Auxiliary Heat Feeder

[mint402]

Replacing/relocating old fuse panel. Existing feeder to heat pump air handler/aux heater is #4 SE cable. Fused at 60 but draw/spec 48A. Using 60 degree column seems like 55 would be the max. Am I missing something, since this isn't the first time I've seen this. Have used 55amp breaker before but not readily available. Have used flex and #6 THHN with 60 breaker....should I be shopping for #3 SE or is there an exception I'm missing? Thanks.

 

[JDBrown]

NEC 240.4(B) allows you to use the "next higher standard overcurrent device rating (above the ampacity of the conductors being protected)" under certain conditions. Based on your post, it appears that you meet those conditions (this is not part of a MWBC supplying receptacles, the conductor ampacity does not correspond to a standard OCPD rating, and the next higher standard OCPD rating is less than 800 amps). The standard ampere ratings are listed in NEC 240.6(A), and 55 amps is not listed -- it jumps from 50 amps to 60 amps.

This rule does not change the conductor ampacity, so your cable would still only be good for 55 amps, but all that means is that your load calculation must not exceed 55 amps even though your overcurrent device is rated for 60 amps.

 

[don_resqcapt19]

What code cycle? I think that the 2008 cycle was the only one that limited SE cable to the 60?C ampacity.
Also #4 is good for 70 amps even in the 60?C column.

 

[curt swartz]

What code cycle? I think that the 2008 cycle was the only one that limited SE cable to the 60?C ampacity.
Also #4 is good for 70 amps even in the 60?C column.

Don, I'm thinking the OP has AL SE cable so the 55 amps would be correct. As JD pointed out he would be ok anyway since the load is less than 55 amp.

The 2011 still has the 60C rating on SE if it installed in insulation. I can't think of any time I have installed SE cable that it was NOT in insulation.

 

[don_resqcapt19]

Don, I'm thinking the OP has AL SE cable so the 55 amps would be correct. As JD pointed out he would be ok anyway since the load is less than 55 amp.

The 2011 still has the 60C rating on SE if it installed in insulation. I can't think of any time I have installed SE cable that it was NOT in insulation.

That would make sense...don't do that type of work so never think about aluminum.
I would not expect a feeder or branch circuit cable to this equipment to be installed in insulation.

 

[augie47]

As others have noted, the 55 amp rating for #4 AL SE in the '11 Code only applies if the SE is installed in insulation.
Also if the 48 amps you mention is the unit rated MCA, your conductor would only need to be rated for that current. The unit should also indicate the MOCP for your selecting your maximum breaker.

 

[infinity]

As others have noted, the 55 amp rating for #4 AL SE in the '11 Code only applies if the SE is installed in insulation.
Also if the 48 amps you mention is the unit rated MCA, your conductor would only need to be rated for that current. The unit should also indicate the MOCP for your selecting your maximum breaker.

I agree the information in the OP is somewhat vague so it's hard to accurately answer the question. If an MCA is involved when sizing the conductors the rating of the OCPD is irrelevant.

 

[mint402]

Thanks everyone....a big help! Sorry, yes it is Al, and it will pass through a little insulation and could get covered down the road....but it looks like it will be ok with a 60amp breaker. I will check inside the unit more closely to see about its rating....there is a table on the outside but they didn't note which heater was installed....I measured the 48A in operation.

 

[kwired]

Thanks everyone....a big help! Sorry, yes it is Al, and it will pass through a little insulation and could get covered down the road....but it looks like it will be ok with a 60amp breaker. I will check inside the unit more closely to see about its rating....there is a table on the outside but they didn't note which heater was installed....I measured the 48A in operation.

As has been said, if unit marked MCA is 48 amps a 55 amp conductor is acceptable, but if 48 amps still needs to be multiplied by 1.25% then you need a 60 amp conductor minimum - which is still ok if you are allowed to use 75C column with your 4 AWG, but if you have to use the 60C column you may be too small with the conductor.

If you are actually measuring 48 amps, the MCA is probably higher than 48, does this circuit also include the blower? Sometimes that can be the killer.