NEC Article 440 branch-circuit sizing for one compressor motor, then the same compressor paired with a condenser fan on a shared feeder — showing why combination-load sizing isn't just 'add the two MCAs together'.
| Compressor rated-load current (RLA) | 18 A |
| Compressor locked-rotor amps (LRA) | 110 A |
| Second load on the feeder — condenser fan RLA | 4 A |
| Standard | NEC Article 440 |
MOCP starts at 175% of RLA, rounds up to the next standard breaker/fuse size, but is capped so it never exceeds 225% of RLA.
NEC combination-load sizing does not simply sum each device's own MCA — instead, the largest motor gets its own 125% factor, and every other load on the same feeder is added at 100% (its full RLA, no margin).
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Single compressor MCA | n/a (sizing basis) | 22.5 A | ✓ PASS |
| Single compressor MOCP | ≤ 40.5 A cap | 35 A | ✓ PASS |
| Combination-load MCA (compressor + fan) | n/a (sizing basis) | 26.5 A | ✓ PASS |
| Combination-load MOCP | n/a (sizing basis) | 39 A | ✓ PASS |
Key insight: A frequent sizing mistake is adding each device's own already-125%-margined MCA together for a shared feeder — that double-counts the safety margin on every motor except the largest, producing an oversized (and non-compliant with the Article 440 method) result. NEC combination-load sizing applies the 125% factor exactly once, to the single largest motor, and full RLA to everything else on that feeder.
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Open HVAC Electrical Sizing calculator →The 125% factor exists to keep the branch-circuit conductor comfortably above one motor's starting and running behavior — once that largest motor's margin is included, the other loads on the same feeder are already running loads (not the one setting the peak starting/thermal condition), so NEC 440.22(A) only requires them to be counted at their full rated current rather than re-applying the margin to each one individually.
No — a VFD's input and output circuits are sized independently of each other and of any combination-load calculation, because the VFD itself decouples the fixed-frequency supply side from the variable-frequency motor side; this calculator's separate VFD section applies the 125% (NEC) or IEC margin factor directly to the drive's rated input current and to the motor's RLA on the output side, each on its own.