What Is the Voltage Drop for 1/0 AWG at 21A and 100 Feet?

Running 21A through 1/0 AWG copper for 100 feet on a single-phase / DC circuit produces a 0.5124-volt drop. On a 120V source that is 0.427%; on 240V it is 0.2135%. 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.

1/0 AWG, 21A, 100ft · single-phase / DC

0.5124 V drop (0.427% on 120V)

On 120V circuit0.427%

On 240V circuit0.2135%

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

1/0 AWG

Amps

Feet

Drop @ 120V

0.51V (0.43%)

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 × 100 × 21 × 0.122) ÷ 1000 = 0.5124 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: (0.5124 ÷ 120) × 100 = 0.427%

On 240V: (0.5124 ÷ 240) × 100 = 0.2135%

How This Estimate Changes with Run Length and Gauge

Gauge Check

1/0 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 1/0 AWG at 21A at different distances:

Distance	Drop (V)	% on 120V	% on 240V	NEC (120V)
25ft	0.1281V	0.1067%	0.0534%	OK
50ft	0.2562V	0.2135%	0.1067%	OK
75ft	0.3843V	0.3203%	0.1601%	OK
100ft	0.5124V	0.427%	0.2135%	OK
150ft	0.7686V	0.6405%	0.3203%	OK
200ft	1.02V	0.854%	0.427%	OK
300ft	1.54V	1.28%	0.6405%	OK

Same Run, Different Wire Gauges

How does wire gauge affect voltage drop for 21A at 100 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)
1/0 AWG	0.5124V	0.427%	0.2135%	OK
2/0 AWG	0.4061V	0.3385%	0.1692%	OK
3/0 AWG	0.3217V	0.2681%	0.1341%	OK
4/0 AWG	0.2554V	0.2128%	0.1064%	OK
250 kcmil	0.2163V	0.1803%	0.0901%	OK
300 kcmil	0.1802V	0.1502%	0.0751%	OK

Related Calculations

cable Wire size: 21A, 100ft bolt Watts to amps calculator functions Voltage drop formula

Frequently Asked Questions

What is the voltage drop for 1/0 AWG at 21A and 100ft?expand_more

1/0 AWG carrying 21A over 100ft has a 0.5124V drop (0.427% on 120V). Reference: 0.2135% on 240V.

Does 1/0 AWG meet the 3% drop target at 21A and 100ft on 120V?expand_more

On 120V, this run sits at 0.427%, 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.

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 0.2135% on 240V versus 0.427% on 120V.

What happens if voltage drop is too high?expand_more

Motors run hotter and can have trouble starting under load. Incandescent and halogen lighting dims. Some electronics misbehave at the low end of their input tolerance. Energy is wasted as I²R heating in the conductor. These are performance issues; high drop is not itself a code violation unless the specific installation cites a hard limit.

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.