Breaker sizing after derating

C

clemver

Member
Location
PA
Occupation
Electrician
Once you derate a conductor, are you then able to round up to the next available breaker size?

ex: Have 30A breakers that I need to derate for conduit fill, by 50%. #8 at 90degree is 55A. Calc gives you 27.5A. Is that still good for a 30A breaker with 90degree wire?
 
Yes, so long as it meets the requirements of 240.4(B) and there isn't another section that prohibits rounding up like feeder taps (240.21). What you describe is "above the ampacity of the conductors" since your ampacity, for that conductor, after derating, is 27.5A.

From the NEC 2020 -
"
240.4 (B) -
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.

(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed 800 amperes.
"
 
jim dungar said:
Why not size your conductors based on your load?
Click to expand...
Not sure I follow. Need to relocate a panel, installing a junction trough at existing panel, and FMC to panel. Ill have 16ccc's in each. The way I laid it out I need to derate the wiring in those conduits by 50%. So for my 30A breakers I'm trying to size #8
 
also keep 210.19 in mind.....
(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served
ie: if your load is 30 amps you conductor ampacityshould be 30 amp minimum.
 
clemver said:
Not sure I follow. Need to relocate a panel, installing a junction trough at existing panel, and FMC to panel. Ill have 16ccc's in each. The way I laid it out I need to derate the wiring in those conduits by 50%. So for my 30A breakers I'm trying to size #8
Click to expand...
How long is the FMC ??
 
Elect117 said:
Yes, so long as it meets the requirements of 240.4(B) and there isn't another section that prohibits rounding up like feeder taps (240.21). What you describe is "above the ampacity of the conductors" since your ampacity, for that conductor, after derating, is 27.5A.

From the NEC 2020 -
"
240.4 (B) -
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.

(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed 800 amperes.
"
Click to expand...

augie47 said:
How long is the FMC ??
Click to expand...
Sadly over 24" :mad:
 
clemver said:
Not sure I follow. Need to relocate a panel, installing a junction trough at existing panel, and FMC to panel. Ill have 16ccc's in each. The way I laid it out I need to derate the wiring in those conduits by 50%. So for my 30A breakers I'm trying to size #8
Click to expand...
Okay, this makes sense.
Just be sure 27.5A is enough for your load, as mentioned by augie47 in post #5.
 
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