EGC size

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petersonra

petersonra

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engineer
Something that occurred to me.

I wrote an email some time ago that I never actually sent.

I mentioned that the EGC is sized according to the OCPD, and that in some cases such as with motors, the OCPD is larger than "normal" so the EGC needs to be larger than the ampacity of the protected conductors might otherwise indicate.

In thinking about this, I asked myself a question. How would this apply to motor circuit protectors? They are the sole SC protection for a motor circuit that has an MCP, so would not the same thing apply in such a circuit? I am suggesting it appears like for motors using an MCP you would have to always size the EGC at the same size as the main conductors.

But, I can't say i have ever seen anyone actually do this. Am I missing something here?
 
petersonra said:
Something that occurred to me.

I wrote an email some time ago that I never actually sent.

I mentioned that the EGC is sized according to the OCPD, and that in some cases such as with motors, the OCPD is larger than "normal" so the EGC needs to be larger than the ampacity of the protected conductors might otherwise indicate.

In thinking about this, I asked myself a question. How would this apply to motor circuit protectors? They are the sole SC protection for a motor circuit that has an MCP, so would not the same thing apply in such a circuit? I am suggesting it appears like for motors using an MCP you would have to always size the EGC at the same size as the main conductors.

But, I can't say i have ever seen anyone actually do this. Am I missing something here?
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Take a look at this: http://forums.mikeholt.com/showthread.php?t=145929&p=1406671&highlight=#post1406671
 
I think that thought has crossed all our minds but it is clear that the egc never needs to be larger than the circuit conductors
 
Dennis Alwon said:
I think that thought has crossed all our minds but it is clear that the egc never needs to be larger than the circuit conductors
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I think I said that.

Its not a big deal for circuits with #10 conductors and smaller since the EGC will be the same size as the circuit conductors anyway, but once you get above #10, it appears to me that the use of a MCP means as a practical matter that the EGC has to also be sized the same as the circuit conductors.
 
hmmmm. never noticed this before.

(D) Motor Circuits. Where the overcurrent device consists
of an instantaneous trip circuit breaker or a motor short-circuit
protector, as allowed in 430.52, the equipment grounding conductor
size shall be permitted to be based on the rating of the
motor overload protective device but shall not be less than the
size shown in Table 250.122.
Click to expand...

so i guess if you use an MCP you can size the egc off the overload, but not if you have a time delay breaker. what kind of logic is that?
 
Sorry I knew you know that I was just saying many of us knew it but I did not read it closely enough. IMO. If the OCPD is sized larger than the conductors than I would agree that the EGC must be sized accordingly up to the size of the circuit conductors. So yes I would think there are times when the egc may have to be the same size as the circuit conductors
 
Bob this changed a bit in 2008 are you using 2005NEC

(D) Motor Circuits. Equipment grounding conductors for motor circuits shall be sized in accordance with (D)(1) or (D)(2).
(1) General. The equipment grounding conductor size shall not be smaller than determined by 250.122(A) based on the rating of the branch-circuit short-circuit and ground-fault protective device.
(2) Instantaneous-Trip Circuit Breaker and Motor Short-Circuit Protector. Where the overcurrent device is an instantaneous-trip circuit breaker or a motor short-circuit protector, the equipment grounding conductor shall be sized not smaller than that given by 250.122(A) using the maximum permitted rating of a dual element time-delay fuse selected for branch-circuit short-circuit and ground-fault protection in accordance with 430.52(C)(1), Exception No. 1.
Click to expand...
 
petersonra said:
I only have the 2005 code in PDF. I suppose I should see if the boss will spring for an electronic copy of 2011. She is kind of cheap sometimes.
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They did not make an electronic copy for 2011 instead they launched a website (NEC Plus) and you must pay subscription to use it - It does have many more features than previous electronic versions of NEC.

There are of course just PDF versions of 2011 out there, and not that hard to find one for free. NFPA even offered free download of PDF in many hard copy versions of 2011 NEC.
 
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