Size conductors based on 80% of OCPD?

[jstpie5]

I know this has been discussed ad nauseam, but I want to document this so I don't end up searching this topic for 30 minutes next time it comes up.

Can you legally size conductors based on 80% of a standard breaker?

Example, an industrial location is installing a panelboard with a 400A 80% rated main breaker that currently only has 200A of continuous load.

Do you need to size the feeder conductors to carry 400A (ignore any correction factors for this debate) or can one assume that the breaker should not be loaded to more than 80% of continuous load thus the conductors could be sized to carry 320A?

 

[david luchini]

Do you need to size the feeder conductors to carry 400A (ignore any correction factors for this debate) or can one assume that the breaker should not be loaded to more than 80% of continuous load thus the conductors could be sized to carry 320A?

The conductors would need to be sized for 400A (or protected at the "next higher standard rating.") See 240.4.

 

[d0nut]

The confusion stems from thinking about the sizing backwards. Since the equipment comes in standard sizes, it is tempting to start with that size and try to apply the demand factors, continuous loading, etc. to that equipment. Since you are trying to apply the factors at the wrong part of the calculation, it is easy to get stuck in a loop where you increase the equipment, apply the factors, increase the equipment again, reapply the factors, to infinity.

The way the calculations work is from the load, not the equipment. First you calculate the load, applying demand factors, adjustment for continuous load, etc. Once you have that number, you size the conductors to have at least that ampacity after applying your factors for higher ambient, multiple current carrying conductors, and whatever else applies to your situation. Then you select an overcurrent device to protect the conductors. You can increase the feeders, breakers, panels, etc. to get to a standard size, for future growth, whatever you want, but you do not have to reapply any adjustments after you have done the load calculation.

Once you do the load calculation, you have your minimum number. No additional adjustments need to be made afterwards. They are already all included.