Next Higher Up rule with Continuous Loads

[hitehm]

Got a test question that I'm not agreeing with but now I'm questioning it. Here it is:

"What is the MAX standard size OCPD that may be used to protect a 4/0 THWN conductor that is not serving a motor load?"

Seems simple enough. The ampacity of 4/0 THWN is 230A. There are many loads that are continuous that are not motors so I add the 125% for a possible continuous load which brings it to 287.5A. Nothing stated says it doesn't meet the requirements of 240.4b so next size up is 300A. The answer they have is 250A which implies they used the next size up rule but did not apply the continuous. So my question is, Does the 125% continuous load increase negate the next size up rule?

 

[jaggedben]

There's no reason you have to consider the continuous factor at all here, as it's presumed the conductor size has already accounted for that, if needed.

The reason that the motor load is mentioned is that there are rules that allow higher OCPDs for motors. That also has nothing to do with continuous or not.

Generally the process goes like this:
1) Establish the circuit current. Nameplate for non-continuous loads, or 125% or nameplate for continous loads (if not meeting an exception).
2) Choose a conductor that meets or exceeds the circuit current requirement.
3) Choose an OCPD that protects the conductor.

The question starts with steps (1) and (2) already done.

 

[jaggedben]

That said, they should have told you the material (copper or aluminum) of the 4/0.

 

[don_resqcapt19]

Got a test question that I'm not agreeing with but now I'm questioning it. Here it is:

"What is the MAX standard size OCPD that may be used to protect a 4/0 THWN conductor that is not serving a motor load?"

Seems simple enough. The ampacity of 4/0 THWN is 230A. There are many loads that are continuous that are not motors so I add the 125% for a possible continuous load which brings it to 287.5A. Nothing stated says it doesn't meet the requirements of 240.4b so next size up is 300A. The answer they have is 250A which implies they used the next size up rule but did not apply the continuous. So my question is, Does the 125% continuous load increase negate the next size up rule?

You could never use a conductor with an ampacity of 230 amps for that load. That load would require a conductor with an ampacity of 287.5 amps.

Assuming the load on the conductor is 230 amps or less, the answer key is correct.

Remember that 240.4(B) does not change the ampacity of the conductor, it just permits you to use an oversized overcurrent protective device.

 

[hitehm]

Ah yes, ALL correct, thanks everyone. Not sure what the heck I was thinking but I was definitely thinking of it backwards. Of course, a 4/0 conductor has a max of 230A. That right there should've told me that the OCPD can only go to the next size up. Not sure why I thought of the continuous loads factoring in. Total senior brain fart. Thanks again guys!