40 Amp Breaker — Wire Size, Max Load & NEC Guide
A 40 Amp breaker requires 8 AWG copper wire. Max continuous load: 32A (80% rule). Max watts: 3,840W at 120V or 7,680W at 240V.
40 Amp Breaker — Complete NEC Guide
40A circuits serve EV chargers (32A continuous = 40A breaker at 125%), small ranges, and hot tubs. Requires 8 AWG copper minimum with a double-pole breaker.
The 80% Rule (NEC 210.20)
A 40 Amp breaker has a maximum continuous load of 32A (40A × 80%). Continuous loads are those expected to run for 3 hours or more. If your load exceeds 32A continuously, upsize to the next standard breaker size. Non-continuous loads can use the full 40A rating.
Wire Sizing (NEC 310.16)
Per NEC Table 310.16 at 75°C: a 40 Amp breaker requires 8 AWG copper conductor minimum. For aluminum, upsize two gauges. For runs exceeding 75 feet, use 6 AWG to keep voltage drop under 3%. Verify with our Wire Size Calculator and Voltage Drop Calculator.
Common Uses for 40 Amp Circuits
Electric ranges (small), EV chargers (32A), hot tubs.
Disclaimer: This guide is for reference. All electrical work must be verified by a licensed electrician. Local codes may differ from NEC.
Frequently Asked Questions
All Breaker Sizes — Wire & Load Reference
| Breaker | Wire | Max Cont. | Max W (240V) |
|---|---|---|---|
| 15 Amp | 14 AWG | 12A | 2,880W |
| 20 Amp | 12 AWG | 16A | 3,840W |
| 25 Amp | 10 AWG | 20A | 4,800W |
| 30 Amp | 10 AWG | 24A | 5,760W |
| 40 Amp | 8 AWG | 32A | 7,680W |
| 50 Amp | 6 AWG | 40A | 9,600W |
| 60 Amp | 6 AWG | 48A | 11,520W |
| 70 Amp | 4 AWG | 56A | 13,440W |
| 80 Amp | 3 AWG | 64A | 15,360W |
| 100 Amp | 1 AWG | 80A | 19,200W |
| 125 Amp | 1/0 AWG | 100A | 24,000W |
| 150 Amp | 2/0 AWG | 120A | 28,800W |
| 200 Amp | 3/0 AWG | 160A | 38,400W |
| 300 Amp | 350 MCM | 240A | 57,600W |
| 400 Amp | 500 MCM | 320A | 76,800W |