NEC/ Overcurrent

Omer · Oct 18, 2017

is there on the NEC (the part we are usually tested on) a requirement to go for the (next lower level) on the overcurrent protection (overload, ground fault and short circuit) for conductors and motors? (branch circuit, feeders).

All I have seen is (next upper level)

FPar · Oct 18, 2017

I keep seeing it too ...

user 34010 · Oct 18, 2017

I don't know about NEC, but in my industry when we size overcurrent protection we always size it equal to or less than the Amps.

Stephen2awesome · Oct 19, 2017

The overload is can not go above a certain amperage. So, using NEC 430.32, at 68A with the whatever percentage used for service factor/temp/all other motors, it will be 60A breaker. "This device shall be selected to trip or shall be rated AT NO MORE THAN the following percent of the motor nameplate full-load current rating." NEC 240 has the standard sizes and 65 is not one.

Omer · Oct 19, 2017

Stephen2awesome said:
The overload is can not go above a certain amperage. So, using NEC 430.32, at 68A with the whatever percentage used for service factor/temp/all other motors, it will be 60A breaker. "This device shall be selected to trip or shall be rated AT NO MORE THAN the following percent of the motor nameplate full-load current rating." NEC 240 has the standard sizes and 65 is not one.

@Stephen2awesome

Thanks..

This is an interesting one.

So for the motor overload I should go for the next lower level. But for the conductor I am allowed to go for the next upper level, right?

is it applicable also for the short circuit/ground fault?

Stephen2awesome · Oct 19, 2017

Sizing conductors for motors is 430.22 for a single motor. continuous duty is 125% of the FLA in the Motor tables. Several motors is 430.24. Example: (4) motors, (1) is 25HP, the other three are 5HP. Size 125% of the FLA of the 25HP motor and then add the FLA 's of the 5HP. For conductor sizing only.

Sizing conductors yes you will need to go to the next available size because of the different ampacities. For 27A, you'll need to go to 10 AWG since it is rated for 30A @ 60degrees. But look out for the insulation (THHN or TW) and terminal temperature ratings (60, 75, 90) as well.

Short circuit/ground fault is 430.52(C) using the 430.52 table. Depends on the type of motor and type of fuse or breaker.

rg1 · Oct 21, 2017

More I read and discuss NEC, more confused I am. Perhaps this happens when you want to explain science in arts way , you have to use so many ifs and buts. Coming to the point, even though 430.32.A.1 says "not more than......." ; 430.32.C permits a higher capacity overload device if selection by 430.32.A does not withstand FL current of motor. So according to me you can always take the higher value for overload purpose of a motor. For conductor also it is similar. I am poor at NEC and so take my opinion as pinch of salt. However I have a out of box thought- when in confusion, toss a coin to find out- if NCEES allows coins inside the exam hall!!!

Stephen2awesome · Oct 21, 2017

rg1 said:
More I read and discuss NEC, more confused I am. Perhaps this happens when you want to explain science in arts way , you have to use so many ifs and buts. Coming to the point, even though 430.32.A.1 says "not more than......." ; 430.32.C permits a higher capacity overload device if selection by 430.32.A does not withstand FL current of motor. So according to me you can always take the higher value for overload purpose of a motor. For conductor also it is similar. I am poor at NEC and so take my opinion as pinch of salt. However I have a out of box thought- when in confusion, toss a coin to find out- if NCEES allows coins inside the exam hall!!!

If the motor doesn't start then yes use the percentages in 430.32.C

rg1 · Oct 21, 2017

Stephen2awesome said:
If the motor doesn't start then yes use the percentages in 430.32.C

Yes, you are true. There is a question in NCEES sample Qs -#510. The answer is according to your suggested way. Thanks a lot Omer for raising the issue and to all contributors in this thread. I really learned it today, at least to get correct answer in the Exam. But for this thread I was going with my mind made up for next higher rating.

sayed · Oct 22, 2017

i wouldn't worry about the NEC too much. I actually thought i bombed the exam because i was lost on most of the problems i had (i think i had about 12 in my test).

I honestly think that the graders curved this for me. Cuz literally, using the nec isn't engineering, it's contractor stuff. It wouldn't have been fair to fail me for missing the non-engineering topic on my test. (I needed to pass the PE test in order to qualify for the contractor test which I needed to study NEC for anyways)

Bonsai · Oct 22, 2017

sayed said:
i wouldn't worry about the NEC too much. I actually thought i bombed the exam because i was lost on most of the problems i had (i think i had about 12 in my test).

I honestly think that the graders curved this for me. Cuz literally, using the nec isn't engineering, it's contractor stuff. It wouldn't have been fair to fail me for missing the non-engineering topic on my test. (I needed to pass the PE test in order to qualify for the contractor test which I needed to study NEC for anyways)

The NEC is quite applicable to engineering. If I didn't put the right conductors and breakers on my design drawings, I'd end up with quite a bit of egg on my face when it came time for the client to review my drawings. I'd lose my consulting job pretty quickly if I couldn't reference the NEC. Studying for the exam has given me a new level of understanding and appreciation for those standards.

Just because it's not as technical doesn't mean it's not important.

That said, the NEC does spell out whether you should go up or down quite clearly. Keep an eye out for qualifiers like "no more than" and "no less than" when looking in the appropriate articles. DO NOT IGNORE FOOTNOTES, EXCEPTIONS, AND INFORMATIONAL NOTES. NOT EVEN IN TABLES. They can be absolutely VITAL to coming up with the correct answer.

rg1 · Oct 22, 2017

Bonsai said:
The NEC is quite applicable to engineering. If I didn't put the right conductors and breakers on my design drawings, I'd end up with quite a bit of egg on my face when it came time for the client to review my drawings. I'd lose my consulting job pretty quickly if I couldn't reference the NEC. Studying for the exam has given me a new level of understanding and appreciation for those standards.

Just because it's not as technical doesn't mean it's not important.

That said, the NEC does spell out whether you should go up or down quite clearly. Keep an eye out for qualifiers like "no more than" and "no less than" when looking in the appropriate articles. DO NOT IGNORE FOOTNOTES, EXCEPTIONS, AND INFORMATIONAL NOTES. NOT EVEN IN TABLES. They can be absolutely VITAL to coming up with the correct answer.

Makes sense. Now a new question for consideration.

If a motor with 60A needs a feeder ckt of 1.25*60=72A and a capacitor rated for 60A is connected somewhere near motor terminals ( A hypothetical case for understanding purpose only), will the motor feeder ckt conductors now required be 1.35*60 = 81A rated or 72 A rated. ( reference Articles 460.8.A and 430.22), if yes why, if no why?

Thanks .

Stephen2awesome · Oct 22, 2017

rg1 said:
Makes sense. Now a new question for consideration.

If a motor with 60A needs a feeder ckt of 1.25*60=72A and a capacitor rated for 60A is connected somewhere near motor terminals ( A hypothetical case for understanding purpose only), will the motor feeder ckt conductors now required be 1.35*60 = 81A rated or 72 A rated. ( reference Articles 460.8.A and 430.22), if yes why, if no why?

Thanks .

NEC 430.27 says conductors must comply with 460.8 and 460.9. 460.8 says to size the conductors 135% of rated capacitor current. So this section will have more authority than the 125% requirement just because of that capacitor.

Stephen2awesome · Oct 22, 2017

sayed said:
i wouldn't worry about the NEC too much. I actually thought i bombed the exam because i was lost on most of the problems i had (i think i had about 12 in my test).

I honestly think that the graders curved this for me. Cuz literally, using the nec isn't engineering, it's contractor stuff. It wouldn't have been fair to fail me for missing the non-engineering topic on my test. (I needed to pass the PE test in order to qualify for the contractor test which I needed to study NEC for anyways)

I disagree as well. For a fully functional system in real world applications, you want to design a safe electrical system that meets the NEC and safety standards while saving money and having enough capacity to handle whatever load a client adds.

Engineering isn't just creating circuits or devices, its also designing systems that work to meet a clients needs and also other factors.

rg1 · Oct 22, 2017

Stephen2awesome said:
NEC 430.27 says conductors must comply with 460.8 and 460.9. 460.8 says to size the conductors 135% of rated capacitor current. So this section will have more authority than the 125% requirement just because of that capacitor.

I too thought on similar lines. Thanks for the explanation.

sayed · Oct 22, 2017

Stephen2awesome said:
I disagree as well. For a fully functional system in real world applications, you want to design a safe electrical system that meets the NEC and safety standards while saving money and having enough capacity to handle whatever load a client adds.

Engineering isn't just creating circuits or devices, its also designing systems that work to meet a clients needs and also other factors.

maybe it is different in other states, but in my state contractors are the ones who crunch the numbers and size wires, panels, etc. and uses the NEC heavily.

In fact, most projects don't require an engineer to sign off on electrical (building departments allow the plan sheets to remain blank on the electrical).

Also, i stated "i wouldn't worry about the NEC too much." This implies having a working knowledge but not dwelling on the NEC trying to master it.

sayed · Oct 22, 2017

Bonsai said:
The NEC is quite applicable to engineering. If I didn't put the right conductors and breakers on my design drawings, I'd end up with quite a bit of egg on my face when it came time for the client to review my drawings. I'd lose my consulting job pretty quickly if I couldn't reference the NEC. Studying for the exam has given me a new level of understanding and appreciation for those standards.

Of course it is applicable to engineering. But using the NEC in itself is not engineering. That is why an electrician who worked with the NEC for decades is not an engineer.

Hopefully the difference is clear.

Stephen2awesome · Oct 22, 2017

sayed said:
maybe it is different in other states, but in my state contractors are the ones who crunch the numbers and size wires, panels, etc. and uses the NEC heavily.

In fact, most projects don't require an engineer to sign off on electrical (building departments allow the plan sheets to remain blank on the electrical).

Also, i stated "i wouldn't worry about the NEC too much." This implies having a working knowledge but not dwelling on the NEC trying to master it.

I'm curious, which state is that? Most projects I've done(depending on size) require PE's to sign drawings for construction. Just recently, contractors installed panels without a revised drawing so I had to go into the panel schedules that I made for the construction documents and re-calculate the loads of what they did install so that they didn't have to cut slab and re-install larger panels to accommodate the additional load.

I've done projects in multiple states as well, MI, FL, AL, AR for example all required PE signed construction documents.

rg1 · Oct 22, 2017

Trying to define NEC

NEC is a standardised Engineering document prepared by Engineers to be followed by non Engineers.

When a non Engineer refers it, he simply follows it, when an Engineer refers it, he follows it with reasoning and sometimes questions it because one can not tame Engg into one book, there are many cases in real life which are new to the book. Engineer knows how to play with articles to get answer to the new situation while non Engineer needs help of an Engineer there.

rg1 · Oct 22, 2017

@Stephen2awesome How do you explain #CI-I-69. where he asks largest allowable current for O/C device to protect 300kcmil, (TW) 60 degree Celsius, Aluminium Conductors in steel conduit feeding a 277V load. From table I get Ampacity as 195A. Now which option is true for answer 180A or 200A and why. I thought 200A as it next higher level, but CI says 180 ( Does not explain). Your help is required here. Thanks

NEC/ Overcurrent

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