Wire Sizing

What Size Wire for 40 Amp 100 Feet Away

Pre-calculated NEC-compliant wire size for 40 amps at 100 feet on a 240V single-phase circuit. Immediate answer below.

Recommended Wire Size

Copper

8 AWG

Voltage drop: 2.59% (6.22V)

Voltage at load: 233.8V

Controlling factor: Ampacity (NEC 310.16)

Aluminum

6 AWG

Voltage drop: 2.69% (6.46V)

Voltage at load: 233.5V

Controlling factor: Voltage Drop (3% limit)

Based on NEC Table 310.16 (75°C column), single-phase, 240V, 3% max voltage drop, one-way distance 100 ft.

Wire Comparison Table

Wire Size	Ampacity	V-Drop (V)	V-Drop (%)	V at Load
14 AWG	15A	25.12V	10.47%	214.9V
12 AWG	20A	15.84V	6.6%	224.2V
10 AWG	30A	9.92V	4.13%	230.1V
8 AWGREC	50A	6.22V	2.59%	233.8V
6 AWG	65A	3.93V	1.64%	236.1V
4 AWG	85A	2.46V	1.03%	237.5V
3 AWG	100A	1.96V	0.82%	238V
2 AWG	115A	1.55V	0.65%	238.4V
1 AWG	130A	1.23V	0.51%	238.8V
1/0 AWG	150A	0.98V	0.41%	239V
2/0 AWG	175A	0.77V	0.32%	239.2V
3/0 AWG	200A	0.61V	0.26%	239.4V
4/0 AWG	230A	0.49V	0.2%	239.5V
250 kcmil	255A	0.41V	0.17%	239.6V
300 kcmil	285A	0.34V	0.14%	239.7V
350 kcmil	310A	0.29V	0.12%	239.7V
400 kcmil	335A	0.26V	0.11%	239.7V
500 kcmil	380A	0.21V	0.09%	239.8V

Table shows all standard AWG/kcmil sizes for 40A at 100 ft on 240V (copper). Green check = meets both ampacity and 3% voltage drop. REC = smallest compliant size.

Key Considerations

For a 40A circuit at 100 feet on 240V, both ampacity and voltage drop must be satisfied. NEC recommends keeping the total voltage drop (feeder + branch) under 5%, with no more than 3% on either segment alone.

Copper conductors at 75°C column (THHN/THWN-2) are assumed. If using 60°C terminations, derate the ampacity to the 60°C column values. Verify the temperature rating of your terminals.

Frequently Asked Questions

What size wire do I need for a 40 amp service at 100 feet?▾

For a 40A/240V circuit at 100 feet, use 8 AWG copper (2.59% drop) or 6 AWG aluminum. This keeps voltage drop within the NEC-recommended 3% limit.

Can I use aluminum wire for a 40 amp circuit?▾

Yes. Aluminum wire is a cost-effective alternative for larger circuits. For 40A at 100 feet, use 6 AWG aluminum (2.69% voltage drop). Aluminum requires AL-rated connectors and anti-oxidant compound at all terminations.

What happens if the voltage drop exceeds 3%?▾

Voltage drop above 3% reduces equipment efficiency, can cause motors to overheat, and may violate NEC 210.19(A) Informational Note No. 4. For a 40A circuit at 100 feet on 240V, exceeding 3% means the load receives less than 232.8V, which can damage sensitive electronics and shorten appliance lifespan.

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Need a Custom Calculation?

Different voltage, three-phase, or custom max drop percentage? Use our interactive calculator.

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