Wire size question

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  #1  
Old 04-04-13, 10:59 PM
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Wire size question

I am running a circuit for a auxiliary heat source for a heat pump. This would be a 15kw electric unit for when our outside temperatures drop below freezing. I was told by the heat pump contractor that we needed 2 circuits run: 6/2 nm-b w/g and a 10/2 nm-b w/g for the electric air handler unit (90 amps) and 10/2 nm-b w/g for the heat pump (30amps)
Since the ampacity tables I have since looked at state that 6/2 is only capable of handling 55amps, my question is am I missing something here? It seems to me that 4/2 would have been the appropriate wire size. I have purchased the 6/2 wire already so am posting in hopes that someone could give me advice. Thanks so much.
 
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Old 04-05-13, 07:49 AM
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You are fine. I just had a 20KW aux. and heat pump installed and ran two 6-2wg cables and one 10 gauge. What you have is a 10KW strip and a 5KW and yes, 6-2 is rated at 55 amps but you are allowed to move up to 60 amp breaker since a 55 amp is not made. A 50 amp breaker would not do as that would be too close. (You need to allow extra capacity on heating circuits) You would be pulling around 43 amps and 60 amp breaker would be best. These cables are now made using 90C rated materials too so no worries.....
 
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Old 04-05-13, 09:37 AM
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What is the 90 amp rating? The largest element at 10,000 watts divided by 240 volts = about 41.6 amp draw. The smaller one should draw about 20.8 amps. If they both come on together you are right around 62.5 amps.

Table 310.16 in the NEC shows copper 6awg rated at 60 degrees C is good for 55 amps, but the 75 and 90 degree rated cconductors are good for 65 and 75 respectively. Keep in mind, NM-B isn't listed above that chart, but 334.112 says NM-B is a 90 degree rated sheathing so your 6awg should be good for 75 amps.
 
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Old 04-05-13, 03:46 PM
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Table 310.16 in the NEC shows copper 6awg rated at 60 degrees C is good for 55 amps, but the 75 and 90 degree rated conductors are good for 65 and 75 respectively.
And #6 AWG copper can be used to supply 65 or 75 amps if all of the terminals for that run are also rated for 75[SUP]o[/SUP]C or 90[SUP]o[/SUP]C.
 
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Old 04-05-13, 06:27 PM
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I just had a 20KW aux. and heat pump installed and ran two 6-2wg cables and one 10 gauge
Jim, what was the 10 gauge for? I believe a 2 circuit 20 KW electric furnace just needs two 60 amp circuits? Could it have been 25 KW aux heat?
 
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Old 04-05-13, 10:06 PM
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Oops....Sorry about the confusion. That wire was for the heat pump itself as I had to run a new 10/2wg (UF)cable to the a/c pullout box. The old cable was romex and was exposed to the elements. Hard a a rock. The heat strips are both 10KW so I ran 2 - 6-2wg romex cables to the air handler per the installers instructions.
 
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Old 05-02-13, 04:53 PM
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Wire size question thanks

Thanks to all of you who responded. Good to know that the electric furnace has a 10kw and 5 kw strip inside. I was not sure how the 15kw rating was obtained. Thanks for notes on the wire sizing. Really appreciate the help!
 
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Old 05-02-13, 05:53 PM
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Table 310.16 in the NEC shows copper 6awg rated at 60 degrees C is good for 55 amps, but the 75 and 90 degree rated conductors are good for 65 and 75 respectively. Keep in mind, NM-B isn't listed above that chart, but 334.112 says NM-B is a 90 degree rated sheathing so your 6awg should be good for 75 amps.
NM-b is not listed in that chart because it is a cable, not a conductor. Art. 334.80 tells us that you may use the 90 degree for derating and such. 334.80 also tells us that the ampacity for NM cable is limited to the 60 degree column after all adjustments.

Therefore, #6 NM-b is limited to 55 amps. Or a 60 amp overcurrent device.
 
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Old 05-02-13, 06:04 PM
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The 15 Kw rating is the combination of the 10 Kw and 5 Kw strip heaters.
 
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