NEC OCPD on a transformer question

[Sparky Bill PE]

Solution said "it isn't allowed to go up in size when picking a OCPD". I can't find in the code where that isn't allowed. I would naturally think you would want to upsize your OCPD so you know you don't undersize it. I still don't see how if you have a max current of 313A how you can use a 300 OCPD and it not trip automatically. 

 

[DilutedAr18_PE]

Solution said "it isn't allowed to go up in size when picking a OCPD". I can't find in the code where that isn't allowed. I would naturally think you would want to upsize your OCPD so you know you don't undersize it. I still don't see how if you have a max current of 313A how you can use a 300 OCPD and it not trip automatically. 

View attachment 19187

View attachment 19188

It is because the 300% for fuses or the 250% is the maximum overcurrent protective device. You can size the fuse or circuit breaker as small as you want, but it might trip depending upon your loads and inrush currents.

 

[Sparky Bill PE]

It is because the 300% for fuses or the 250% is the maximum overcurrent protective device. You can size the fuse or circuit breaker as small as you want, but it might trip depending upon your loads and inrush currents.

I'm still just as lost as I was when I posted the question. I don't see how you calculate your ampacity as 313, and then "downsize" the OCPD to 300 amps. I'm sure it's some little rule or exception somewhere. 

I read somewhere that the cables "now" (not before) are designed with a 80% load factor incorporated so they are always "less ampacity than the table says", but again, couldn't find anything in codebook. 

 

[speakeelsy PE]

I'm still just as lost as I was when I posted the question. I don't see how you calculate your ampacity as 313, and then "downsize" the OCPD to 300 amps. I'm sure it's some little rule or exception somewhere. 

The ampacity of your load is 100%. Your ocpd needs to be bigger than that. The chart gives you the maximum size. So the 300% rating is the largest it can be. If that 300% rating is an odd size like 313A, you can go smaller but not bigger. So youll pick the 300A ocpd cause that is the largest standard size less than 313A.

In certain cases you are allowed to upsize your ocpd to the next standard size. So again if your ocpd 300% rating is 313A, you could upsize to a 350A ocpd.

Code has lots of notes and exceptions. Check out Zach's code section quizzes, im pretty sure a few situations like this are in there.

 

[Sparky Bill PE]

The ampacity of your load is 100%. Your ocpd needs to be bigger than that. The chart gives you the maximum size. So the 300% rating is the largest it can be. If that 300% rating is an odd size like 313A, you can go smaller but not bigger. So youll pick the 300A ocpd cause that is the largest standard size less than 313A.

In certain cases you are allowed to upsize your ocpd to the next standard size. So again if your ocpd 300% rating is 313A, you could upsize to a 350A ocpd.

Code has lots of notes and exceptions. Check out Zach's code section quizzes, im pretty sure a few situations like this are in there.

Well dang said and explained perfectly for my understanding. Makes perfect sense. Yes I've worked all of Zach's quizzes, twice. My goal is to redo the circuits/nec/protection modules probably next week since we are now closer to the test. That is a great idea and man do I love his website!