MCA 20.1 amps

[Greg1707]

I was looking at the name plate on an AC outdoor unit today and noted the MCA was listed as 20.1 amps. It seems to be quite common to have the MCA as 15.1 amps etc. Is there a back story to this? Surely, the equipment could be designed for 20 amps? Or could it not operate safely with conductors rated for 20 amp?

 

[infinity]

Sounds like you need a conductor with a minimum of 20.1 amps. I would guess that the math just worked out that way.

 

[kwired]

Most of the time* the MCA is simply RLA of the compressor x 1.25 plus the FLA of the fan, and any other loads if present - it just happens to work out that way.

In the past the same BTU unit would have been less efficient and would have had a higher MCA

* sometimes we don't really know how they arrived at MCA, as is being discussed in another thread.

 

[topgone]

Most of the time* the MCA is simply RLA of the compressor x 1.25 plus the FLA of the fan, and any other loads if present - it just happens to work out that way.

In the past the same BTU unit would have been less efficient and would have had a higher MCA

* sometimes we don't really know how they arrived at MCA, as is being discussed in another thread.

It would be best to visit UL 1995 to undestand how these HVAC terms are calculated.

 

[steve66]

I was looking at the name plate on an AC outdoor unit today and noted the MCA was listed as 20.1 amps. It seems to be quite common to have the MCA as 15.1 amps etc. Is there a back story to this? Surely, the equipment could be designed for 20 amps? Or could it not operate safely with conductors rated for 20 amp?

I'm wondering if 220.5(B) would allow one to drop the 0.1 fraction and connect this to a 20A branch circuit?

 

[LarryFine]

I'm wondering if 220.5(B) would allow one to drop the 0.1 fraction and connect this to a 20A branch circuit?

The conductor and breaker sizes need not match on HVAC equipment. The question is really what size must the conductors be, 12 or 14 cu?

 

[jumper]

I'm wondering if 220.5(B) would allow one to drop the 0.1 fraction and connect this to a 20A branch circuit?

IMO, the conductors must be rated per nameplate value as speced. The guy (PE/ME/EE) who signed off on that and gave his stamp says: MCA of conductors is 20.1A.

If the PE who speced it wanted that section of the the NEC to be field applied, it defeats the purpose of an article 440 mandated nameplate and info required, no?

The conductor and breaker sizes need not match on HVAC equipment

Agree. Different parameters of protection needed with OLs considered.

. The question is really what size must the conductors be, 12 or 14 cu?

Rated for 20.1A per name plate specs. Design issues and other considerations will determine needed size, no?

 

[Dale001289]

Most of the time* the MCA is simply RLA of the compressor x 1.25 plus the FLA of the fan, and any other loads if present - it just happens to work out that way.

In the past the same BTU unit would have been less efficient and would have had a higher MCA

* sometimes we don't really know how they arrived at MCA, as is being discussed in another thread.

Suppose your equipment is not marked with minimumcircuit ampacity. For each single motor compressor, use a branch-circuitconductor ampacity not less than 125 percent of the motor-compressor current.Protect branch-circuit conductors against short circuits and ground faultsbetween 175 percent and 225 percent of the rated-load current. See 440.22(A).
RLA is a mathematical calculationrequired to meet Underwriters Laboratories (UL) approval. The compressormanufacturer must run a series of tests to determine the Maximum ContinuousAmps (MCA) before the overload trips. Once that has been determined, UL says todivide the MCA by 1.56 to determine the RLA.