Branch circuit derating...

[bcorbin]

Here's the scenario...

I've got six dedicated 20-amp branch circuits, each feeding a receptacle.

Due to space issues, they are all being routed down a wall through a single 1-1/4" EMT conduit.

Please tell me if I'm looking at this correctly.

12 CCC's: derate to 70%.

25A x 0.7 = 17.5A.

Can I use the "next standard size higher" rule to keep the circuit breakers feeding these circuits at 20 amps?

The client insists on 20-amp circuit vs. 15-amp, so it's either this or provide #10's.

 

[jumper]

What wire are you using? Why are you not using the 90 degree column for derating?

 

[raider1]

Here's the scenario...

I've got six dedicated 20-amp branch circuits, each feeding a receptacle.

Due to space issues, they are all being routed down a wall through a single 1-1/4" EMT conduit.

Please tell me if I'm looking at this correctly.

12 CCC's: derate to 70%.

25A x 0.7 = 17.5A.

Can I use the "next standard size higher" rule to keep the circuit breakers feeding these circuits at 20 amps?

The client insists on 20-amp circuit vs. 15-amp, so it's either this or provide #10's.

Are you using single receptacles or duplex receptacles?

Keep in mind that 240.4(B) can't be used for conductors that are part of a branch circuit supplying more than 1 receptacle for cord and plug connected portable loads. (See 240.4(B)(1))

Chris

 

[raider1]

Also Table 310.15(B)(2)(a) for 12 CCC's you must derate to 50% not 70%.

So even if you use the 90 degree column of 310.16 the ampacity of #12 is 30 amps with the 50% ampacity adjustment you get 15 amps and since that is a standard OCPD rating you can't use the next size up rule.

Chris

 

[bcorbin]

#12's.

The receptacles are rated only 60C. Even if they were rated 75C (though I haven't seen any), the #12 would only be 25 amp.

My main question is if I can use the "next size higher" rule here. I use it on feeders all the time, but I rarely have to specify specifics for branch circuits.

Oops, several people posted while I was posting...give me a minute to digest it all...

 

[bcorbin]

Gah...thanks, Raider.

I was operating under the illusion that there was load diversity and was using Table B.310.11 in Annex B.

I think I had better prepare for worst case and just use 310.15(B)(2)(a).

By the way, they are duplex receptacles.

I think rather than have them switch to #10's, I will just have them try to squeeze two 3/4" conduits in the place of the single 1-1/4" and split it 6 CCC's each. That should be slightly cheaper than changing six circuits to #10's.

Thanks, again...always a learning experience.

 

[Dennis Alwon]

Even if the receptacles are rated 60C the wire is rated 90C, in most cases, so you can derate from the 90C as long as the final result is not more than the 60C rating.

 

[david luchini]

Are you using single receptacles or duplex receptacles?

Keep in mind that 240.4(B) can't be used for conductors that are part of a branch circuit supplying more than 1 receptacle for cord and plug connected portable loads. (See 240.4(B)(1))

Chris

Chris, this is a great point as 240.4(B)(1) is probably often overlooked, but I would interpret a single duplex receptacle as being OK under 240.4(B)(1). It doesn't say more than one receptacle, it says more than one outlet. A single duplex receptacle is only one outlet.

 

[bcorbin]

Even if the receptacles are rated 60C the wire is rated 90C, in most cases, so you can derate from the 90C as long as the final result is not more than the 60C rating.

I always thought the temperature rating of the wire had to take into account what was connected to both ends. I really need more study time these days....

 

[Dennis Alwon]

I always thought the temperature rating of the wire had to take into account what was connected to both ends. I really need more study time these days....

The wire is 90C so you start there but the end ampacity must not be more then the weakest link. In this case the rating of the recep. at 60C. It should not be affected by #12 or #14 wire anyway.

 

[Dennis Alwon]

This may help-- ref. art. 110.14 (C)

 

[jumper]

Here another graphic, but you can use the 90 degree column for derating if the location and wire type allows.

 

[raider1]

Chris, this is a great point as 240.4(B)(1) is probably often overlooked, but I would interpret a single duplex receptacle as being OK under 240.4(B)(1). It doesn't say more than one receptacle, it says more than one outlet. A single duplex receptacle is only one outlet.

Actually in the 2011 NEC 240.4(B)(1) does in fact say "The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord and plug connected portable loads." I added to bold to show the new language in that section. So by a strict reading a duplex receptacle is 2 receptacles and would violate this section.

Chris

 

[david luchini]

Actually in the 2011 NEC 240.4(B)(1) does in fact say "The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord and plug connected portable loads." I added to bold to show the new language in that section. So by a strict reading a duplex receptacle is 2 receptacles and would violate this section.

Chris

Thanks for the updated language. CT is still under 2005, and 2008 still says "outlet."

 

[Smart $]

I always thought the temperature rating of the wire had to take into account what was connected to both ends. I really need more study time these days....

It does.


Getting back to the situation at hand, is the "stuffed" portion of the conductor runs less than 10 ft, or less than 10% of the run length if less than 100'...??? If so, you can apply 310.15(A)(2) Exception, and the installtion will be compliant.