A/c

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cowboyjwc

cowboyjwc

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I don't see this a lot, but I just had a long discussion with an engineer about sizing overcurrent for an A/C unit.

(Some of these numbers are made up so don't do the math)

Min Cir Ampacity = 22amps (which equates to #10 conductors)
Max Fuse or Hacr breaker = 30 (Fused disconnect on roof)

The problem was this.

He had those #10 conductors coming off of a 60 amp breaker saying that he can size the overcurrent 200% and the conductors 125%.

I say that all of the calculations have been done and the overcurrent device cannot be larger than 30 amps to protect the conductors. If he had a nonfused disconnect on the roof the largest size the overcurrent device could be is 30 amps per the nameplate.

Am I missing something?
 
Actually you could use #12 wire for 22 amps using 240.4 and fuse it at least 175% and I believe as much as 225%. 440.22
 
Dennis Alwon said:
Actually you could use #12 wire for 22 amps using 240.4 and fuse it at least 175% and I believe as much as 225%. 440.22
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Dennis, what cowboy is saying is that there is already a nameplate on the unit, and the engineer is doing the calculations twice...
 
Dennis Alwon said:
Actually you could use #12 wire for 22 amps using 240.4 and fuse it at least 175% and I believe as much as 225%. 440.22
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Not if the unit is labeled with MCA and Max Breaker / fuse size.

See 440.6(A)
 
iwire said:
I think he is out of his tree.

Did he mention what section of the NEC allows this?
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I think he was trying to use Article 430 for his argument, but he didn't give me an exact code section.

He also used the old "this is only the second or third time anyone has questioned me on this."
 
cowboyjwc said:
Yes, that's exactly what I'm saying.
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Sorry I missed that and I agree you cannot go over the nameplate but you can still use #12 but I don't think that is pertinent to your question. Sorry.
 
Dennis Alwon said:
Sorry I missed that and I agree you cannot go over the nameplate but you can still use #12 but I don't think that is pertinent to your question. Sorry.
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It wasn't, but that does bring up another question and it is also mentioned in the article that stickboy attached.

If you use #12 can you install a 30 amp breaker or can you only install a 25 amp breaker?
 
cowboyjwc said:
It wasn't, but that does bring up another question and it is also mentioned in the article that stickboy attached.

If you use #12 can you install a 30 amp breaker or can you only install a 25 amp breaker?
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You can use a 30 amp breaker, IMO
 
I have seen #12 with a 40 amp protection. Since the motor has an overload protection in it, then the breaker is only for short circuit and ground fault protection. At least that is my understanding.
 
I have installed 30 amp breakers on # 14awg, the overload protection is built into the motor... Its pretty hard to melt the wire when the motor is not running... :smile:
 
I don't see it as a real big safety problem, 10 AWG is the EGC size for a 60 amp OCPD anyway and there is no real way to overload the conductor.

Still IMO it is a violation.

I also really wonder what benefit this EE sees in the installation. :confused: :roll:
 
chris kennedy said:
Would there be warranty issues if the manufactures MOP is exceeded?
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Probably so but Cowboys example is not a true example if a unit has a 22 amp Min cir. amp the max OCP is generally 175% .

Thus that unit would be marked 22 amp MCA
and 38.5 max OCP -- The unit would probably say 35 or 40 amps but you could still use #12 wire on that setup.
 
Dennis Alwon said:
Probably so but Cowboys example is not a true example if a unit has a 22 amp Min cir. amp the max OCP is generally 175% .

Thus that unit would be marked 22 amp MCA
and 38.5 max OCP -- The unit would probably say 35 or 40 amps but you could still use #12 wire on that setup.
Click to expand...



Just ignore...
 
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