10 Ton A/C

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wmeek

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Electrician
Trying to size conductors and a N/F disconnect and a OCPD for a 10 Ton AC 120/240 3ph . Nameplate says it has a MCA of 51.9. So do I take 51.9 x 125% = 64.8A go up to a #6 Thhn for conductor and 51.9 x 115%= 59.6A go with a 60A for disconnect sizing. Also for OCPD 51.9x125% = 64.87A go up to a 70A breaker
 
Nameplate did not have Max OCPD on it only had MCA. So with that said #6 Thhn and a 60A breaker and a 60A N/F disconnect would be code compliant.
 
I was also thinking 115% of MCA for disconnect. 51.9A x 115% = 59.6A so 60 amp disconnect should work. I will go to a 70A breaker .
I should be code compliant with a #6 thhn , 60A NF disconnect and a 70A breaker
 
jumper said:
Disconnect size is 115% of MCA I think, but that breaker is probably be undersized and nuisance trip.
Click to expand...

The disconnect would be based on 115% of the sum of all currents at rated load conditions, not 115% of MCA (440.12(B)(2)).

I agree the breaker may be undersized.
 
david luchini said:
The disconnect would be based on 115% of the sum of all currents at rated load conditions, not 115% of MCA (440.12(B)(2)).

I agree the breaker may be undersized.
Click to expand...

440.12(B)(2) leads me to 440.12(B)(1) which has this:

(1) Ampere Rating. The ampere rating shall be at least
115 percent of the nameplate rated-load current or branchcircuit
selection current, whichever is greater.

I thought that was MCA, part in bold, and assumed that it would be the greater of the two.

I seem to be missing something.
 
This unit has two 5Ton compressors on it. I was told by AC contractor that both compressors would never be able to start at same time.That should make a big difference on the inrush current
 
jumper said:
440.12(B)(2) leads me to 440.12(B)(1) which has this:

(1) Ampere Rating. The ampere rating shall be at least
115 percent of the nameplate rated-load current or branchcircuit
selection current, whichever is greater.

I thought that was MCA, part in bold, and assumed that it would be the greater of the two.

I seem to be missing something.
Click to expand...

Branch circuit selection current is not MCA. Branch circuit selection current is used to determine the MCA (that is to say, MCA=125% of branch circuit selection current - see 440.32.)
 
I found the manufactures literature for this unit, and it calls for a maximun circuit breaker of 60A. Its 120/240V
3ph unit.
 
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