Sizing a breaker below MOCP rating

[ElecEngbw]

I have an HVAC unit with a MCA of 74A and MOCP of 90A. I have an existing 80A breaker I would like to use for this unit. How low below the MOCP can I size my breaker?

 

[Jraef]

As low as you like. But the lower you go, the higher the risk of nuisance tripping.

 

[kingpb]

I need clarification - 125% of 74A is 92.5A, which normally you would go to a 100A breaker. But, 440.52 says not more than, so 90A would match the MOCP. But the quandary is, I am not supposed to load a breaker to more than 80%, which in this case I am.

Is the NEC assuming that it will not run continuously? If so, that's a heck of an assumption.

 

[infinity]

I need clarification - 125% of 74A is 92.5A, which normally you would go to a 100A breaker. But, 440.52 says not more than, so 90A would match the MOCP. But the quandary is, I am not supposed to load a breaker to more than 80%, which in this case I am.

Is the NEC assuming that it will not run continuously? If so, that's a heck of an assumption.

The 125% has already been factored in 74 * 125% = 92.5. Just look at the nameplate and choose an OCPD between the MCA and the MOCPD.

 

[Npstewart]

I agree, the MCA by definition has 125% built in.

RLA * 1.25 = MCA

 

[anbm]

MCA = minimum circuit ampacity = recommended conductor size?

I would get the actual motor full load amp (FLA) and size CB based on article 430.

 

[ElecEngbw]

Great information. Thanks

 

[infinity]

MCA = minimum circuit ampacity = recommended conductor size?

I would get the actual motor full load amp (FLA) and size CB based on article 430.

440.4(A) would require you to use the nameplate information.

 

[anbm]

Agreed.