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. That is why 8 AWG NM-B and 8 AWG THHN can both serve 40 A.
What size wire for 40 amps?
A 40 A circuit takes 8 AWG NM-B in the wall or 8 AWG copper THHN in conduit (75 °C), with a 10 AWG ground; aluminum needs 8 AWG. At an 32 A load the copper reaches 144 ft at 240 V (3 % drop). Used for ranges, cooktops and 32 A EV chargers.
02 · Ampacity · NEC 2023 Table 310.16
8 AWG copper · 90 °C insulation, 75 °C terminals, 30 °C, 3 conductors
USE:
8 AWG COPPER · 50 A ALLOWABLE · 50 A MAX BREAKER
Allowable
50 A
8 AWG Cu
Max breaker
50 A
ampacity ≥ breaker
For your load
8 AWG
40 A
8 AWG copper carries 55 A after derating from the 90 °C column and 50 A at its 75 °C terminals, so it is good for 50 A on a breaker up to 50 A.
- Meets NEC 2023 Table 310.16 at 75°C — verify with your local AHJ/inspector110.14(C)
- Load 40 A ≤ 50 A310.14
Derating math
- Table 310.16 at 90 °C
- 55 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
- 55 A
- Terminal limit (75 °C)
- 50 A110.14(C)
- Allowable
- 50 Athe terminals decide
8 AWG
- Copper 60 / 75 / 90 °C
- 40 / 50 / 55 ANM-B uses the 60 °C column (334.80)
- Aluminum 60 / 75 / 90 °C
- 35 / 40 / 45 A
- Largest breaker (Cu, 75 °C)
- 50 Aampacity ≥ breaker
- Solid diameter
- 0.128"3.26 mm · ASTM B258 formula
- Stranded (7 strands)
- 0.146"3.71 mm · Table 8
- THHN outside
- 0.216"5.49 mm · 0.0366 in² · Table 5
- Area
- 16,510 cmil8.37 mm² · nearest IEC 10 mm²
- DC resistance, 75 °C
- 0.778 Ω/1000 ftaluminum 1.28 Ω/1000 ft · Table 8
- Copper weight
- 50.0 lb/1000 ft
40 A circuit at a glance
Wire
- Copper NM-B (60 °C)
- 8 AWG
- Copper THHN (75 °C)
- 8 AWG
- Aluminum (75 °C)
- 8 AWG
Ground & breaker
- Copper ground
- 10 AWG
- Aluminum ground
- 8 AWG
- Continuous load max
- 32 A
- Breaker
- 40 A
Distance (3 %)
- Cu at 120 V, 32 A
- 72 ft
- Cu at 240 V, 32 A
- 144 ft
- Al at 240 V, 32 A
- 87 ft
40 A circuit tables 9 rows
Sort any column, print a shop sheet, download CSV. Click a row to load it in the calculator.
| Actions | |||||
|---|---|---|---|---|---|
| 8 AWG | 8 AWG | 10 AWG | 144 ft | ||
| 8 AWG | 8 AWG | 10 AWG | 144 ft | ||
| 8 AWG | 8 AWG | 8 AWG | 87 ft |
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
40 amps:
which wire, which method.
A 40 A circuit takes 8 AWG NM-B in the wall or 8 AWG copper THHN in conduit (75 °C), with a 10 AWG ground; aluminum needs 8 AWG. At an 32 A load the copper reaches 144 ft at 240 V (3 % drop). Used for ranges, cooktops and 32 A EV chargers.
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 40 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 40 amp breaker?
+−
8 AWG copper NM-B in the wall, 8 AWG copper THHN in conduit or 8 AWG aluminum, all with ampacity at least 40 A (NEC 2023 Table 310.16).
02Can I use 8 AWG wire on a 40 amp breaker in NM-B cable?
+−
Yes: 8 AWG NM-B is rated 40 A at 60 °C.
03What size ground wire for 40 amps?
+−
10 AWG copper or 8 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 40 amp circuit?
+−
At 32 A (80 %) on 8 AWG copper, 144 ft one way at 240 V or 72 ft at 120 V before the drop passes 3 %. Beyond that, go up a size.
05What is the largest continuous load on a 40 amp circuit?
+−
32 A: a continuous load (3 hours or more) may use only 80 % of the breaker rating (the 125 % rule).