heat pump calculation ?'s

[texie]

I have never been clear if branch circuit selection current and minimum circuit ampacity are the same. Like others, I have never seen a unit marked with branch circuit selection current. I am however firmly in the camp that says MCA means just that-you need conductors with the stated MCA and the max. breaker can be as stated. I am often amazed how many otherwise experienced electricians don't follow this concept. Even more shocking is the number of inspectors who don't understand this concept and insist that the conductor ampacity must be sized to the overcurrent device. Any inspectors care to comment?

 

[dwellins]

Thank you for all your input. I have decided to go with a 150A subpanel.

 

[bob]

Bob, you very well may be right, I was basing my numbers on the wording throughout Art 440 that states:
protective device having a rating or setting not exceeding 175 percent of the motor-compressor rated-load current or branch-circuit selection current, whichever is greater,

Augie
After reading all of this information, do you have a new out look on this subject?

 

[bob]

bump

 

[david luchini]

I don't recall ever seeing the BCSC on a nameplate as required by 440.4(C)...

I think this is part of the confusion. In general, 440.4(C) doesn't required a BCSC to be listed on nameplates of all equipment that has a hermetic refrigerant motor-compressor.

It requires the BCSC to be listed on (hrm-c) equipment that has a protective equipment that permits continuous current in excess of the specified percentage of nameplate rated-load current given in 440.52(B)(2) or (B)(4).

Personally, I don't know what type of equipment would require a continuous current in excess of 156% of the compressor rated load current.

 

[augie47]

Augie
After reading all of this information, do you have a new out look on this subject?

Not really.... I will have to put myself in the still confused category
but I'm working on it.....
The whole thing is muddy for as texie and kwired noted, I too don't recall seeing BCSC on a nameplate despite that requirement in 440.4(C) but as pointed out in the thread you referenced in Post #18, 440.4(C) states: having a protection system that is approved for use with the motor-compressor that it protects and that permits continuous current in excess of the specified percentage of nameplate rated-load current given in 440.52(B)(2) or (B)(4) shall also be marked with a branch-circuit selection current that complies with 440.52(B)(2) or (B)(4). This marking shall be provided by the equipment manufacturer and shall be on the nameplate(s) where the rated-load current(s) appears.
which appears to me to say you don't need the BCSC rating unless the current exceeds that in 440.52(B)(2)-(4) (156%)
.. so we normally don't see BCSC as its not always required.....


so now we are back to using the BCSC or the motor compressor Rated Load Current and if the BCSC is not there as it is not needed, I assume we use the RLC. RLC I assume is RLA in this case...so dwellins calculations in post #1 are correct.

Now my head hurts worse

I would like to add that I personally appreciate everyones input on the subject. It has been a enlightening trip even if a bit confusing.....

 

[david luchini]

so now we are back to using the BCSC or the motor compressor Rated Load Current and if the BCSC is not there as it is not needed, I assume we use the RLC. RLC I assume is RLA in this case...so dwellins calculations in post #1 are correct.

Now my head hurts worse

The calculations in Post #1 are not correct. Note that each unit has an MCA of 29.3 and an RLA of 23.2, so there must be 0.3A of load in addition to the motor compressor. The MCA for the feeder should be 125% of the largest compressor plus 100% of the other loads, or 123.3A (instead of 121.8A).

And the feeder MOCP must not be larger than the largest permitted OCPD for on compressor, plus the sum of the other load currents. This would be a 125A c/b rather than a 150A c/b.

 

[augie47]

The calculations in Post #1 are not correct. Note that each unit has an MCA of 29.3 and an RLA of 23.2, so there must be 0.3A of load in addition to the motor compressor. The MCA for the feeder should be 125% of the largest compressor plus 100% of the other loads, or 123.3A (instead of 121.8A).

And the feeder MOCP must not be larger than the largest permitted OCPD for on compressor, plus the sum of the other load currents. This would be a 125A c/b rather than a 150A c/b.

That didn't help my headache....
You are one sharp cookie
I agree with the feeder MCA, but come up with a different number on the MOCP (134.9).. what did I miss ?

 

[david luchini]

That didn't help my headache....
You are one sharp cookie
I agree with the feeder MCA, but come up with a different number on the MOCP (134.9).. what did I miss ?

I came up with 139.3 for the MOCP, but it must go down to the next size, so 125A. Interestingly, the unit MOCP is 40, but the max allowable OCPD for a 23.2 RLA motor-compressor is 45, so I used that in my calculation. See post #13.

 

[curt swartz]

I guess I don't understand what is being said. Why is there a MOCP for the feeder? The feeder can be any size as long as each unit is protected properly.

 

[david luchini]

I guess I don't understand what is being said. Why is there a MOCP for the feeder? The feeder can be any size as long as each unit is protected properly.

The feeder conductors can be protected at above their ampacity in accordance with 440, and 430. If the feeder MCA calculates to 145.5 and the feeder MOCP calculates to 177, then you could use #1/0Awg on a 175A c/b.

Of course, nothing prevents you from using #2/0awg on the 175A c/b. Or nothing would prevent you from using #3/0awg on a 200A c/b to feed the heat pump units. But the feeder MCA/MOCP calculations may allow you to use a smaller conductor for a given circuit breaker.