Derating current carrying conductors.

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Jon Montgomery

New member
Location
Davis, Ca
I have a 2"x21" grc nipple connected to a 2" L.B. Table 310.15(B)(2)(a) exception #3 says that derating shall not apply in nipples less then 24". The inspector is telling me that the combination of the nipple and the L.B. have to be less then the 24". Any thoughts?
 

bob

Senior Member
Location
Alabama
Table 310.15(B)(2)(a) exception #3 states that the adjustment factors do not apply to conductors in raceways that do not exceed 24 inches. An LB is a conduit body not a raceway. However, You may want consider Ken's advice.
 

Smart $

Esteemed Member
Location
Ohio
Table 310.15(B)(2)(a) exception #3 states that the adjustment factors do not apply to conductors in raceways that do not exceed 24 inches. An LB is a conduit body not a raceway. However, You may want consider Ken's advice.
While I concur that technically the LB is not a raceway, if the number of current-carrying conductors is the same entering and exiting the LB, I can see where the intent of 2011 NEC's 310.15(B)(3)(a)(2) should not apply. Now if it were a "tee" and CCC's split off after entering, I would challenge the inspector.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I have a 2"x21" grc nipple connected to a 2" L.B. Table 310.15(B)(2)(a) exception #3 says that derating shall not apply in nipples less then 24". The inspector is telling me that the combination of the nipple and the L.B. have to be less then the 24". Any thoughts?
I agree with Smart. If the conduit total run is more than 2' then I would derate. Not an argument you can easily win.

Let's take it further and have LB's back to back with 1' nipple in between each LB. Now your run is 3' feet but no nipple is more than 1'. I think the idea here is to derate
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I agree with Smart. If the conduit total run is more than 2' then I would derate. Not an argument you can easily win.

Let's take it further and have LB's back to back with 1' nipple in between each LB. Now your run is 3' feet but no nipple is more than 1'. I think the idea here is to derate

If the LB's in your example were boxes would you be required to apply derating factors?
 
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Smart $

Esteemed Member
Location
Ohio
What is the total end to end length of the conductors in question?
I'm going to guess that you're thinking about applying 310.15(A)(2) Exception...

...but it wouldn't be end-to-end length of the conductors. It would be the length of the higher adacent ampacity, and that length would have to be 20' or greater.
 

kwired

Electron manager
Location
NE Nebraska
I agree with Smart. If the conduit total run is more than 2' then I would derate. Not an argument you can easily win.

Let's take it further and have LB's back to back with 1' nipple in between each LB. Now your run is 3' feet but no nipple is more than 1'. I think the idea here is to derate

What about installing a junction box every two feet? This allows you to forget about derating, and you can also use 60% fill for the raceway:)
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
What about installing a junction box every two feet? This allows you to forget about derating, and you can also use 60% fill for the raceway:)

Apparently my last post did not post. I specifically stated if you had 1' nipples and a box and continue for 100' would you not derate the conductors?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Apparently my last post did not post. I specifically stated if you had 1' nipples and a box and continue for 100' would you not derate the conductors?

The way the code section is written I don't see how you would be required to derate in your example.
 

kwired

Electron manager
Location
NE Nebraska
Apparently my last post did not post. I specifically stated if you had 1' nipples and a box and continue for 100' would you not derate the conductors?
Sorry, I kind of took your idea and ran with it.

Would a person need to derate if there was a junction box every 24" or less? Maybe not. You have a place to possibly get rid of some heat every 24" inches or less that you would not have with raceway alone.
 

Smart $

Esteemed Member
Location
Ohio
...

Would a person need to derate if there was a junction box every 24" or less? Maybe not. You have a place to possibly get rid of some heat every 24" inches or less that you would not have with raceway alone.
Some, yes. Enough that derating would not be necessary, who knows?

If you put a condulet every 24" without branching off, would you dissipate more heat? ...or less?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Some, yes. Enough that derating would not be necessary, who knows?

If you put a condulet every 24" without branching off, would you dissipate more heat? ...or less?

I think that the argument here is that the NEC wording does not support the practical applicaiton. In your example the conduit bodies would do little to disapate heat.
 
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