What size wire do I need for a 40 amp breaker?

8 AWG copper or 6 AWG aluminum, per NEC Table 310.16 at the 75°C column. The equipment grounding conductor is 10 AWG copper per NEC Table 250.122.

What is the maximum continuous load on a 40 amp breaker?

32 amps. NEC 210.19(A)(1) and 210.20(A) require the circuit to be rated at 125% of a continuous load — one that runs for three hours or more — which works out to 80% of the breaker rating available for continuous use. Non-continuous loads may use the full 40 A.

What size ground wire for a 40 amp breaker?

10 AWG copper or 8 AWG aluminum, from NEC Table 250.122. The table lists maximum overcurrent device ratings, so you round up to the next listed row. If the circuit conductors are upsized for voltage drop, NEC 250.122(B) requires the ground to be upsized proportionally too.

Can I use 8 AWG NM-B cable on a 40 amp breaker?

Yes. NEC 334.80 requires NM-B ampacity to be taken from the 60°C column of Table 310.16, where 8 AWG copper is rated 40 A. That is why NM-B sometimes needs a larger conductor than THHN in conduit for the same breaker.

40 Amp Breaker Wire Size

Copper Wire

8 AWG

Aluminum Wire

6 AWG

Ground (Copper)

10 AWG

Double-pole · 240 V · Max continuous load 32 A · NEC Tables 310.16 and 250.122

40 Amp Circuit — Specification Summary

Item	Value
Breaker type	Double-pole, 240 V
Recommended copper conductor	8 AWG
Recommended aluminum conductor	6 AWG
Minimum per NEC Table 310.16, 75°C	8 AWG copper (50 A)
Minimum NM-B cable, 60°C column	8 AWG copper (40 A)
Equipment grounding conductor	10 AWG copper / 8 AWG aluminum (NEC Table 250.122)
Maximum continuous load (125% rule)	32 A
Maximum non-continuous load	40 A

Recommended conductor sizes follow the 75°C column of NEC Table 310.16, the column that matches the termination rating of virtually all modern breakers and lugs (NEC 110.14(C)).

Common Uses for 40 A Circuits

Electric ranges and cooktops, 32 A Level 2 EV chargers, larger central air conditioning, electric furnaces

What a 40 Amp Breaker Powers

Load	Notes
Electric range / cooktop	40 A is common for cooktops and smaller ranges
Level 2 EV charger (32 A)	32 × 1.25 = 40 A circuit — NEC 625.41
Central air conditioner	Size from the nameplate MCA and MOCP
Electric furnace / air handler	Often multiple circuits

Always size a circuit from the equipment nameplate or a load calculation, not from a general table. Air conditioners and heat pumps in particular carry a minimum circuit ampacity (MCA) and maximum overcurrent protective device (MOCP) rating that override the general rules.

NEC requirements for this circuit

40 A is where the 125% continuous-load rule starts to bite. An EV charger or an electric heater runs for more than three hours, so the circuit must be rated at 125% of the load: a 32 A charger needs a 40 A circuit, not a 32 A one.

Outdoor and garage receptacle outlets up to 250 V require GFCI protection under NEC 210.8(A). Hard-wired appliances are generally exempt unless a specific article requires it.

Voltage Drop by Distance — 8 AWG Copper at 40 A

One-way run	Drop	% of 240V	Verdict
25 ft	1.56 V	0.6%	Within the 3% branch-circuit target
50 ft	3.11 V	1.3%	Within the 3% branch-circuit target
75 ft	4.67 V	1.9%	Within the 3% branch-circuit target
100 ft	6.22 V	2.6%	Within the 3% branch-circuit target
150 ft	9.34 V	3.9%	Over 3% — upsize one gauge
200 ft	12.45 V	5.2%	Well over 3% — upsize two gauges or more

Single-phase, 240V, full 40 A load, 0.778 Ω/1000 ft DC resistance (NEC Chapter 9, Table 8) at 75°C terminations. NEC 210.19(A) Informational Note recommends keeping branch-circuit drop at or below 3%, and 5% total across feeder plus branch circuit. See the voltage drop formula for the arithmetic.

The 80% rule: a continuous load is one expected to run for three hours or more — EV chargers, electric heat, and most commercial lighting. NEC 210.20(A) requires the overcurrent device to be rated at not less than 125% of that continuous load, which leaves 32 A usable on a 40 A breaker. A breaker that trips after an hour or two under a steady load is usually a continuous-load sizing mistake, not a faulty breaker.

Other Breaker Sizes

15A 20A 30A 40A 50A 60A 100A 200A

Wire size for 40 amps — full page → 8 AWG full specifications → NEC Table 250.122 — ground wire size → NEC Table 310.16 — conductor ampacity → Wire size calculator →