Current Protection for derated conductor amps

[JEFF MILLAR]

Current carrying conductors have been corrected for all applicable NEC ampacity derating factors.
How do you size the overcurrent protection.
Is this baced on the corrected ampacity x 1.25 %.
Rounded up to the next size if there is no standard size .

 

[xptpcrewx]

Current carrying conductors have been corrected for all applicable NEC ampacity derating factors.
How do you size the overcurrent protection.
Is this baced on the corrected ampacity x 1.25 %.
Rounded up to the next size if there is no standard size .

OCPD minimum rating/setting is based on 125% of continuous load + 100% of non-continuous load. The OCPD rating/setting must also be checked to protect the conductor (based on conditions of use).

 

[JEFF MILLAR]

Sorry for the post i understand now. The overcurrent protection device must be sized to protect at the derated amps.
And can be rounded up to the next standard size if there is no standard size overcurrent protection device.

 

[xptpcrewx]

Sorry for the post i understand now. The overcurrent protection device must be sized to protect at the derated amps.
And can be rounded up to the next standard size if there is no standard size overcurrent protection device.

No problem. Assuming the conductors are properly sized for the load, sizing the OCPD based on the load usually takes care of everything.


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[infinity]

Sorry for the post i understand now. The overcurrent protection device must be sized to protect at the derated amps.
And can be rounded up to the next standard size if there is no standard size overcurrent protection device.

That's correct up to 800 amps. After that the next size up rule doesn't apply.

 

[Carultch]

That's correct up to 800 amps. After that the next size up rule doesn't apply.

240.4(B) is the code reference for more details. Essentially, it means that the derated ampacity can round up to the next standard size that you are using, even if just fractions of an ampere above the previous size. It can't be an exact match to the previous size. It also has to at least be sized for the calculated load. So you can use 500 kcmil Cu for a 380A and less load on a 400A OCPD, but if you have a 390A load, you'd need 600 kcmil Cu, or paralleled conductors to get at least 390A of ampacity.

Also, it doesn't apply for taps and analogously transformer secondary conductors, governed by 240.21(B) and (C) respectively. For these kinds of conductors that are not yet protected at their ampacity, you have to have at least as much conductor ampacity as you have OCPD at the end of the tap/xfmr 2nd'ry circuit. This leads to situations where you get to have #1 Cu on the feeder, while needing to have #1/0 Cu on the tap.