead of 12 AWG for this run?expand_more

This run is at 3.96% on 120V, past the 3% branch-circuit drop target. If you want to land under 3% at 3A over 400ft on 120V, the smallest gauge in our table that clears it is 10 AWG at 2.48%. Going up one size from 12 AWG is not always enough, each AWG step only drops the resistance by roughly 20-25%, so on long runs or high currents you often have to skip one or two sizes to meet the target. NEC 210.19(A) Informational Note 4 frames 3% as a recommendation, not a code requirement, so the right answer for you also depends on the load (motor startup, sensitive electronics) and how much drop is tolerable.

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.