Minimum circuit ampacity calculator
Enter the compressor RLA and the fan and blower FLA from the nameplate to get the minimum circuit ampacity, the maximum fuse or breaker, and the copper wire size, worked the way NEC Article 440 does it.
Inputs
Result
How the MCA adds up
Every motor at its nameplate current, the largest at 125%, and electric heat at 125%.
| Load | Nameplate A | Counted as A |
|---|
What MCA and MOCP mean on a nameplate
Every condenser, heat pump and air handler carries two electrical figures. Minimum circuit ampacity (MCA) is the smallest current the branch circuit's wire must be able to carry. Maximum overcurrent protection (MOCP), printed as "Max fuse/breaker", is the largest fuse or breaker you may put on it. Wire is sized to the first, the breaker can be anything up to the second.
Why a small wire on a big breaker is legal here
A unit with an 18.6 A compressor and a 1.2 A fan has an MCA of 24.45 A and a maximum breaker of 40 A. That means 12 AWG copper on a 40 A breaker, which looks wrong to anyone used to the 20 A limit for 12 AWG. It is correct.
The ordinary limits in 240.4(D) protect wire from overload. A compressor has its own overload protection built in, so the breaker on its circuit only has to protect against short circuits and ground faults, and it has to be large enough to let the compressor start. NEC 440.22 therefore lets the breaker exceed the small-conductor limits, and the wire only has to carry the MCA.
175% or 225%?
The code starts at 175% of the largest motor's rated current and allows up to 225% if the smaller breaker trips on start. Equipment nameplates usually print the 225% figure, rounded down, as the maximum. Using the nameplate maximum is permitted and gives the compressor the most room to start; a smaller standard breaker that still holds the MCA is also fine.
Heat strips change the picture
Electric heat is a continuous resistive load with no starting surge, so it is counted at 125% and the 225% allowance does nothing for it. On an air handler with large strips the 225% figure can come out below the MCA. The calculator then moves the breaker up to the next standard size at or above the MCA and tells you so. For these units the nameplate is the authority; heat circuits also fall under Article 424.
Questions
What is minimum circuit ampacity?
The smallest current the branch-circuit conductors to a piece of equipment must be able to carry. For air conditioning it is the sum of the motor currents with the largest increased by 25%, plus any electric heat at 125%. Size the wire to it; do not size the breaker to it.
How do I calculate MCA for an air conditioner?
Take the compressor's rated load amps (RLA), multiply by 1.25, and add the condenser fan's full load amps (FLA). A compressor at 18.6 A with a 1.2 A fan gives 1.25 × 18.6 + 1.2 = 24.45 A.
What is the difference between MCA and MOCP?
MCA sizes the wire; MOCP, the max fuse or breaker, sizes the protection. MOCP is always the larger figure, often much larger, because the breaker on a compressor circuit has to let the compressor start, while its own internal overload protects it from overheating.
Can I use a 40 amp breaker on 12 gauge wire for an AC unit?
Yes, if the unit's MCA is 25 A or less with 75 °C terminals and its maximum fuse/breaker is 40 A or more. NEC 440.22 allows a compressor circuit's breaker to exceed the normal 20 A limit for 12 AWG, because the wire is protected from overload by the compressor's own protection.
Should I use the max breaker size or a smaller one?
Either is allowed, as long as the breaker is at least large enough to carry the MCA and no larger than the maximum. The nameplate maximum gives the compressor the most room to start without nuisance trips, which is why most installers use it.
Why does my air handler's breaker look too small for the heat strips?
It may not be. Heat strips are counted at 125% in the MCA and get no starting allowance, so on a heat-heavy air handler the breaker is sized up from the MCA rather than down from 225%. Follow the nameplate, which covers each heat kit option separately.
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