Why multiply a continuous load by 125 percent?

NEC 210.20(A) and 215.3 require the overcurrent device and conductor for a continuous load, one running three hours or more, to be rated at least 125 percent of the load. This prevents nuisance tripping and overheating from sustained current at the breaker terminals.

Why is a motor breaker so much larger than the motor FLA?

Motors draw a large inrush current at startup, so NEC 430.52 sizes the short-circuit and ground-fault device well above running current, such as 250 percent of FLA for an inverse-time breaker. Overload protection at the starter, sized near 115 to 125 percent, handles sustained overcurrent separately.

What are MCA and MOCP on HVAC equipment?

Minimum Circuit Ampacity sets the smallest conductor and breaker the equipment may use, while Maximum Overcurrent Protective Device is the largest breaker allowed. NEC 110.3(B) makes these nameplate values mandatory, so the breaker must be at least the MCA-driven size and no larger than the MOCP.

Does this enforce small-conductor limits?

For resistive loads the tool reminds you of NEC 240.4(D): 14 AWG copper is capped at 15 A, 12 AWG at 20 A, and 10 AWG at 30 A regardless of the calculated value. Always apply that cap before finalizing the breaker.

Why round up to a standard size?

Breakers are only manufactured in the standard ampere ratings listed in NEC 240.6. For non-motor loads 240.4(B) lets you round up to the next standard size when the conductor allows, and 430.52 explicitly permits rounding up for motors.

Circuit Breaker Sizing Calculator

Choosing the overcurrent device is governed by different NEC articles depending on whether the load is resistive, a motor, or packaged HVAC equipment. This calculator applies the right rule for each and returns a real standard breaker size.

How it works

Each mode uses its own NEC rule, then snaps to a standard size from NEC 240.6:

resistive: sized = load × (continuous ? 1.25 : 1.00),  round up to standard
motor:     sized = FLA × device multiplier (250/300/175/800%), round up
hvac:      breaker = largest standard ≤ MOCP, must be ≥ MCA

The standard sizes are 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100 amps and up. Motor multipliers come from Table 430.52 by device type.

Example and notes

A 16 A continuous resistive load sizes to 20 A (16 × 1.25), landing exactly on a standard 20 A breaker. A motor with 28 A FLA on an inverse-time breaker computes to 70 A (28 × 2.5). A heat pump with a 21.5 A MCA and 35 A MOCP takes a 35 A breaker, the largest standard size not exceeding the MOCP while still covering the MCA. Remember the resistive mode does not auto-apply 240.4(D) small-conductor caps, so verify the breaker against the actual wire gauge.