[KimWilcox]

How do I calculate how much load I can place on a nn amp circuit breaker?
My knowns would be voltage and watts per hour.
How to I allow for (amp draw) when a shop motor or freezer compressor spins up at initial start?

[stanfortyman]

How to I allow for (amp draw) when a shop motor or freezer compressor spins up at initial start?

You don't. The breaker takes care of that.

[tkrussell]

A circuit breaker needs to be higher in rating than the load of the unit being served.

For a standard single phase motor, nameplate data, such as wattage or amps cannot be used for these calculations, only the horsepower rating is used, and for a single phase motor, refer to the amperages listed in the NEC Table 430.148.

A standard inverse time circuit breaker feeding standard single phase motor can be no larger than 250% of the running load in amps of that motor.

The branch circuit wiring must be th minimum of 125% of the running load.

So, for example, Table 430.148 lists a 1 HP motor at 120 volts at 16 amps.

16 x 1.25 = 20, so #12 wire minimum is allowed.

16 x 2.5 = 40, so a 40 amp maximum circuit breaker is allowed.

The 40 amp breaker is now rated to handle the "initial start" ,or called Locked Rotor Amperage. The wire is sized to handle the running load.


A 40 amp breaker on #12 wire? I am sure to get some comments on this.

A refrigerator falls under Article 440, and the breaker is sized differently, due to the inherent higher Locked Rotor Amps that all refrigeration compressors have. Here the nameplate current, and not horsepower is used.

The wire is still to be a minimum of 125% of the running load, and the circuit breaker can be a 175% of the running load, up to a maximum of 225% if necessary to start the unit.

So, with my example of a 16 amp unit, the wire can still be #12 copper wire, but the breaker can be a 30 amp CB at 175%, or a 40 amp CB at 225%, which under normal conditions, should not be needed..

[stanfortyman]

A 40 amp breaker on #12 wire? I am sure to get some comments on this.

I'll comment that you are totally correct.

Now I'll give my opinion.
I am of the strong opinion that Articles 430, 440, 630, etc have NO place in a residence. The rules that allow us to use "non-typical" breaker/wire sizes do NOT apply when wiring a RECEPTACLE in a residence.
That's right, you are wiring a receptacle, NOT a welder, refrigerator, or compressor motor.


Like I said in my original reply. If you have a tool or appliance that can plug into a standard 15A/125v receptacle the circuit breaker IS designed to handle ANY startup load that appliance or tool might have.