Air conditioning compressor nameplate data

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tonype

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Sometimes I see A/C units with specs on Min. circuit amps, min. OCP rating and max OCP rating, with the "min" somewhere between the other two. However, sometimes, there are not "min. OCP ratings.

For a unit with a 20.1 min. circuit amps and a 35 max OCP spec, would a 20-amp breaker be OK? My opinion is now, because it is less than the min circuit amps.

For a unit with a 19.4 min circuit amps and a 30 max OCP spec, would a 20-amp breaker also be OK. My opinion is yes.

Is there some formula for this?
 
tonype said:
Sometimes I see A/C units with specs on Min. circuit amps, min. OCP rating and max OCP rating, with the "min" somewhere between the other two. However, sometimes, there are not "min. OCP ratings.

For a unit with a 20.1 min. circuit amps and a 35 max OCP spec, would a 20-amp breaker be OK? My opinion is now, because it is less than the min circuit amps.

For a unit with a 19.4 min circuit amps and a 30 max OCP spec, would a 20-amp breaker also be OK. My opinion is yes.

Is there some formula for this?
Click to expand...

If we figure out how they determine this MCA it makes more sense. They have done calculations from 430 for us to come up with MCA. The marked MCA is 125% of largest compressor rated load plus all other loads.

It is similar for the overcurrent device size they have marked.

If you place a 20 amp breaker on either example you gave it would not be a code violation, but you may have trouble holding that breaker when the compressor starts. The starting current should be low enough for typical 30 or 35 amp inverse time breakers to hold in those applications.
 
kwired said:
If we figure out how they determine this MCA it makes more sense. They have done calculations from 430 for us to come up with MCA. The marked MCA is 125% of largest compressor rated load plus all other loads.

It is similar for the overcurrent device size they have marked.

If you place a 20 amp breaker on either example you gave it would not be a code violation, but you may have trouble holding that breaker when the compressor starts. The starting current should be low enough for typical 30 or 35 amp inverse time breakers to hold in those applications.
Click to expand...

Thanks - this helps alot
 
tonype said:
Sometimes I see A/C units with specs on Min. circuit amps, min. OCP rating and max OCP rating, with the "min" somewhere between the other two. However, sometimes, there are not "min. OCP ratings.

For a unit with a 20.1 min. circuit amps and a 35 max OCP spec, would a 20-amp breaker be OK? My opinion is now, because it is less than the min circuit amps.

For a unit with a 19.4 min circuit amps and a 30 max OCP spec, would a 20-amp breaker also be OK. My opinion is yes.

Is there some formula for this?
Click to expand...

I think it is important to realize that Minimum Circuit Ampacity (MCA) relates only to the conductor size and Max Over Current Protection (MOCP) relates only to the breaker (or fuse) size. In the case of MCA, the 125% has already been figured in and you just need to select the conductor from Table 310.15 and the appropriate temp column, regardless of the breaker you intend to use. The breaker can be any value up to the MOCP. I typically would use the max OCP or close to it to avoid nuisance trips. I think it is very unlikely that in your example that the 20 amp breaker would reliably start the unit. Keep in mind that the conductor capacity does not need to match the breaker ampacity.
 
tonype said:
Just thinking some more - 20.1 amp would require #10 AWG?
Click to expand...

Nope. #12 assuming THHN and raceway.

One could argue #14 based on:

220.5 Calculations
(B) Fractions of an Ampere. Where calculations result in
a fraction of an ampere that is less than 0.5, such fractions
shall be permitted to be dropped.

But since you did not actually do the calcs, I wouldn't.
 
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tonype said:
Just thinking some more - 20.1 amp would require #10 AWG?
Click to expand...

If allowed to use 75?C conductors - no. If required to use 60?C conductors - yes. Not sure you can drop the fraction of an amp or not - since it was not a result of any calculation done in the field. If you can drop the fraction then as jumper mentioned you could run 14AWG if it is 75?C conductor.
 
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