Three columns, three answers
NM-B cable is held to the 60 °C column, THHN in conduit on 60/75 °C breakers uses 75 °C. NM-B is made only up to 2 AWG copper, so a 100 A circuit runs as THHN, XHHW or SER.
What size wire for 100 amps?
A 100 A feeder takes 3 AWG copper or 1 AWG aluminum at 75 °C with a 8 AWG copper ground. A house service (or the feeder carrying the whole house) may use the 83 % rule: 4 AWG copper or 2 AWG aluminum. A 100 A subpanel feeder does not get it.
02 · Ampacity · NEC 2023 Table 310.16
3 AWG copper · 90 °C insulation, 75 °C terminals, 30 °C, 3 conductors
USE:
3 AWG COPPER · 100 A ALLOWABLE · 100 A MAX BREAKER
Allowable
100 A
3 AWG Cu
Max breaker
100 A
ampacity ≥ breaker
For your load
3 AWG
100 A
3 AWG copper carries 115 A after derating from the 90 °C column and 100 A at its 75 °C terminals, so it is good for 100 A on a breaker up to 100 A.
- Meets NEC 2023 Table 310.16 at 75°C — verify with your local AHJ/inspector110.14(C)
- Load 100 A ≤ 100 A310.14
Derating math
- Table 310.16 at 90 °C
- 115 Aderating starts at the insulation rating
- Ambient factor
- × 1.0030 °C, Table 310.15(B)(1)
- Bundling factor
- × 1.003 current-carrying conductors, 310.15(C)(1)
- Derated
- 115 A
- Terminal limit (75 °C)
- 100 A110.14(C)
- Allowable
- 100 Athe terminals decide
3 AWG
- Copper 60 / 75 / 90 °C
- 85 / 100 / 115 ANM-B uses the 60 °C column (334.80)
- Aluminum 60 / 75 / 90 °C
- 65 / 75 / 85 A
- Largest breaker (Cu, 75 °C)
- 100 Aampacity ≥ breaker
- Solid diameter
- 0.229"5.83 mm · ASTM B258 formula
- Stranded (7 strands)
- 0.260"6.60 mm · Table 8
- THHN outside
- 0.352"8.94 mm · 0.0973 in² · Table 5
- Area
- 52,620 cmil26.66 mm² · nearest IEC 25 mm²
- DC resistance, 75 °C
- 0.245 Ω/1000 ftaluminum 0.403 Ω/1000 ft · Table 8
- Copper weight
- 159.3 lb/1000 ft
100 A circuit at a glance
Wire
- Copper NM-B (60 °C)
- 2 AWG (next size up)
- Copper THHN (75 °C)
- 3 AWG
- Aluminum (75 °C)
- 1 AWG
Ground & breaker
- Copper ground
- 8 AWG
- Aluminum ground
- 6 AWG
- Continuous load max
- 80 A
- Breaker
- 100 A
Distance (3 %)
- Cu at 120 V, 80 A
- 91 ft
- Cu at 240 V, 80 A
- 183 ft
- Al at 240 V, 80 A
- 177 ft
100 A circuit tables 12 rows
Sort any column, print a shop sheet, download CSV. Click a row to load it in the calculator.
| Actions | |||||
|---|---|---|---|---|---|
| — | 2 AWG | 8 AWG | 292 ft | ||
| 3 AWG | 3 AWG | 8 AWG | 183 ft | ||
| 1 AWG | 1 AWG | 6 AWG | 177 ft |
A dwelling’s 100 A service, or the feeder carrying its whole load, may use 83 % (310.12): 4 AWG copper or 2 AWG aluminum.
Feet one way (the current goes out and back). Ampacity is a separate check: a long run may need a heavier wire for drop, never a lighter one for heat.
05 / Field guide
100 amps:
feeder or service?
A 100 A feeder takes 3 AWG copper or 1 AWG aluminum at 75 °C with an 8 AWG copper ground. A house service (or the feeder carrying the whole house) may use the 83 % rule: 4 AWG copper or 2 AWG aluminum. A 100 A subpanel feeder does not get it.
Breaker rules
240.4(B) lets a conductor whose ampacity is not a standard size take the next breaker up (not on multi-outlet receptacle circuits). The headline here uses ampacity at least 100 A.
Distance
The answer above is the minimum for heat. On a long run upsize for voltage drop, and grow the ground with it: the table below gives the longest 240 V run at 3 % for each candidate size.
Questions
Answered plainly.
01What size wire for a 100 amp breaker?
+−
3 AWG copper THHN in conduit or 1 AWG aluminum, all with ampacity at least 100 A (NEC 2023 Table 310.16).
02Can I use 3 AWG wire on a 100 amp breaker in NM-B cable?
+−
Not at the ampacity rule: NM-B is limited to the 60 °C column (334.80), where 3 AWG carries 85 A; use conduit.
03What size ground wire for 100 amps?
+−
8 AWG copper or 6 AWG aluminum (Table 250.122), larger in proportion if the circuit wire is upsized for distance (250.122(B)).
04How far can I run a 100 amp circuit?
+−
At 80 A (80 %) on 3 AWG copper, 183 ft one way at 240 V or 91 ft at 120 V before the drop passes 3 %. Beyond that, go up a size.
05What is the largest continuous load on a 100 amp circuit?
+−
80 A: a continuous load (3 hours or more) may use only 80 % of the breaker rating (the 125 % rule).
06Can 4 AWG copper be used for 100 amps?
+−
For a dwelling’s 100 A service, or the feeder carrying the whole dwelling, yes (310.12, 83 %). For a 100 A subpanel feeder, no: use 3 AWG copper or 1 AWG aluminum.