4 AWG at 100A and 50 Feet: Ampacity-Invalid Reference Calculation

Reference voltage-drop calculation only. 4 AWG is NEC-capped at 85A branch-circuit OCP per NEC 240.4(D) (75°C ampacity 85A), so 100A on this gauge fails the ampacity check before voltage drop ever enters the conversation. Do not use the number below as an install spec. As a load-side reference, 4 AWG copper at 100 amps over 50 feet on a single-phase / DC circuit would drop 3.08 volts (2.57% on 120V, 1.28% on 240V).

4 AWG, 100A, 50ft · single-phase / DC · reference only, ampacity-invalid

3.08 V drop (2.57% on 120V)

On 120V circuit2.57%

On 240V circuit1.28%

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

3.08V (2.57%)

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 × 100 × 0.308) ÷ 1000 = 3.08 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: (3.08 ÷ 120) × 100 = 2.57%

On 240V: (3.08 ÷ 240) × 100 = 1.28%

How This Estimate Changes with Run Length and Gauge

Gauge That Meets the 3% Target

4 AWG can't carry the 100A load in the first place, its branch-circuit OCP cap is 85A under typical conditions. The smallest gauge in our table that clears both the ampacity cap and the 3% drop target at these inputs is 3 AWG. Run the full wire-size calculator for run length, material, and drop-target variations.

Impact of Distance

Voltage drop is proportional to distance. Here is 4 AWG at 100A at different distances:

Distance	Drop (V)	% on 120V	% on 240V	NEC (120V)
25ft	1.54V	1.28%	0.6417%	OK
50ft	3.08V	2.57%	1.28%	OK
75ft	4.62V	3.85%	1.93%	Caution
100ft	6.16V	5.13%	2.57%	Past 5%
150ft	9.24V	7.7%	3.85%	Past 5%
200ft	12.32V	10.27%	5.13%	Past 5%
300ft	18.48V	15.4%	7.7%	Past 5%

Same Run, Different Wire Gauges

How does wire gauge affect voltage drop for 100A at 50 feet on 120V single-phase / DC? Only gauges whose branch-circuit OCP cap is at or above the 100A 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	2.45V	2.04%	1.02%	OK
2 AWG	1.94V	1.62%	0.8083%	OK
1 AWG	1.54V	1.28%	0.6417%	OK
1/0 AWG	1.22V	1.02%	0.5083%	OK
2/0 AWG	0.967V	0.8058%	0.4029%	OK
3/0 AWG	0.766V	0.6383%	0.3192%	OK
4/0 AWG	0.608V	0.5067%	0.2533%	OK
250 kcmil	0.515V	0.4292%	0.2146%	OK
300 kcmil	0.429V	0.3575%	0.1788%	OK
350 kcmil	0.367V	0.3058%	0.1529%	OK
500 kcmil	0.258V	0.215%	0.1075%	OK
750 kcmil	0.171V	0.1425%	0.0713%	OK

Related Calculations

cable Wire size: 100A, 50ft bolt Watts to amps calculator functions Voltage drop formula

Frequently Asked Questions

What is the voltage drop for 4 AWG at 100A and 50ft?expand_more

4 AWG carrying 100A over 50ft has a 3.08V drop (2.57% on 120V), but 4 AWG is NEC-capped at 85A branch-circuit OCP per NEC 240.4(D), so 100A on 4 AWG is ampacity-invalid and the drop figure above is a reference calculation only, not an install spec. Reference: 1.28% on 240V.

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 1.28% on 240V versus 2.57% on 120V.

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 (2.57% 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.

Does 4 AWG meet the 3% drop target at 100A and 50ft on 120V?expand_more

On 120V, this run sits at 2.57%, 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. However, 4 AWG is ampacity-invalid for 100A (NEC 240.4(D) caps 4 AWG at 85A branch-circuit OCP), so this is a reference drop calculation only and the 3% check does not make this a valid install.

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.

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.