Refresh My Memory

Status
Not open for further replies.
DBoone

DBoone

Senior Member
Location
Mississippi
Occupation
General Contractor
Is using NM the only time we are limited from the very beginning to a max ampacity? (60 degree column)

Example, if pulling #12 THHN through pipe and we have 75 degree terminals, we could use a 25A breaker as long as derating did not take us below 25A.
 
DBoone said:
Is using NM the only time we are limited from the very beginning to a max ampacity? (60 degree column)

Example, if pulling #12 THHN through pipe and we have 75 degree terminals, we could use a 25A breaker as long as derating did not take us below 25A.
Click to expand...

Even with THHN in a raceway you still need to comply with 240.4(D). So the answer is, maybe. :)
 
DBoone said:
Is using NM the only time we are limited from the very beginning to a max ampacity? (60 degree column)

Example, if pulling #12 THHN through pipe and we have 75 degree terminals, we could use a 25A breaker as long as derating did not take us below 25A.
Click to expand...

Sort of. You have to remember the small conductor rule, 240.4(D). For example, you could use #12 with a 25 amp breaker with loads as in (E) or (G) but not say, to a receptacle.
 
Thanks guys.. On a side note, why are we limited to 60 degrees for NM?

Does it give us a cushion since NM is in insulation most of the time?
 
I believe the main reason is that we really don't know what kind of insulation is used for the conductors on an NM cable assembly. It sure resembles THHN/THWN but it isn't marked.
 
A/A Fuel GTX said:
I believe the main reason is that we really don't know what kind of insulation is used for the conductors on an NM cable assembly. It sure resembles THHN/THWN but it isn't marked.
Click to expand...

I thought the reason we can derate using the 90 degree column was because the insulation is rated 90 degrees.
 
I think there is confusion here. Yes nm has 90C conductors and you can use the 90C to start but must use no more than 60C for the final ampacity. So 90 C then derate- final ampacity cannot be more than 60C.

I also believe the reason it is limited to 60C is because of the insulation that it will generally see.
 
PHP:
texie said:
Ah, but that is only true for MC and single conductors. You don't get that luxury with NM, it is rated 60 degree, period.
Click to expand...

One of the world's great mysteries.... What type of insulation is used in NM... Will we ever know! :happysad:
 
Dennis Alwon said:
I think there is confusion here. Yes nm has 90C conductors and you can use the 90C to start but must use no more than 60C for the final ampacity. So 90 C then derate- final ampacity cannot be more than 60C.
Click to expand...

I agree and good luck trying to find a legitimate reason for the 60? C limit. :roll:
 
This is only my guess, but I think they want NM cables to operate at a lower overall temperature than other cables or raceways. Metallic sheathed cables have a metal sheath to sink some of the heat inside, same for metal raceways, non metallic raceways have some air space within the raceway for heat movement, but a fairly tightly wrapped NM cable is going to have similar temp outside the sheath as inside the sheath compared to other wiring methods. There is probably some exceptions but this is just my opinion as being the general reason.
 
Status
Not open for further replies.
Top