400 Amp Breaker — Wire Size, Max Load & NEC Guide
A 400 Amp breaker requires 500 MCM copper wire. Max continuous load: 320A (80% rule). Max watts: 38,400W at 120V or 76,800W at 240V.
400 Amp Breaker — Complete NEC Guide
400A is the largest common residential service. Typically uses CT (current transformer) metering and requires 500 MCM copper or parallel runs. Often found in homes with electric heating, multiple EV chargers, pools, hot tubs, and workshops.
The 80% Rule (NEC 210.20)
A 400 Amp breaker has a maximum continuous load of 320A (400A × 80%). Continuous loads are those expected to run for 3 hours or more. If your load exceeds 320A continuously, upsize to the next standard breaker size. Non-continuous loads can use the full 400A rating.
Wire Sizing (NEC 310.16)
Per NEC Table 310.16 at 75°C: a 400 Amp breaker requires 500 MCM copper conductor minimum. For aluminum, upsize two gauges. For runs exceeding 75 feet, use 2x 3/0 AWG parallel to keep voltage drop under 3%. Verify with our Wire Size Calculator and Voltage Drop Calculator.
Common Uses for 400 Amp Circuits
Very large homes, commercial, multi-unit residential.
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 |