Locked Rotor Amps versus Min Circuit Amps on A/C unit

[eb1of9]

Two simple questions: on a Carrier home A/C unit, the Locked Rotor Amps (LRA) are listed as 112A while the min circuit Amps are listed as 23.6A and the circuit breaker for the A/C unit is 50 A. Q1: Why isn't the circuit breaker sized to the LRA? Q2: for a stationary generator to supply this load, do I need to find a unit that can handle the LRA?

 

[BrianPetersen]

If the rotor is locked and stays locked, you WANT the circuit breaker to pop, before the motor incinerates itself.

The breaker has to take long enough to interrupt the power, to accommodate the momentary inrush spike.

The generator's ability (or inability) to deal with momentary inrush current is a question for the generator manufacturer.

The generator will already be spinning at the moment you connect the load, and the generator's rotor and the engine should have enough inertia to overcome that spike. The generator will slow down momentarily. For this type of application, this is not a problem. If the generator is an inverter-type generator, the generator speed is not locked to 60 Hz, but the inverter itself has to be able to deal with an inrush spike in the load. And that is the question you need to ask of the manufacturer of the generator.

 

[waross]

The A/C manufacturer has done this before and he knows what he is doing.
Those numbers are based on the Electrical code. The electrical inspector will accept an installation based on those numbers.
Those numbers take into account the tripping characteristics of the breaker and the starting characteristics of the motor.

What is the full load current of the compressor.
Minimum circuit ampacity may be the sum of any fans plus 125% of full load current.
From 23.6 Amps I will assume that the full load current is 23.6A/125% or 18.88 Amps. Round that off to 19 Amps.
The generator should have reserve capacity of 3 times that or 57 Amps.
Take the base load which will be connected when the A/C starts and add 57 Amps.
Note:

For 120 Volt loads, use the Amps of the most heavily loaded line. (ie: if there is a 4 Amp load on line 1 and a 6 Amp load on line 2, use the 6 Amp load as the base load.)​

For Standby generators, use the listed generator current.​

For Prime Power sets, use 110% of listed generator current.​

Bill
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"Why not the best?"
Jimmy Carter