Quick De-Rating Question

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barclayd

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Colorado
24 #10 current carrying conductors (THHN) in a conduit must be de-rated by 45% = 40Ax.45= 18Amps.
Can a 20a breaker be used? Or does the ampacity have to be equal to the breaker?
On the road without a code book.
thanks
db
 
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240.4 says next size up allowed if 800 amp or less.

310.15 has nothing that changes this rule when deration applies.
 
There are two schools of thought voiced on the Forum concerning that. I am on the position that the 20 amp breaker is allowable if the load is not greater than the 18 amp conductor ampacity.
 
One thing that was left out of the discussion is the type of load. Multi-receptacle circuits cannot use the next size up rule so your stuck with 15 amps if you're using receptacles.
 
Good point, Rob.
That was rattling around in my ole brain, but I couldn't hone in.
 
barclayd said:
These are individual, dedicated circuits.
Where does it say the thing about multiple receptacles?
thanks
db
Click to expand...

240.4(B)(1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord and- plug-connected portable loads.

I learned this from this forum about 2 weeks ago and now i get to share my knowledge. :)
 
Another consideration which is seldom brought up is utilizing 310.15(A)(2) Exception, if applicable.
 
barclayd said:
Which is??
(I'm codebookless)
db
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Exception: Where two different ampacities apply to adjacent
portions of a circuit, the higher ampacity shall be
permitted to be used beyond the point of transition, a distance
equal to 3.0 m (10 ft) or 10 percent of the circuit
length figured at the higher ampacity, whichever is less.
 
augie47 said:
There are two schools of thought voiced on the Forum concerning that. I am on the position that the 20 amp breaker is allowable if the load is not greater than the 18 amp conductor ampacity.
Click to expand...

Derating is like taking the installation conditions and adjusting the values in 310.16 to meet those conditions.

If your conductor has an ampacity of 18 you can not load it to 20 whether it was derated or not.

Overcurrent protection is a separate issue. I do see what you are getting at, just wanted to remind people that ampacity and overcurrent protection are two different things.
 
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