What size wire for 40 amps at 200 feet?
Short answer: For a 40 amp load 200 feet away (one way) at 240 V, use 4 AWG copper or 3 AWG aluminum in conduit with 75 °C terminations, which keeps voltage drop at 2.1% and 2.7%. In NM-B (Romex) cable it is 4 AWG copper. The breaker stays 40 A; the extra size is only for distance.
40 amps at 200 feet: wire size by type
| Wire type | Size by ampacity | Size for 200 ft (≤3% drop) | Voltage drop |
|---|---|---|---|
| Copper THHN/THWN-2 in conduit, 75 °C | 8 AWG | 4 AWG | 2.05% |
| Aluminum XHHW-2 in conduit, 75 °C | 8 AWG | 3 AWG | 2.69% |
| Copper NM-B (Romex) cable, 60 °C | 8 AWG | 4 AWG | 2.05% |
| Aluminum SER / NM-B cable, 60 °C | 6 AWG | 3 AWG | 2.69% |
That is one or more sizes above the 8 AWG that ampacity alone allows: at 200 ft the run, not the breaker, decides the gauge. NEC 250.122(B) then requires the equipment ground to be increased in proportion: start from 10 AWG copper for a 40 A breaker.
At 120 volts
The same 40 A at 120 V loses twice the percentage over the same wire, so long 120 V runs need bigger wire:
| Wire type | Size for 200 ft | Voltage drop |
|---|---|---|
| Copper THHN/THWN-2 in conduit, 75 °C | 2 AWG | 2.59% |
| Aluminum XHHW-2 in conduit, 75 °C | 1/0 AWG | 2.68% |
Voltage drop of each gauge over 200 ft
| Wire | Volts lost at 40 A | Drop at 40 A | Drop at 32 A (80%) |
|---|---|---|---|
| 8 AWG copper | 12.22 V | 5.09% | 4.07% |
| 6 AWG copper | 7.86 V | 3.27% | 2.62% |
| 4 AWG copper | 4.93 V | 2.05% | 1.64% |
| 3 AWG copper | 3.92 V | 1.63% | 1.31% |
| 2 AWG copper | 3.10 V | 1.29% | 1.03% |
voltage drop = 2 × length × resistance per 1,000 ft × amps ÷ 1,000
= 2 × 200 × R(4 AWG) × 40 ÷ 1,000 = 4.93 V
percent = 4.93 ÷ 240 × 100 = 2.05%
If the real load is the 80% continuous maximum (32 A, as with an EV charger on a 40 A breaker), copper in conduit can be 6 AWG. 3% is the NEC's recommendation for a branch circuit (Informational Note to 210.19), not a hard rule, but motors, compressors and chargers run better with it.
Same 40 A, other distances
50 ft · 75 ft · 100 ft · 150 ft. Minimum sizes with no distance correction: what size wire for a 40 amp breaker.
Other loads at 200 feet
30 A · 50 A · 60 A · 100 A · 125 A · 150 A · 200 A
Change the numbers in the calculator
Safety. A 40 A circuit or feeder over 200 ft needs a permit almost everywhere in the US. Buried runs have their own depth and conduit rules (NEC 300.5), and equipment nameplates override any general table.