How to derate above 80 deg C

CaptKarnage

Member
Location
Milwaukee, WI
Occupation
Electrical Engineer
I work with large industrial furnaces. We use 200 degree C SRML wire for a lot of devices where the wiring needs to get close to the furnace. I'm waiting on a survey to find out what the actual maximum ambient temperature is close to the furnace. If it turns out it's below 80 deg C, then I don't have an issue. But, if we have ambient temperatures above 80 deg C close to the furnaces where some of these cables are run, how do I derate? Is there a formula behind the NEC tables that can be used for higher temperatures? I would really like to do a "worst case" calculation because the survey may not be done in time for the project.

Background: due to construction, we are having to reroute cable and conduit and in order to fit all conductors into the same conduit we would like to decrease the wire gauge. It's only a 5A circuit, so the electrician doesn't seem concerned, but at high temperatures, it can derate by qutie a bit. I'm using the manufacturer's listed ampacity at 30 degrees C, and for the size, number of conductors and 80 deg C, my derating is down from their listed 40 A to 8.2A and I need to figure out how much further I need to derate if my ambient gets above 80 deg C.

Note: this is not internal wiring in the furnace, this is wiring adjacent to the furnace and does come from feeders normally covered under NEC.
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
As david luchini already said you cannot use a "normal" cable insulation that means up to 90oC [or even 105oC] .You have to think to another cable insulation[150 -250 oC rated]-and I think the equipment could withstand such elevated temperature.
 

CaptKarnage

Member
Location
Milwaukee, WI
Occupation
Electrical Engineer
See Table 310.15(B)(2)(b)
Thank you for pointing me in that direction. However, that brings about a few more questions:

1. Aside from you pointing me towards that table, how do I know it's applicable? I found one of your other posts which makes it clear when you're using the NEC tables for ampacity, you use 310.15(B)(2)(a) or 310.15(B)(2)(b) depending on which original NEC ampacity table you're using. However, the specific cable type I'm using is not in the other ampacity tables, therefore I'm not using them. I'm using the manufacturer's listed ampacity. For that matter, I guess I couldn't be sure that 310.15(B)(2)(a) was applicable, either and I was originally using that. 310.15(B)(2)(b) does seem more appropriate since it does have 200 deg C rating on it and the other table does not - I just want to make sure I'm not violating something.

2. The manufacturer's rating IS based on 30 deg C (similar to 310.15(B)(2)(a)) NOT 40 deg C, so that's part of my confusion. In order to use 310.15(B)(2)(b) should I first readjust from 30 deg C to 40 deg C by dividing by the table value for 30 deg C (I would be dividing by 1.03 in this case - assuming I can use the 200 deg C cable rating, see #3) then apply the derating factor in the table for the actual ambient temperature?

3. I also want to mention, when selecting which rating column to use I know that terminal rating is often the limiting factor, rather than cable. I know the terminals are not rated to 200 deg C (pretty sure they are 90 deg C, but need to double check). But, it is the cable itself that is inches from the furnace hull and might experience ambient temperatures above 80 deg C, the terminations are significantly farther away and experience a much lower ambient temperature that I know never exceeds 65 deg C. So would I be able to use the 200 deg C column or would I have to use the 90 deg C column due to the lower rated terminals?
 

paulengr

Senior Member
The formula is right there in the NEC. It looks simple but there are about 5 pages of tedious correction factors to get an answer. It’s called the Neher McGrath method. The table notes point you to N-M. To put it in perspective most polymers start to melt at 80-85 C. Compounding helps a bit. To get up higher you need to go to Teflon. Above that (150-200 C wiring) the wire itself needs a little nickel and the insulation goes to mineral insulations. It is possible to get insulation good to 1500 C but by that point the lugs (75 C) even in lower temperature areas and practically everything becomes a huge engineering challenge. Worked in iron and steel mills. Been there, done that.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Thank you for pointing me in that direction. However, that brings about a few more questions:

1. Aside from you pointing me towards that table, how do I know it's applicable? I found one of your other posts which makes it clear when you're using the NEC tables for ampacity, you use 310.15(B)(2)(a) or 310.15(B)(2)(b) depending on which original NEC ampacity table you're using. However, the specific cable type I'm using is not in the other ampacity tables, therefore I'm not using them. I'm using the manufacturer's listed ampacity. For that matter, I guess I couldn't be sure that 310.15(B)(2)(a) was applicable, either and I was originally using that. 310.15(B)(2)(b) does seem more appropriate since it does have 200 deg C rating on it and the other table does not - I just want to make sure I'm not violating something.
310.10 says that the conductors described in 310.104 shall be permitted for use in any of the wiring methods covered in Chapter 3...
I don't have any experience with SRML wire, but it seems as it would be type SA wire from Table 310.104(a). I would think you would have to make sure that the SRML wire is listed as Type SA.

2. The manufacturer's rating IS based on 30 deg C (similar to 310.15(B)(2)(a)) NOT 40 deg C, so that's part of my confusion. In order to use 310.15(B)(2)(b) should I first readjust from 30 deg C to 40 deg C by dividing by the table value for 30 deg C (I would be dividing by 1.03 in this case - assuming I can use the 200 deg C cable rating, see #3) then apply the derating factor in the table for the actual ambient temperature?
310.15(A)(1) says that ampacity shall be determined by either the Tables in 310.15(B) or under engineering supervision. You should be using the ampacity tables in the Code rather than manufacturer's listed ampacity.

3. I also want to mention, when selecting which rating column to use I know that terminal rating is often the limiting factor, rather than cable. I know the terminals are not rated to 200 deg C (pretty sure they are 90 deg C, but need to double check). But, it is the cable itself that is inches from the furnace hull and might experience ambient temperatures above 80 deg C, the terminations are significantly farther away and experience a much lower ambient temperature that I know never exceeds 65 deg C. So would I be able to use the 200 deg C column or would I have to use the 90 deg C column due to the lower rated terminals?
Per 110.14(C): The temperature rating associates with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination...
 

CaptKarnage

Member
Location
Milwaukee, WI
Occupation
Electrical Engineer
310.10 says that the conductors described in 310.104 shall be permitted for use in any of the wiring methods covered in Chapter 3...
I don't have any experience with SRML wire, but it seems as it would be type SA wire from Table 310.104(a). I would think you would have to make sure that the SRML wire is listed as Type SA.



310.15(A)(1) says that ampacity shall be determined by either the Tables in 310.15(B) or under engineering supervision. You should be using the ampacity tables in the Code rather than manufacturer's listed ampacity.



Per 110.14(C): The temperature rating associates with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination...
Thank you for your help. I got it figured out.

One thing I don't understand, how can a manufacturer claim a higher ampacity than what's in the NEC? This manufacturer has an ampacity of 54A for 14 AWG SRML wire, but NEC type SA is 36A for 14 AWG. Fortunately, even with the lower ampacity and full derating, we can still protect the circuit with a 5A or 6A fuse. But, how can there be such a huge discrepancy?
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Thank you for your help. I got it figured out.

One thing I don't understand, how can a manufacturer claim a higher ampacity than what's in the NEC? This manufacturer has an ampacity of 54A for 14 AWG SRML wire, but NEC type SA is 36A for 14 AWG. Fortunately, even with the lower ampacity and full derating, we can still protect the circuit with a 5A or 6A fuse. But, how can there be such a huge discrepancy?
What conditions of use is the manufacturer listing for the given ampacity?

See Table 310.15(B)(19) in addition to T310.15(B)(18).
 

CaptKarnage

Member
Location
Milwaukee, WI
Occupation
Electrical Engineer
What conditions of use is the manufacturer listing for the given ampacity?

See Table 310.15(B)(19) in addition to T310.15(B)(18).
Thank you for that. I missed a detail in their sheet. I was using Table 310.18 [2020 edition, equivalent to 310.15(B)(18) in previous editions, if I'm not mistaken] for 200 degree C Type SA for conductors in a raceway. Their values are using Table 310.19 - because they listed single conductor in free air. In that case, the ampacity of the manufacturer matches the NEC.

Can I use the values of 310.18 values since they are being put in a conduit? Or do I need the manufacturer to confirm they can even be placed in a conduit first?
 
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