Raceway in Two Different Ambient Temperatures

[JuliaTruchsess]

I have a raceway partially in 35C and partially in 60C; say, six feet of 35C and two feet of 60C, or 4 feet of 35C and a foot of 60C. Are there any regulatory provisions for this scenario quantifying how much of the run needs to be in 35C before I can derate my conductors per 35C instead of 60C? Or do I have to measure or simulate or calculate an "average ambient" temperature?

 

[drktmplr12]

I have a raceway partially in 35C and partially in 60C; say, six feet of 35C and two feet of 60C, or 4 feet of 35C and a foot of 60C. Are there any regulatory provisions for this scenario quantifying how much of the run needs to be in 35C before I can derate my conductors per 35C instead of 60C? Or do I have to measure or simulate or calculate an "average ambient" temperature?

if one section of conductor is subjected to 60C ambient, i would derate the entire thing.

 

[ramsy]

I have a raceway partially in 35C and partially in 60C..

In new construction AHJ's, and their planning department might help solve this question.

For such short lengths, placing smaller raceway inside larger insulating pipe may also deflect some heat away from short lengths under 10ft. Temperature probe readings would confirm heat shielding efforts.

 

[infinity]

I have a raceway partially in 35C and partially in 60C; say, six feet of 35C and two feet of 60C, or 4 feet of 35C and a foot of 60C. Are there any regulatory provisions for this scenario quantifying how much of the run needs to be in 35C before I can derate my conductors per 35C instead of 60C? Or do I have to measure or simulate or calculate an "average ambient" temperature?

Although they might not apply to short runs there are provisions in the NEC to disregard certain higher ambient temperature sections of a run.

310.15(A)(2) Selection of Ampacity. Where more than one ampacity applies for a given circuit length, the lowest value shall be used.
Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be
permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit
length figured at the higher ampacity, whichever is less.

 

[JuliaTruchsess]

Thanks for the helpful responses! I like the idea of insulation/heat deflection and measurements to verify effectiveness, if permitted by authorities.

Julia

 

[ggunn]

Thanks for the helpful responses! I like the idea of insulation/heat deflection and measurements to verify effectiveness, if permitted by authorities.

Julia

Isn't there a ten foot, ten percent, whichever is less rule?

 

[JuliaTruchsess]

Isn't there a ten foot, ten percent, whichever is less rule?

If there is, I'd love to know more about it

 

[packersparky]

If there is, I'd love to know more about it

See post #4.

 

[JuliaTruchsess]

See post #4.

Yes, sorry, I did see that, I just had trouble parsing it.

"the higher ampacity shall be permitted to be used beyond the point of transition"

seems to be saying that I'm allowed to change to smaller conductors at the point of transition? Like add a junction box and splices? That can't be right, can it?

 

[JuliaTruchsess]

OK, this post

https://forums.mikeholt.com/showthread.php?t=61254&p=458382#post458382

seems to clarify it nicely. If my 60C section is less than 10ft and it's also less than 10% of the total circuit length, I can treat the whole run as if it's in 35C. Is that right?

 

[retirede]

OK, this post

https://forums.mikeholt.com/showthread.php?t=61254&p=458382#post458382

seems to clarify it nicely. If my 60C section is less than 10ft and it's also less than 10% of the total circuit length, I can treat the whole run as if it's in 35C. Is that right?

They way I read it, it’s either 10 FT or 10%, not “and”.

 

[JuliaTruchsess]

They way I read it, it’s either 10 FT or 10%, not “and”.

You're allowed to go 10ft, or you're allowed to go 10%, whichever is less. So if 10% is less than 10ft, you're not allowed to go 10ft, and if 10ft is less than 10%, you're not allowed to go 10%. Therefore whatever distance you want to run at the higher ampacity has to be both less than 10ft and less than 10% of the total circuit. Or else I need more coffee...

 

[retirede]

You're allowed to go 10ft, or you're allowed to go 10%, whichever is less. So if 10% is less than 10ft, you're not allowed to go 10ft, and if 10ft is less than 10%, you're not allowed to go 10%. Therefore whatever distance you want to run at the higher ampacity has to be both less than 10ft and less than 10% of the total circuit. Or else I need more coffee...

You’re correct.

It would have been clearer if the wording was simply “AND” & leave out “whichever is less”.

 

[JuliaTruchsess]

You’re correct.

It would have been clearer if the wording was simply “AND” & leave out “whichever is less”.

I know, it's the most awkwardly-worded thing ever :blink:

 

[electrofelon]

I know, it's the most awkwardly-worded thing ever :blink:

The only poorly worded thing in the NEC :roll: