Max Amps on 6/2 NM-B

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  #1  
Old 08-18-07, 11:28 PM
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Max Amps on 6/2 NM-B

I have a new heat pump I need to wire and I'm hesitant to run what the HVAC guy said too. It has a 15kw backup resistance heater and the plate on the air handler specifies 2 circuits. For 1 circuit it lists the Minimum Circuit Amps as 21.5 and the Max Fuse as 25 amps. That one I can handle, a 10/2 NM-B cable with a 25 amp breaker (if I can find one). It's the other circuit that I think is pushing the limit. It lists the Minimum Circuit Amps as 53.1 and the Max Fuse as 60 amps. The HVAC guy said to feed it with a 6/2 NM-B cable on a 60 amp breaker. But isn't 6/2 NM-B limited to 55 amps? If it was a real short run, I probably feel better about that, but it's not. It's a 75' run in a hot attic. Of course the attic isn't hot in the winter time, when this circuit may come to life. The manual that came with the heater has a table that also says to use 6/6 wire for this circuit with a max wire length of 80'. But still I'm thinking maybe I should either run #6 THHN cable in conduit to it, or step up to #4 wire. What do you think? And do they even make a 4/2 NM-B cable or would I have to use SE cable in conduit?
 
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Old 08-19-07, 12:22 PM
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It doesn't hurt anything other than your wallet to make the conductors larger. Code is _minimum_ standard.

1) 6ga NM has an ampacity of 55A in normal conditions. If this cable is bundled with other cables, or if the ambient temperature _when the conductor is being used_ exceeds 86F, then the ampacity may be reduced. The 'hot attic' is not a problem if it isn't hot when the heat pump runs. If the heat pump requirements are MCA of 53.1A and max OCPD of 60A, then 6ga NM is acceptable.

2) If you choose to run conduit, use individual THHN conductors, _not_ a cable.

3) 4ga and larger NM is available, however you may need to check with several suppliers.

4) If the heat pump specifies a maximum _fuse_ rather than maximum OCPD, then you must use a fuse, not a circuit breaker.

-Jon
 
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Old 08-19-07, 12:25 PM
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They have you by the short hairs when stating amperage on this equipment, by saying 21 amps and 60 amps, but since it is stated on the plates, you have to go by the code and upgrade to the appropriate wire. Every installation I have seen has #6 for the 60 amp circuit (although #6 is rated at 55 amps). I recently upgraded my rental cabin to a 200 amp panel in order to install additional applicances and HVAC. I used #4 in conduit for the run (30'), as we have a new inspector in town, and I didn't want any hassle. As far as the 21 amps, I doubt you will find a 25 amp breaker, but will have to go with a 30, and your #10 cable.
 
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Old 08-19-07, 12:52 PM
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per NEC 2005 240.4 (B) you can use the next standard OCPD above the rating of the conductors (as long as everything meets certain criteria) which your situation does meet..

the standard rating go from 50 to 60 so you would be fine with a 60 amp OCPD.

as well, 25 amps is a standard rating so you should be able to find a 25 amp OCPD. Without knowing what manuf, you need, I have no way of showing this though.
 
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Old 08-19-07, 03:38 PM
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25 amp breakers are obtained at an electrical supply, they are usually not carried by the big boxes. See if you can find some SER copper cable at a local supply house or have them order some. That gives you a 65 amp rating but NM-b will be fine at 55 amps in my opinion.
Building a conduit run in an attic seems uneccessary to me as there really is no physical damage or environmental issues up there. Maybe some heat deration issues but I doubt you need to derate.

If it says fuse and not breaker heed Winnies advice.

Roger
 
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Old 08-19-07, 08:35 PM
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Ok, so he was correct and I am just paranoid. I called an industrial electrician buddy of mine this afternoon and he concurs with you guys, says 6/2 NM-B will be fine and since I already have some on hand left over from a cabin stove earlier this year, that's what I'll use.

The issue with the "fuse" though is new to me. So if it says fuse, it has to be fused and can't use a breaker. Hmmm. I'm not certain of the exact wording as I just jotted down the amp values, but I will look on the unit again and in the manual to see exactly what it says. I do know that it has a set of 60 amp fuses (not a breaker) within the unit itself right where you wire it up. Surely I can use a breaker in the panel though. If I couldn't then how would you use a set fuses in a panel? Do they make a gizmo that a main breaker panel will except for fuses or would you have to install a fused panel feed from the panel? Anyway, that seems rediculous since there is already a set upstream of the panel at the unit itself.
 
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Old 08-19-07, 08:59 PM
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the fuses in the unit are the only fuses you need to employ. You can use a breaker other than that.
 
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