40 Amp Wire Size — Quick Reference
Account for voltage drop on a longer run:
Calculate for Your Circuit Length →Why #8 AWG Copper for a 40-Amp Circuit?
Per NEC Table 310.15(B)(16), #8 AWG copper is rated 40A at 60°C and 50A at 75°C. Paired with a 40A breaker, it is protected well within its 75°C ampacity limit. #8 AWG aluminum is rated only 40A at 75°C — which matches exactly with a 40A breaker, but the next standard size up, #6 AWG aluminum (50A rated), is typically preferred because it provides headroom and is required if any derating applies.
Note: #8 AWG aluminum is technically permissible on a 40A breaker per the 60°C ampacity column (40A), but most engineering practice uses #6 AWG aluminum for 40A circuits to avoid using conductors at their exact rated limit.
Common 40-Amp Circuit Applications
- HVAC condensing unit: Many 2–5 ton residential A/C units have a 40A MCA or MOCP rating — check the nameplate before selecting wire and breaker size
- Heat pump (including mini-split systems): Larger single-zone and multi-zone heat pumps often require a 40A circuit
- Electric range (some models): Some range manufacturers specify a 40A circuit, but 50A is more common
- Small subpanel feeder: 40A feeder to a garage or workshop subpanel
- Well pump: Some larger residential well pump systems use a 40A circuit
- Level 2 EV Charger (32A EVSE): A 32A EVSE charger requires a 40A circuit (125% continuous load rule — NEC 625.42)
Always Check the Equipment Nameplate
For HVAC and heat pump equipment, wire and breaker sized from only the nameplate is more reliable than generically sizing by the circuit amperage. The nameplate shows:
- MCA (Minimum Circuit Ampacity): The minimum conductor ampacity required. Your wire's ampacity must meet or exceed this.
- MOCP (Maximum Overcurrent Protection): The maximum breaker size permitted by the manufacturer. Never exceed this size.
For example, if an HVAC unit shows MCA 32A and MOCP 45A, you would use #8 AWG copper (50A ampacity > 32A MCA) and a 40A or 45A breaker (≤ 45A MOCP). Always defer to the equipment nameplate over generic sizing tables.
Voltage Drop on 40-Amp Circuits
Use the Wire Size Calculator with your actual run length to find the exact minimum AWG for any 40A circuit.
Frequently Asked Questions
Per NEC Table 310.15(B)(16): minimum #8 AWG copper (50A at 75°C) or #6 AWG aluminum (50A at 75°C) for a 40A circuit at standard conditions.
No. #10 AWG copper is rated 35A at 75°C. Using it with a 40A breaker violates NEC 240.4. Use #8 AWG copper (50A rated) as the minimum.
#8 AWG copper or #6 AWG aluminum with a 40A double-pole breaker (not exceeding the nameplate MOCP). Always verify with the equipment nameplate MCA and MOCP values before finalizing wire and breaker size.
#8 AWG copper (50A rated) can be used with a 40A or 50A breaker depending on the circuit. For a 40A circuit, pair with a 40A breaker. The conductor's rated ampacity at 75°C (50A) exceeds the 40A breaker rating, which is correct — the breaker protects the wire.
Approximately 55–60 feet one-way before exceeding the NEC 3% voltage drop recommendation. Upsize to #6 AWG copper for runs up to about 91 feet, or #4 AWG for very long runs.
MCA (Minimum Circuit Ampacity) is the minimum conductor ampacity your supply wiring must have. Your chosen wire's ampacity at the applicable temperature column must be at or above the MCA. MOCP (Maximum Overcurrent Protection) is the largest breaker permitted for the equipment.