What Is the Voltage Drop for 3 AWG at 70A and 50 Feet?

3 AWG copper carrying 70 amps over 50 feet on a single-phase / DC circuit drops 1.72 volts (1.43% 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.

3 AWG, 70A, 50ft · single-phase / DC

1.72 V drop (1.43% on 120V)

On 120V circuit1.43%

On 240V circuit0.7146%

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

3 AWG

Amps

Feet

Drop @ 120V

1.72V (1.43%)

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 × 50 × 70 × 0.245) ÷ 1000 = 1.72 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: (1.72 ÷ 120) × 100 = 1.43%

On 240V: (1.72 ÷ 240) × 100 = 0.7146%

How This Estimate Changes with Run Length and Gauge

Gauge Check

3 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 3 AWG at 70A at different distances:

Distance	Drop (V)	% on 120V	% on 240V	NEC (120V)
25ft	0.8575V	0.7146%	0.3573%	OK
50ft	1.72V	1.43%	0.7146%	OK
75ft	2.57V	2.14%	1.07%	OK
100ft	3.43V	2.86%	1.43%	OK
150ft	5.15V	4.29%	2.14%	Caution
200ft	6.86V	5.72%	2.86%	Past 5%
300ft	10.29V	8.58%	4.29%	Past 5%

Same Run, Different Wire Gauges

How does wire gauge affect voltage drop for 70A at 50 feet on 120V single-phase / DC? Only gauges whose branch-circuit OCP cap is at or above the 70A 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)
3 AWG	1.72V	1.43%	0.7146%	OK
2 AWG	1.36V	1.13%	0.5658%	OK
1 AWG	1.08V	0.8983%	0.4492%	OK
1/0 AWG	0.854V	0.7117%	0.3558%	OK
2/0 AWG	0.6769V	0.5641%	0.282%	OK
3/0 AWG	0.5362V	0.4468%	0.2234%	OK

Related Calculations

cable Wire size: 70A, 50ft bolt Watts to amps calculator functions Voltage drop formula

Frequently Asked Questions

What is the voltage drop for 3 AWG at 70A and 50ft?expand_more

3 AWG carrying 70A over 50ft has a 1.72V drop (1.43% on 120V). Reference: 0.7146% 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 3 AWG meet the 3% drop target at 70A and 50ft on 120V?expand_more

On 120V, this run sits at 1.43%, 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 3 AWG for this run?expand_more

3 AWG already sits within the 3% branch-circuit drop target at these inputs (1.43% 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.

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 0.7146% on 240V versus 1.43% on 120V.

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.