heat dissipation in a gutter.

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Mes,AZ
could someone help me find where it talks about derating and heat dissepation in a gutter. heres what i have; 8- 200amp 3phase 120/208 panels which are existing, its a TI. had to re-route all the homeruns to different panels, we cut all the pipes and put in a gutter that covers all 8 panels. this way we were able to re-route the homeruns without have to re-pipe. we re-pulled new wire and some have to cross over inside the gutter to a different panel. others we piped up and over that had to cross (horizontal) more than 2feet inside the gutter. we seperated all the conductors put them in pipes alone and put nuetrals and grounds going into the panel. My general foreman said something about not being able to this because of heat dissapation, he said grounds, nuetrals and conductors have to go into the same pipe. How is this going to change heat dissapation and where can i find it in the code book?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
300.3(B)

If the conduits are ferrous and you put the hots and neutrals in differnt conduits, there will be inductive heating of the conduits. If there is a substantial amount of current the circuit will fail in short order.
 
Location
Mes,AZ
I was also wondering when is derating necessary? Is it needed indoors for modular furniture in an office building? 28 wires in an 1 1/4" pipe #10 for a 20amp circuit less than 20' long, does this need derating?
 

kwired

Electron manager
Location
NE Nebraska
It is not heat dissipation that is an issue it is the inductive heating effects that Don mentioned that are an issue if all conductors of a circuit are not run through same metal raceway or enclosure. You can not derate for this either, you are supposed to avoid separating conductors of same circuit through metal raceways or enclosures.

General rule is you must derate anytime there are more than 3 current carrying conductors in a raceway, cable, or other situations where they are in close proximity. Lengths of 24 inches or less is generally an exception, but not always - more recent changes do not allow the exception to apply where holes for NM cables are sealed inhibiting movement of heat.

Another place where deration generally doesn't apply: If number of current carrying conductors in an aux gutter is less than 30 the deration factors of 310.15(B)(3)(a) do not apply. see 366.22.
 
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