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Old 05-06-2013, 10:39 AM   #1
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Breaker & wire size


installing a residential 4 ton central AC unit. Name plate states 240V, MCA= 26A, MOP= 45A. The AC unit will be approximately 70 feet from the main distribution panel. I've reviewed NEC & table 310.16, and not sure what size breaker and wire size (45A & #8 wire- 2 conducter, 1 ground)??

any help please?

thank you

Rob
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Old 05-06-2013, 10:52 AM   #2
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Breaker & wire size


The plate on the ac unit should state max amp. That is what you go by.

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Old 05-06-2013, 05:45 PM   #3
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Breaker & wire size


10's on a 40 amp breaker.
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Old 05-06-2013, 05:48 PM   #4
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#10's on a 40 amp breaker is my vote as well....




Quote:
Originally Posted by rcassidy View Post
installing a residential 4 ton central AC unit. Name plate states 240V, MCA= 26A.
310.16 allows 35 amps on #10 AWG

Quote:
Originally Posted by rcassidy View Post
MOP= 45A.
40 amp breaker is more common than a 45A breaker, and will not be an issue for your installation.

Last edited by stickboy1375; 05-06-2013 at 05:51 PM.
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Old 05-06-2013, 11:56 PM   #5
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Breaker & wire size


Quote:
Originally Posted by gregzoll
The plate on the ac unit should state max amp. That is what you go by.
For sizing what? The wire or the breaker?
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Old 05-07-2013, 07:27 AM   #6
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Quote:
Originally Posted by gregzoll View Post
The plate on the ac unit should state max amp. That is what you go by.
I've never seen "max amp" on an AC nameplate. The closest thing to that would be the locked-rotor amps (LRA) and that is NOT what you use to size the conductors or the OCPD.

The nameplate should include the following. Two of which are used to select the right conductors and OCPD:

"Maximum Fuse Size" or "Maximum Fuse or Circuit Breaker Size" (Note: For the former, you have to have fuses on the circuit) (Abbreviated MOP)
"Minimum Circuit Ampacity" (Abbreviated MCA)
"Locked Rotor Amps" (Abbreviated LRA)

Don't confuse LRA with RLA that might also be on the nameplate. The RLA is the Running Load Amps and that is the normal current draw of the unit. The MCA is figured from the RLA.

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Last edited by busman; 05-07-2013 at 08:51 AM. Reason: Additional Information
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Old 05-07-2013, 12:02 PM   #7
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Quote:
Originally Posted by electures View Post
For sizing what? The wire or the breaker?
The breaker. I was at work and posting during break, so did not have time to go into much detail. Also hate trying to post from the iPhone, because the keyboard on those things are not made for adults.
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Old 05-07-2013, 12:09 PM   #8
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Busman, mine has min & max amp rating, min & max voltage on the nameplate, also both my neighbors have the min & max amp/volt rating on the plates. As for the OP, 40 amps is a large amount for a unit. I am either guessing a unit with a heater, large btu size, packaged unit, or older unit.

I ended up pulling #10 for my unit, because at the time, we were looking at both heat pumps & a/c units, and ended up getting a good deal on a 13seer Armstrong a/c unit, that someone had ordered, but never got picked up, so my guy that does my hvac, got it at about half off, due to the warehouse that he gets all his hvac stuff from here in town, could not send it back to Armstrong, and wanted to get it out of the shop.

If I ever go with a heat pump, easy change over from a 20 amp breaker to 30 amp, because I already have the #10 pulled.
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Old 05-07-2013, 12:34 PM   #9
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Quote:
Originally Posted by gregzoll View Post
Busman, mine has min & max amp rating, min & max voltage on the nameplate, also both my neighbors have the min & max amp/volt rating on the plates. As for the OP, 40 amps is a large amount for a unit. I am either guessing a unit with a heater, large btu size, packaged unit, or older unit.

I ended up pulling #10 for my unit, because at the time, we were looking at both heat pumps & a/c units, and ended up getting a good deal on a 13seer Armstrong a/c unit, that someone had ordered, but never got picked up, so my guy that does my hvac, got it at about half off, due to the warehouse that he gets all his hvac stuff from here in town, could not send it back to Armstrong, and wanted to get it out of the shop.

If I ever go with a heat pump, easy change over from a 20 amp breaker to 30 amp, because I already have the #10 pulled.
Yes, there are a lot of other numbers on most nameplates. I only bothered to cover the ones of interest to the OP. You mention that yours has min and max amp rating. I'm going to bet that that is min and max OCPD size. To call is "amp rating" is confusing. It's true that the rating of a branch circuit is determined by the rating of the OCPD, but I think it's less confusing to call it the required sized of the OCPD.

The only thing the OP really need to know is that the conductors need to meet or exceed the MCA when properly adjusted and derated and the OCPD needs to be between the minimum and maximum OCPD values listed and it needs to be the proper type (fuse or circuit breaker). He then just needs to account for voltage drop.

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Old 05-07-2013, 12:51 PM   #10
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Breaker & wire size


That is true on the voltage drop. Of course, most would have just gone out and got the #8 and not think about it. It would have cost them a little more, but would not have done any damage going with the larger size wire, other than their pocket book.

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