Circuit Ampacity

A

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
Been listening to MH’s NEC calculation stuff and very interesting.

Example I cam across on my drawings.

3P-100A fused switch in dist board that has 90A fuses is feeding AC unit with a 3P-100A unfused switch on it.

The FLA of the AC unit is 40A and the MCA is 50A.

Questions.

  1. I’d think the AC unit would be a continuous load and multiplied by 125%. Seems like they did take that into consideration with MCA of 50A, correct?
  2. With the circuit ampacity being 50A I still need to have wire ampacity for at least 90A?
 
1) MCA includes a 125% factor on the largest motor, you don't need to further apply 125%. Since the FLA of the compressor is 40A, and 125% * 40A = 50A, apparently this outdoor unit has no other loads.

2) Depends on the AC unit's MOCP, which should be marked on it. Per 440.22(A), it will be either 175% * 40A = 70A, or 225% * 40A = 90A if the former is "not sufficient for the starting current of the motor." So if the unit is marked MOCP 90A, then you can use 50A ampacity conductors. [The idea is that the unit has its own overload protection, so the 90A fuses are just for short circuit protection.] If the unit is marked MOCP 70A, then you would need to provide 90A ampacity conductors, or reduce the fuses to 70A and provide 50A conductors.

Cheers, Wayne
 
wwhitney said:
1) MCA includes a 125% factor on the largest motor, you don't need to further apply 125%. Since the FLA of the compressor is 40A, and 125% * 40A = 50A, apparently this outdoor unit has no other loads.

2) Depends on the AC unit's MOCP, which should be marked on it. Per 440.22(A), it will be either 175% * 40A = 70A, or 225% * 40A = 90A if the former is "not sufficient for the starting current of the motor." So if the unit is marked MOCP 90A, then you can use 50A ampacity conductors. [The idea is that the unit has its own overload protection, so the 90A fuses are just for short circuit protection.] If the unit is marked MOCP 70A, then you would need to provide 90A ampacity conductors, or reduce the fuses to 70A and provide 50A conductors.

Cheers,
Click to expand...
 
wwhitney said:
1) MCA includes a 125% factor on the largest motor, you don't need to further apply 125%. Since the FLA of the compressor is 40A, and 125% * 40A = 50A, apparently this outdoor unit has no other loads.

2) Depends on the AC unit's MOCP, which should be marked on it. Per 440.22(A), it will be either 175% * 40A = 70A, or 225% * 40A = 90A if the former is "not sufficient for the starting current of the motor." So if the unit is marked MOCP 90A, then you can use 50A ampacity conductors. [The idea is that the unit has its own overload protection, so the 90A fuses are just for short circuit protection.] If the unit is marked MOCP 70A, then you would need to provide 90A ampacity conductors, or reduce the fuses to 70A and provide 50A conductors.

Cheers, Wayne
Click to expand...
Thanks. Just saw they are calling for 3#2's and #8.

What is MOCP? motor overcurrent protection?
 
What am I missing here? You said if the unit is marked MOCP 90, you can use 50 ampacity conductors but if it's marked MOCP 70 you have to use 90 ampacity conductors. How does this make sense?
 
wwhitney said:
[The idea is that the unit has its own overload protection, so the 90A fuses are just for short circuit protection.] If the unit is marked MOCP 70A, then you would need to provide 90A ampacity conductors, or reduce the fuses to 70A and provide 50A conductors.
Click to expand...
Please explain this!
It goes against what you wrote before this statement.
The MCA and MOCP will be on the unit nameplate. You can choose to go with a lower fuse/breaker, but that wouldn't make you change the size of the conductors. You can't go above the Max OCP.
 
Alwayslearningelec said:
I don't follow.

OCPD @ the distribution board feeding unit is a 3P-100A switch fused at 90A.
THe unfused disconnect at the unit is a 3P-100A.
FLA on unit is 40A
MCA on unit is 50A.
Click to expand...
You don't need to be concerned with the FLA as it has been figured in. Your nameplate should have a "Max Fuse or Breaker" size listed on the nameplate.
MCA=Minimum Circuit Ampacity = the conductor size
MOCP=Maximum Over Current Protection= Max Breaker/Fuse
 
Little Bill said:
Please explain this!
It goes against what you wrote before this statement.
The MCA and MOCP will be on the unit nameplate. You can choose to go with a lower fuse/breaker, but that wouldn't make you change the size of the conductors. You can't go above the Max OCP.
Click to expand...
I would agree with wwhitney. If you follow the rules you noted in your post #9, wire = MCA, OCP_ does not exceed nameplate, then all is fine.
but if you choose to install a larger OCP for the feeder then the conductor would have to be sized for that OCP/

If is nameplate is MCA =50 and MOCP = 90 then he is fine with a 50 amp conductor but if his nameplate MOCP is 70 and he has a 90 at the panel he would need a 90 amp conductor to the disconnect.
 
augie47 said:
If is nameplate is MCA =50 and MOCP = 90 then he is fine with a 50 amp conductor but if his nameplate MOCP is 70 and he has a 90 at the panel he would need a 90 amp conductor to the disconnect.
Click to expand...
Precisely. And in the latter case, you would also need to provide a 70A or smaller OCPD at the unit. Perhaps the confusion stemmed from my not mentioning that.

Cheers,
Wayne

P.S. To make it a little more complicated, if the MOCP is 70 and if the 90A fuses also supply 20A of additional load, then the feeder would only need to have 70A amapacity. Which would suggest that even without another load, the feeder ampacity should only be required to be 70A, not 90A--the case where there is no other load is no worse than the case where the other load is present but off. However, the conductor sizing rules don't provide that allowance.
 
augie47 said:
I would agree with wwhitney. If you follow the rules you noted in your post #9, wire = MCA, OCP_ does not exceed nameplate, then all is fine.
but if you choose to install a larger OCP for the feeder then the conductor would have to be sized for that OCP/

If is nameplate is MCA =50 and MOCP = 90 then he is fine with a 50 amp conductor but if his nameplate MOCP is 70 and he has a 90 at the panel he would need a 90 amp conductor to the disconnect.
Click to expand...
I can see that, but it wasn't exactly clear from his writing. But we, in the field, or even on drawings, would not be changing the 175%, or 225%, to accommodate starting current, that is done for us and listed on the nameplate by the mfg.
 
wwhitney said:
Precisely. And in the latter case, you would also need to provide a 70A or smaller OCPD at the unit. Perhaps the confusion stemmed from my not mentioning that.

Cheers,
Wayne

P.S. To make it a little more complicated, if the MOCP is 70 and if the 90A fuses also supply 20A of additional load, then the feeder would only need to have 70A amapacity. Which would suggest that even without another load, the feeder ampacity should only be required to be 70A, not 90A--the case where there is no other load is no worse than the case where the other load is present but off. However, the conductor sizing rules don't provide that allowance.
Click to expand...
Yes, it would have been good to mention that!
 
I see little Bill finally answered your question. BTW, FLA is full load amperage. FLA and MCA are different because of the 125% rule you cited. To expand on Little Bill a bit... I generally see units with 4 possible maximum markings. It may say, "Max fuse/BKR" It may have separate "Max fuse" and/or max bkr" that are different sizes, it may merely have "MOCP". If it is either max fuse or max bkr without listing the other then it must be protected by the device stated
 
Alwayslearningelec said:
Thanks. Just saw they are calling for 3#2's and #8.
Click to expand...
Since #2s are good for 115A, that would suggest the conductors have been intentionally upsized, e.g. due to circuit length. If that's not the case, it may be an error or a place for economizing.

Alwayslearningelec said:
What is MOCP? motor overcurrent protection?
Click to expand...
Maximum overcurrent protection.

Cheers, Wayne
 
For AlwaysLearning your post is #9 the most relevant:

You don't need to be concerned with the FLA as it has been figured in. Your nameplate should have a "Max Fuse or Breaker" size listed on the nameplate.
MCA=Minimum Circuit Ampacity = the conductor size
MOCP=Maximum Over Current Protection= Max Breaker/Fuse
 
Little Bill said:
I can see that, but it wasn't exactly clear from his writing.
Click to expand...
Sorry to have been unclear.

Little Bill said:
But we, in the field, or even on drawings, would not be changing the 175%, or 225%, to accommodate starting current, that is done for us and listed on the nameplate by the mfg.
Click to expand...
OK, but in this case the OP is not in the field (he's an estimator) and has provided incomplete nameplate information for the unit. So the two primary possibilities are that the unit is marked MOCP 70A or MOCP 90A, and there are two different conclusions for these two cases.

Cheers, Wayne
 
Alwayslearningelec said:
I don't follow.

OCPD @ the distribution board feeding unit is a 3P-100A switch fused at 90A.
THe unfused disconnect at the unit is a 3P-100A.
FLA on unit is 40A
MCA on unit is 50A.
Click to expand...
No possible answer until you give us the maximum overcurrent protective device shown on the equipment label.
You size the conductor to the MCA and the overcurrent MOCP.
 
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