Derating,

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You start derating #10 from the ampacity given in T310.16. So if you're using THHN, you derate starting at 40 amps. But yes, if the ampacity is lowered, the breaker size must go down.
 
brother said:
when you derate conductors because of number in conduit, do you have to lower the OCP?? for example a #10 wire can be on a 30 amp breaker normally, but at 70% derating its at 28 amps , so do you have to lower the breaker to a 20 amp?? or use a lower amp fuse like a 25 amp??
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In your example, if the #10 is derated to 28amps you can use the next higher standard OCPD which would be 30amps., 240.4(B) if all of the conditions are met.
 
Michael15956 said:
In your example, if the #10 is derated to 28amps you can use the next higher standard OCPD which would be 30amps., 240.4(B) if all of the conditions are met.
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Thanks for the info!! looks like it should be OK to have the #10 on a 30 amp breaker. this is for the heating and ventilation unit, not portable loads or recepticles.We had put 8 conductors in the conduit, so the derating put us at 28 amps. But i still saw the 30 amp breaker and was just concerned.

Ive notice on alot of jobs alot electricians do NOT even do any derating, and alot of inspectors dont seem to catch it or dont care. Im just curious , as do others do any derating on their jobs or they just let it go?? You can be 'unknown' if you wish. :)
 
We always derate. But I have seen many jobs done by others that have 1" or 1.25" homeruns stuffed with conductors. To my knowledge none of those buildings have burned down yet. In the real world my guess is that load diversity can safeguard over stuffed raceways when derating hasn't been applied.
 
infinity said:
In the real world my guess is that load diversity can safeguard over stuffed raceways when derating hasn't been applied.
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Come on Trevor, we have all seen raceways that where hot enough to warm our lunches.

IMO derating is up to the discretion of the EE or installer. I could argue loads on my conductors all day with AHJ.

70%, 60% what ever it takes.
 
chris kennedy said:
Come on Trevor, we have all seen raceways that where hot enough to warm our lunches.

IMO derating is up to the discretion of the EE or installer. I could argue loads on my conductors all day with AHJ.

70%, 60% what ever it takes.
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And even those raceways haven't burned down the building. My point was that many electricians simply do not derate and for some reason flames aren't coming out of the windows. I'm not condoning disregarding derating, but as Brother stated there are many others that do disregard the derating requirements of the NEC.
 
infinity said:
In the real world my guess is that load diversity can safeguard over stuffed raceways when derating hasn't been applied.
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Tell me about load diversity.

ie, I have a restaurant kitchen going on. The "chef's table" (more like an island) has:
18 - 60w lamps (broken in 3x6 lamp arrangements)
3 UC refrigerators 20A@240v ea.
3 OH heaters 20A@240v ea.
2 - convience outlets 20A@120v(same circuit)
1 - Plate warmer 20A@240V
1 - "Hot Well" 15@240v
1 - "Portable Oven" 15A@120v
1 - double broiler 30A@240v
(These are NOT the actual ratings of the equipment...but rather what the panel schedule calls for)

How could load diversity help us out here?
 
celtic said:
Tell me about load diversity.

ie, I have a restaurant kitchen going on. The "chef's table" (more like an island) has:
18 - 60w lamps (broken in 3x6 lamp arrangements)
3 UC refrigerators 20A@240v ea.
3 OH heaters 20A@240v ea.
2 - convience outlets 20A@120v(same circuit)
1 - Plate warmer 20A@240V
1 - "Hot Well" 15@240v
1 - "Portable Oven" 15A@120v
1 - double broiler 30A@240v
(These are NOT the actual ratings of the equipment...but rather what the panel schedule calls for)

How could load diversity help us out here?
Click to expand...


I was just using load diversity as a plausible explanation at to why overstuffed raceways with out derated conductors are not starting fires on a regular basis. For example the NYC electrical code at one time had a maximum derating value of 70% for any raceway fill. In old buildings there are thousands of conduits stuffed with conductors and they're still there functioning on a daily basis. My point was that if you put 20 #12's CCC's in a 1" conduit and don't derate it's likely that nothing catastrophic will happen.
 
infinity said:
My point was that if you put 20 #12's CCC's in a 1" conduit and don't derate it's likely that nothing catastrophic will happen.
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You should have told me that last week ....I put 12 - 14 #10's in two 1" conduits...and that 30A unit was a PITA to figure in w/o having to goto #8's.

:D


I derate like a madman.
 
Years ago I had a service call in a copy center. After three years in thier new location, they started tripping breakers and couldn't figure out why. In the 'back copy room' were 15 copy machines.... the big ones they use when a customer comes in and says "I need 100,000 copies by tomorrow!". So they crank up all 15 machines and run them all night.

Someone had originally installed a 2" pipe from the panel to a 20x20 pull box in the ceiling above the copy room and wagon-wheeled 1/2" and 3/4 conduits from there. All 15 copy machines were run in that 2" run with AWG12s, as well as the lighting, GP circuits and other copy machines. 90% of the building load was in that 2".

Turns out, there were 15 full boats (45 circuits!) in that pipe, and the insulation was melting off some of them. I abandonded the original 2" and piped several smaller conduits in.

'Twas a nightmare doing it, and I wish I had a camera back then... you guys would have either laughed your heads off or chucked your supper.
 
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