4 AWG at 100A and 100 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. The reference math on the single-phase / DC basis gives a 6.16-volt drop (5.13% on 120V). For an install at 100A, start with a conductor whose NEC 240.4(D) branch-circuit OCP cap is at or above 100A, then check voltage drop against that gauge.

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

6.16 V drop (5.13% on 120V)

On 120V circuit5.13%

On 240V circuit2.57%

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

6.16V (5.13%)

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 × 100 × 0.308) ÷ 1000 = 6.16 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: (6.16 ÷ 120) × 100 = 5.13%

On 240V: (6.16 ÷ 240) × 100 = 2.57%

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 1 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 100 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	4.9V	4.08%	2.04%	Caution
2 AWG	3.88V	3.23%	1.62%	Caution
1 AWG	3.08V	2.57%	1.28%	OK
1/0 AWG	2.44V	2.03%	1.02%	OK
2/0 AWG	1.93V	1.61%	0.8058%	OK
3/0 AWG	1.53V	1.28%	0.6383%	OK
4/0 AWG	1.22V	1.01%	0.5067%	OK
250 kcmil	1.03V	0.8583%	0.4292%	OK
300 kcmil	0.858V	0.715%	0.3575%	OK
350 kcmil	0.734V	0.6117%	0.3058%	OK
500 kcmil	0.516V	0.43%	0.215%	OK
750 kcmil	0.342V	0.285%	0.1425%	OK

Related Calculations

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

Frequently Asked Questions

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

4 AWG carrying 100A over 100ft has a 6.16V drop (5.13% 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: 2.57% 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 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.

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.

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.

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.