Question on MCA, Circuit Breaker Sizing, and Wire Size

[warraich]

Hi Everyone,

I have a quick question regarding MCA, circuit breaker sizing, and conductor size.

If a piece of equipment lists the MCA (Minimum Circuit Ampacity) as 13.3A, my understanding is that the circuit breaker should be at least 15A. The datasheet also lists the "Maximum Breaker" size as 20A. Based on this, I believe a 15A breaker would be acceptable.

I am planning to use #12 AWG copper, but the datasheet mentions that the "Terminal Block to Accept" is #10 AWG. This part is confusing me a little — can I still use #12 AWG if the ampacity is adequate, or do I need to upsize to #10 AWG just to match the terminal block?

Similarly, for an MCA of 17.7A, I believe I should use a 20A breaker. Again, I'm currently planning to use #12 AWG.

Can anyone confirm if my understanding is correct?

Thanks in advance for your help!

 

[tom baker]

Minimum circuit ampacity is conductor size. For the first example 14 AWG on a 20 amp CB is fine

 

[LarryFine]

If a piece of equipment lists the MCA (Minimum Circuit Ampacity) as 13.3A, my understanding is that the circuit breaker should be at least 15A.

No, the wire ampacity should be at least 15a.

The datasheet also lists the "Maximum Breaker" size as 20A. Based on this, I believe a 15A breaker would be acceptable.

If the unit will start on 15a, yes; if not, then 20a.

I am planning to use #12 AWG copper, but the datasheet mentions that the "Terminal Block to Accept" is #10 AWG.

That's the largest wire the terminals are sized to accept.

Similarly, for an MCA of 17.7A, I believe I should use a 20A breaker. Again, I'm currently planning to use #12 AWG.

Again, the minimum ampacity determines the wire size.

You can use any breaker between that which will start it and the MOC protection size.

 

[infinity]

MCA 13.3 amps, 20 amp OCPD, #14 AWG.
MCA 17.7 amps, 30 amp OCPD, #14 AWG (not NM cable)

 

[Tainted]

In my opinion always use the maximum breaker size, it's not worth the risk to go lower than the max breaker. What if the efficiency of the equipment gets lower over time driving up the current?

 

[warraich]

thanks everyone for your input. I have attached another table which shows the equipment FLA.

Hopefully, this clarifies things a bit more. Can we comfortably say that a 15A breaker for circuit 1 and 20A breaker for circuit 2 will be acceptable?

Note: The supply fan is being powered by a VFD

 

[infinity]

Can we comfortably say that a 15A breaker for circuit 1 and 20A breaker for circuit 2 will be acceptable?

Yes. They both meet the minimum ampacity but why not just use what's listed as the MaxOCPD? The conductor sizes will be the same either way.

 

[LarryFine]

Hopefully, this clarifies things a bit more. Can we comfortably say that a 15A breaker for circuit 1 and 20A breaker for circuit 2 will be acceptable?

Most of us would not be comfortable doing that for fear of nuisance tripping.

We use MCA to size the wire and MOCP to size the breaker without further thought.

You seem to be concerned with violating the standard small-wire breaker limits.

See 240.4(D), then 240.4(G), then 440, Parts III and IV if you need more clarification.