What Size Wire for 30 Amps? NEC Requirements
NEC Wire Size Rules for 30 Amps
NEC 240.4 requires overcurrent protection (breaker) to match wire ampacity. Key rules:
- Standard loads: 10 AWG copper = 30A (60°C column)
- Continuous loads (3+ hours): Wire must be rated for 125% of load. 30A × 1.25 = 37.5A → use 8 AWG copper (40A rated)
- Temperature rating: Match wire rating to breaker terminal rating. Most residential breakers are rated 60°C or 75°C
- Conduit fill: In conduit with 4+ current-carrying conductors, derate ampacity by 80%
Common Applications for 30-Amp Circuits
| Application | Voltage | Wire Size | Conductor |
|---|---|---|---|
| Electric dryer | 240V | 10 AWG (10/3 w/ ground) | Copper NM-B |
| Window AC (240V) | 240V | 10 AWG | Copper |
| Small water heater | 240V | 10 AWG | Copper |
| Workshop sub-panel | 240V | 10 AWG (or 8 AWG for long runs) | Copper or aluminum |
Frequently Asked Questions
What size wire for 30 amps?
10 AWG copper wire is rated for 30 amps at 60°C (NEC Table 310.16). For continuous loads (3+ hours), NEC requires wire rated for 125% of load = 37.5A → use 8 AWG copper.
Can I use 12 AWG wire on a 30 amp breaker?
No. 12 AWG is rated for only 20 amps. Using it on a 30-amp breaker violates NEC 240.4 and creates a fire hazard. The breaker won't trip before the wire overheats.
What is a 30 amp breaker used for?
30-amp breakers are used for: electric dryers, 240V window ACs, small water heaters, and some workshop equipment. The circuit requires 10 AWG copper wire minimum.
Disclaimer: For preliminary design only. Verify against local codes and professional engineering requirements.