Do I need to derate?

[blueheels2]

Mounted a new meter panel combo and I was going to go through the crawl space with a sub panel feeder and 2 separate HVAC ckts. Realized today that you can't get there from here. Now I must mount a 200 amp panel in the house and run the feeders through the attic. My first thought was to derate for 140 degree attic temp but this comes at a significant cost increase and after reading a few threads up here I'm not sure anyone does and I'm not sure the inspectors require it. I do have an option of going underground and back into the house but it will have over 360 deg of bends so I would have to put a pullbox in the yard and I am trying to avoid that. What say you?

 

[Little Bill]

Mounted a new meter panel combo and I was going to go through the crawl space with a sub panel feeder and 2 separate HVAC ckts. Realized today that you can't get there from here. Now I must mount a 200 amp panel in the house and run the feeders through the attic. My first thought was to derate for 140 degree attic temp but this comes at a significant cost increase and after reading a few threads up here I'm not sure anyone does and I'm not sure the inspectors require it. I do have an option of going underground and back into the house but it will have over 360 deg of bends so I would have to put a pullbox in the yard and I am trying to avoid that. What say you?

I would say no. Because if you used the logic for derating in the attic, every piece of NM ran through the attic would have to be derated. I don't see anyone doing that nor have I heard of an inspector requiring it.

 

[infinity]

IMO your feeder through the attic would require ambient temperature correction. The question is what temperature would you use to perform the calculation?

 

[George Stolz]

Any chance of running under the insulation?

 

[Dennis Alwon]

I have never seen anyone derate for attic temp. Can you come out of the top of the combo unit outside and run up to the attic? From there set a jb or sleeve UF in the conduit to the attic.

 

[blueheels2]

That's what I'm going to do Dennis. I talked to the inspector today and he said he would not require derating. My plan is to run 4/0 THWN AL out of the top of the meter and LB into the attic. I'm going to go ahead and pipe it all the way. Reason for this is I can get 180 amps out of the wire and round up to the next standard rating (200). This way I can use the feed through lugs on the meter main. This make sense to you guys. See any errors in my logic?

 

[Dennis Alwon]

That's the way to do it. I have not use se cable in years so I almost always pipe my large feeders- esp. 4/0 alum.

 

[blueheels2]

Schedule 80 or 40 across the attic?

 

[Dennis Alwon]

Schedule 80 or 40 across the attic?

schedule 40-- I can't imagine 80 would be required.

 

[blueheels2]

Thanks Dennis I just don't want to fail inspection.

 

[kwired]

I would say no. Because if you used the logic for derating in the attic, every piece of NM ran through the attic would have to be derated. I don't see anyone doing that nor have I heard of an inspector requiring it.

If your only deration of the conductor is ambient temperature and lets say you have a 12 AWG NM-B cable, and a max ambient temp of 140?F your deration factor is .71. Still gives you more than 20 amps ampacity for the 12 AWG so as long as you have no other deration factors to apply you are OK. 14 AWG and 6 AWG works out also. If you are running 10 or 8 you could have problems but for motors or air conditioning it may work anyway as overcurrent protection is allowed to be higher than conductor ampacity with these loads.

Oh yeah, why do you think NM in an attic does not need to be derated for ambient temperature? If it is 140? in there why is an attic different than any other space that is also 140?? It probably does get overlooked by many inspectors or they realize it is acceptable and never mention anything.

 

[Finite10]

Carolina attics must be HOT! 0.71 seems like a big correction factor until you said you're feeding HVAC. Then it seems you could take advantage of 440 parts 3 and 4. Laying it under the insullation is a good idea.

 

[Volta]

I don't know, I've crawled under some insulation up there and it felt warm to me!

 

[Dennis Alwon]

I
Oh yeah, why do you think NM in an attic does not need to be derated for ambient temperature? If it is 140? in there why is an attic different than any other space that is also 140?? It probably does get overlooked by many inspectors or they realize it is acceptable and never mention anything.

I don't think the issue is whether it needs to or not but rather what temp. do you use. I know there are charts that give temp. for areas of the country but do you use 140 F if only one day out of the year gets that hot. Do you use an average? Here is the chart for different areas but it does not give attic temps. If the house is in the shade it is different then when there are no trees.

 

[Volta]

I don't think the issue is whether it needs to or not but rather what temp. do you use. I know there are charts that give temp. for areas of the country but do you use 140 F if only one day out of the year gets that hot. Do you use an average? Here is the chart for different areas but it does not give attic temps. If the house is in the shade it is different then when there are no trees.

In my opinion, one should use the hottest temp, or at least the hottest 'x' hour average, I'd think x=3 is fair and reasonable. Here in Columbus I think that ambient outdoor temp is expected to be 90 deg F, personally I think it should be considered 10-15 higher, but that's what I received as an answer when I asked.

 

[kwired]

I don't think the issue is whether it needs to or not but rather what temp. do you use. I know there are charts that give temp. for areas of the country but do you use 140 F if only one day out of the year gets that hot. Do you use an average? Here is the chart for different areas but it does not give attic temps. If the house is in the shade it is different then when there are no trees.

I totally agree with you. How to determine what ambient temp should be used may be nearly impossible. A house that is shaded by a tree may(most likely will) eventually have that tree removed and then everything changes. Type of roofing can make some difference also but that can also be changed someday.

I think at the very least a person should not run cables in this space in a manner that will require further deration.

Most NM cables in an attic supplying general lighting circuits probably are seldom loaded to 15 or 20 amps and for the operating temperature of the conductors themselves, it may not be much of a problem, but the conductor ampacity and applied overcurrent protection still may not comply with code.

 

[blueheels2]

I was thinking about derating at 140 deg and that had me using 4/0 THWN CU which was quite expensive. That caused my call to the inspector and he said not needed. I assume their line of thinking is that at 71%derating that still leaves 125 amps on wire rated for 180. They figure not often will a single family dwelling pull this much.

 

[kwired]

I was thinking about derating at 140 deg and that had me using 4/0 THWN CU which was quite expensive. That caused my call to the inspector and he said not needed. I assume their line of thinking is that at 71%derating that still leaves 125 amps on wire rated for 180. They figure not often will a single family dwelling pull this much.

It probably will not pull that much very often but do you think you could talk him into allowing 10 AWG to be run to a 50 amp range receptacle on a 50 amp breaker? It is in many ways a similar situation. Why not put a 125 amp breaker on it if that is what the final ampacity is?

 

[blueheels2]

I see what you are saying but under that line of reasoning shouldn't we do away with 310.15 (B)(6).

 

[George Stolz]

The ambient would be based on the year round average, not the record. I'm not sure how you'd figure that. I'd recommend running on the roof, at least you'll know how much to derate.