Room Air Conditioner Branch Circuit Calculator

The 80% and 50% rules (440.62(B) and 440.62(C))

A cord-and-plug room air conditioner is a continuous motor load on a branch circuit that often serves other things, so NEC 440.62 caps how much of the circuit the unit may take. On a dedicated circuit - the room air conditioner is the only load - the marked rating may not exceed 80% of the branch-circuit rating (440.62(B)). Where the same circuit also feeds lighting outlets, general-use receptacles, or other appliances, the limit drops to 50% (440.62(C)), because the other loads share the circuit's capacity.

Reading the rule backward gives the minimum circuit: the branch-circuit rating must be at least the marked rating divided by 0.80 (dedicated) or 0.50 (shared), rounded up to the next standard branch-circuit rating (15, 20, 30, 40, or 50 A per NEC 210.3). A 12 A unit needs a 15 A dedicated circuit, but a 30 A circuit if it must share with the room's lighting.

What counts as a room air conditioner (440.62(A))

Section 440.62 applies only to a unit that meets all of 440.62(A): it is cord- and attachment-plug-connected; its rating is 40 A, 250 V, single phase or less; the total rated-load current is marked on the nameplate rather than the separate compressor and fan motor currents; and the branch-circuit short-circuit and ground-fault protective device does not exceed the branch-circuit conductor ampacity or the receptacle rating, whichever is less. A permanently connected (hard-wired) unit, anything over 40 A or 250 V, or three-phase equipment falls outside 440.62 and is sized by the general Article 440 rules for hermetic-compressor equipment - use the MCA and MOCP calculator.

Worked examples

• 12 A unit, dedicated circuit. 12 A / 0.80 = 15 A, and 0.80 x 15 = 12 A, so a 15 A dedicated circuit is the minimum. This is the familiar "12 amp window unit on a 15 amp circuit."
• 12 A unit sharing with room lighting. The 50% rule applies: 12 A / 0.50 = 24 A. A 20 A circuit allows only 0.50 x 20 = 10 A, so the minimum is a 30 A circuit (0.50 x 30 = 15 A). Sharing the circuit forces a bigger breaker for the same unit.
• 16 A unit, dedicated circuit. 16 A / 0.80 = 20 A, and 0.80 x 20 = 16 A exactly, so a 20 A dedicated circuit is the minimum.
• 40 A unit sharing a circuit. 40 A / 0.50 = 80 A. No standard branch circuit up to 50 A meets the 50% limit (0.50 x 50 = 25 A), so a 40 A room air conditioner must be on its own dedicated circuit, where it needs a 50 A circuit.

Frequently asked questions

What size breaker for a window air conditioner?

NEC 440.62 sizes the circuit from the unit's marked rating. On a dedicated circuit the marked rating may not exceed 80% of the circuit rating, so a 12 A unit needs at least a 15 A circuit. On a shared circuit the limit is 50%. Enter the marked rating above for the minimum standard rating, then size the conductor with the Wire Size calculator.

Can a room air conditioner share a circuit with outlets?

Yes, but under the 50% rule (440.62(C)) the unit's marked rating may not exceed half the branch-circuit rating, which usually forces a larger circuit than a dedicated one would. A dedicated circuit for the room air conditioner is often the simpler and cheaper choice.

Do I still have to size the wire?

Yes. This tool applies the 440.62 percentage rule to find the minimum branch-circuit rating. NEC 440.62(A)(4) also requires the overcurrent device not to exceed the conductor ampacity or the receptacle rating - size the conductor with the Wire Size or Wire Ampacity calculator and confirm the receptacle.

Which NEC edition is this?

NEC 2020. The 440.62 room-air-conditioner rule and the 80%/50% branch-circuit limits are unchanged across the 2017, 2020, and 2023 editions. Always confirm the edition your jurisdiction has adopted.

Related calculators

• MCA & MOCP Calculator: size central A/C and hermetic-compressor equipment per NEC Article 440 (440.33/440.22).
• Breaker Size Calculator: standard NEC 240.6(A) breaker for a load, with the 125% continuous rule.
• Wire Size Calculator: size the branch-circuit conductor by ampacity and voltage drop.
• Continuous Load Calculator: the 125% / 80% continuous-load rule for a general branch circuit or feeder.