De-rating and sizing of breaker

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serg75

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Salinas, CA, Monterey
Here is the question... I have an existing 1-1/2 (over 24") conduit with with 35 #10 THHN, all CCC's supplying 20amp loads, will this pass inspection?

#10 THHN good for 40amps
31-40 adjusted to 40% = 16amps
next breaker size 20 amps

I have seen this pass in the past and but I'm getting different answers when asking around. Would greatly appreciate you're feed back.
 
If the wire is rated 16 amps then it cannot supply loads more than 16 amps. You stated the loads are 20 amps.. are they continuous?
 
What kind of loads? If there are multi-receptacles involved you cannot use the next size up rule.

240.4(B) Overcurrent Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above
the ampacity of the conductors being protected) shall be
permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a branch
circuit supplying more than one receptacle for cord-
and-plug-connected portable loads.
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The code reference is in post #3. Since you have an adjusted ampacity of 16 amps you cannot go up to the next size OCPD which is 20 amps so you're stuck with a 15 amp OCPD. If you have 20 amp receptacles they would need to be replaced with 15 amp receptacles on the 15 amp circuits. Not sure where you're getting the 80% trip value.
 
This circuit was originally designed to be protected by a 20 amp CB. This is a TI job and there are existing conduits I would like to use, but like all older builds, everything is jammed packed.
 
infinity said:
The code reference is in post #3. Since you have an adjusted ampacity of 16 amps you cannot go up to the next size OCPD which is 20 amps so you're stuck with a 15 amp OCPD. If you have 20 amp receptacles they would need to be replaced with 15 amp receptacles on the 15 amp circuits. Not sure where you're getting the 80% trip value.
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Thank You,
 
If you're using #10 THHN just limit the number of CCC's in any raceway to 20 and you'll still be good with a 20 amp OCPD.
 
serg75 said:
This is what I'm having a hard time wrapping my head around. If it is known that the breaker will trip at 80% of it value, I would think that a 20 amp circuit would be correct. This is good stuff, thanks for the feed back.
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Welcome to The Forum. I think you are confusing the maximum continuous load that you can put on a 20 amp circuit, which is 16 amps, with circuit breaker trip values... The two are completely unrelated. I'm guessing if the wiring is already run, that will be much easier to change out the 20 amp Breakers to 15 than to repull all the wiring, assuming that you have run the wiring and have the latitude to give the customer 15-amp circuits versus 20
 
JFletcher said:
Welcome to The Forum. I think you are confusing the maximum continuous load that you can put on a 20 amp circuit, which is 16 amps, with circuit breaker trip values... The two are completely unrelated. I'm guessing if the wiring is already run, that will be much easier to change out the 20 amp Breakers to 15 than to repull all the wiring, assuming that you have run the wiring and have the latitude to give the customer 15-amp circuits versus 20
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Yes that is correct, wiring is already pulled. Trying to use existing raceways if possible.
 
serg75 said:
Yes that is correct, wiring is already pulled. Trying to use existing raceways if possible.
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You have a few options. If there is no possibility of adding extra raceways, consolidating some circuits may be an option, multiwire Branch circuits are another. If you have enough room in the raceways, using number 8 wire maybe an option. If the receptacles are already 15 amp, the easiest and cheapest thing would be to change the breakers... however it seldom works out that you can do this. If you change any lighting circuits out to 15 amp Breakers, they will only be able to carry a 12 amp load... That is the 80% continuous load I previously mentioned.

Depending on the build specs and language of your contract, you have anywhere from a minor mess to a pretty big mess to fix. Good luck with it
 
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