What size wire for 125 amps at 100 feet?
Short answer: For a 125 amp load 100 feet away (one way) at 240 V, use 1 AWG copper or 2/0 AWG aluminum in conduit with 75 °C terminations, which keeps voltage drop at 1.6% and 1.7%. In NM-B (Romex) cable it is 1/0 AWG copper. The breaker stays 125 A; distance does not force a larger size.
125 amps at 100 feet: wire size by type
| Wire type | Size by ampacity | Size for 100 ft (≤3% drop) | Voltage drop |
|---|---|---|---|
| Copper THHN/THWN-2 in conduit, 75 °C | 1 AWG | 1 AWG | 1.60% |
| Aluminum XHHW-2 in conduit, 75 °C | 2/0 AWG | 2/0 AWG | 1.66% |
| Copper NM-B (Romex) cable, 60 °C | 1/0 AWG | 1/0 AWG | 1.27% |
| Aluminum SER / NM-B cable, 60 °C | 3/0 AWG | 3/0 AWG | 1.31% |
At 100 ft, voltage drop stays under 3% with the ampacity size, so distance does not force a bigger wire. NEC 250.122(B) then requires the equipment ground to be increased in proportion: start from 6 AWG copper for a 125 A breaker.
Voltage drop of each gauge over 100 ft
| Wire | Volts lost at 125 A | Drop at 125 A | Drop at 100 A (80%) |
|---|---|---|---|
| 1 AWG copper | 3.85 V | 1.60% | 1.28% |
| 1/0 AWG copper | 3.05 V | 1.27% | 1.02% |
| 2/0 AWG copper | 2.42 V | 1.01% | 0.81% |
| 3/0 AWG copper | 1.92 V | 0.80% | 0.64% |
| 4/0 AWG copper | 1.52 V | 0.63% | 0.51% |
voltage drop = 2 × length × resistance per 1,000 ft × amps ÷ 1,000
= 2 × 100 × R(1 AWG) × 125 ÷ 1,000 = 3.85 V
percent = 3.85 ÷ 240 × 100 = 1.60%
If the real load is the 80% continuous maximum (100 A, as with an EV charger on a 125 A breaker), copper in conduit can be 3 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 125 A, other distances
50 ft · 75 ft · 150 ft · 200 ft. Minimum sizes with no distance correction: what size wire for a 125 amp breaker.
Other loads at 100 feet
30 A · 40 A · 50 A · 60 A · 100 A · 150 A · 200 A
Change the numbers in the calculator
Safety. A 125 A circuit or feeder over 100 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.