Wire size scenario
For a 100 amps copper circuit at 240 V single-phase, NEC 310.16 ampacity needs 3 AWG at 75 °C. Over 100 feet, voltage drop bumps the recommendation to 3 AWG (about 2.04% drop). Adjust the run below.
Recommended wire size
—Enter a load, length, and voltage to size the conductor.
| Ampacity needs | — |
|---|---|
| Voltage drop needs | — |
| Recommended (larger of the two) | — |
| Ampacity of recommended | — |
| Voltage drop of recommended | — |
At 240 V single-phase over 100 feet, a 100 amps copper load needs 3 AWG (about 2.04% drop). Here ampacity governs: copper 3 AWG is needed to carry 100 amps at 75 °C, and it comfortably meets the 3% voltage-drop target over 100 feet (about 2.04%).
This recommendation covers ampacity and voltage drop only, for copper at 75 °C. It does not apply ambient-temperature or conductor-count derating, the 125% factor for continuous loads, conduit fill, or terminal ratings — any of which can push the size up. Aluminum of the same load typically goes up one or two sizes. Always confirm with a licensed electrician and your local code (AHJ).
Ampacity per NEC 310.16; voltage drop per NEC Ch.9 Table 8 (75 °C copper).
One hundred amps usually points to a feeder, subpanel, or service-related project rather than a single appliance branch circuit. That expands the job to load calculations, neutral sizing, grounding and bonding, disconnects, and possibly dwelling-specific allowances.
Use this result to compare conductor performance over the 100-foot reference run, not to order a complete feeder. A calculated ampacity size can still be wrong for a particular wiring method, terminal rating, service application, or local amendment.
Code focus: The engine uses the general ampacity table and voltage-drop math. It intentionally does not apply dwelling service or feeder allowances, parallel-conductor design, or local service rules.
| One-way length | Ampacity minimum | Recommended size | Governing check |
|---|---|---|---|
| 25 ft | 3 AWG | 3 AWG | ampacity |
| 50 ft | 3 AWG | 3 AWG | ampacity |
| 100 ft | 3 AWG | 3 AWG | ampacity |
| 150 ft | 3 AWG | 2 AWG | voltage drop |
For 100 amps of copper at 240 V, NEC 310.16 ampacity at 75 °C needs 3 AWG. Over a 100 feet run, voltage drop calls for 4 AWG, so the recommended size is 3 AWG — the larger of the two. Aluminum, derating, continuous load, and local code may change this; confirm with a licensed electrician.
On a short run where voltage drop isn't a factor, 100 amps of copper needs 3 AWG at 75 °C by ampacity alone (NEC 310.16). Longer runs need a larger size for voltage drop — this page assumes 100 feet.
Both. The conductor must carry the current (ampacity, NEC Table 310.16) and keep voltage drop within target (NEC Chapter 9 Table 8). The recommended size is the larger of the two. It still doesn't replace ambient or conductor-count derating, the 125% continuous-load factor, conduit fill, or terminal-rating checks — verify with your local code (AHJ).
Source links and page assumptions reviewed .
Estimate only, based on NEC 310.16 ampacity and NEC Chapter 9 Table 8 voltage drop. Not a substitute for a licensed electrician or your local code (AHJ). It does not fully model ambient or conductor-count derating, the 125% continuous-load factor, conduit fill, or terminal ratings. Verify against the current NEC and local code before wiring. Based on NEC 310.16 and NEC Ch.9 Table 8.