equipment grounding

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pomps

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I have a 800 amp service feeding a transfer switch, from the transfer switch to a splice cabinet with 4 sets of 4/0 wires in emt what size equipment ground should i be running with each set ( in each pipe) and what code to back your answer please. thanks
 
Re: equipment grounding

Since your conduit is EMT no separate EGC is required. If you choose to use one than it must be sized according to the OCPD ahead of the transfer switch. Assuming that the OCPD is 800 amps than each of the 4 conduits would require a #1/0 Cu EGC sized for 800 amps as in table 250.122. This is required by 250.122(F)(1).
 
Re: equipment grounding

Just remember that the table (250.122) does not consider or take into account abnormally high available fault currents. Check to ensure the withstand rating is not exceeded when using the Table minimums.
 
Re: equipment grounding

Originally posted by bphgravity:
Just remember that the table (250.122) does not consider or take into account abnormally high available fault currents. Check to ensure the withstand rating is not exceeded when using the Table minimums.
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Where does the NEC require more than 250.122?

If what you say is a concern than the NEC should have a section (or at least an FPN) stating that.

What you say makes great design sense but I do not see it as an NEC requirement.
 
Re: equipment grounding

Bob, there is a note to Table 250.122 that requires compliance with 250.4(A)(5). There are also sections 110.9 and 110.10, which require the circuit impedance to be low enough to not be damaged.
 
Re: equipment grounding

"Where does the NEC require more than 250."122?"

Lets review the history of the note to 250.122
It used to state the EGC may need to be increased in size for voltage drop
Then it was clarifed that there are other conditions that may require increasing the size.
Finally the 2002 NEC changed it from may to shall.
The free software program "GEMI" from the Steel Tube Institute is based on studies by the UofGeorgia on the size of the EGC.
Typically a GRC conduit longer than 300 ft will need the size of the EGC increased to reduce the impedance.
 
Re: equipment grounding

Personally I think it is a bunch of BS.

Can anyone say they have seen a EGC sized per 250.122 not perform as expected?

But beyond that what is the criteria for selecting the 'right' size EGC.

You can not enforce what is not spelled out.

We go round and round about 250.4(A)(5) but that only says what it has to do, not how it is done.

How quick must the OCP open?

How much available fault current will table 250.122 work with?

This to me seems much like the requirements for arc flash that can not be done as the available fault current can not be determined. :p
 
Re: equipment grounding

I agree with iwire, that is a design issue and those questions better get answered at plan review, not at inspection, has anybody had a job that because of fault currents they had to increase the EGC? I am just curious
 
Re: equipment grounding

Bob,
This to me seems much like the requirements for arc flash that can not be done as the available fault current can not be determined.
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Not really. With the arc flash PPE calculations, if you use a fault current that is too high, your PPE may not be provide the required protection. With the selection of the EGC, all an excessive fault current will do is to oversize the conductor, no hazard will result.
I think that with high available fault current systems, it would be possible for an EGC to be damaged by the current flow before the OCPD opened the circuit, but it would not be common.
Don
 
Re: equipment grounding

Don your missing my point.

There is nothing in any of the sections given in this thread that requires the EGC to be larger than 250.122 requires.

We can say 250.4(A)(5) requires an effective ground fault path but the NEC does not spell out the criteria that provides an effective fault path.

250.122 Size of Equipment Grounding Conductors.
(A) General. Copper, aluminum, or copper-clad aluminum equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122 but shall not be required to be larger than the circuit conductors supplying the equipment. Where a raceway or a cable armor or sheath is used as the equipment grounding conductor, as provided in 250.118 and 250.134(A), it shall comply with 250.4(A)(5) or 250.4(B)(4).
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Than 250.122(B), (C), (D) (E), (F) all make adjustments to the general rule.

There is no 250.122(G) that says you must go bigger if certain conditions are present. :p
 
Re: equipment grounding

Originally posted by ryan_618:
Bob, there is a note to Table 250.122 that requires compliance with 250.4(A)(5). There are also sections 110.9 and 110.10, which require the circuit impedance to be low enough to not be damaged.
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I already have addressed what I think of 250.(A)(5).

110.9 does not apply to conductors as it has to do with items that interrupt current as in breakers, fuses, switches etc.

110.10 does not require that the conductors are not damaged during a fault.

That section says

without extensive damage to the electrical components of the circuit.
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As an EGC can be bare to concern one self with overheated EGC insulation seem fruitless.

And again I ask has anyone seen an EGC not do it's job if installed properly and sized per 250.122?
 
Re: equipment grounding

Originally posted by ryan_618:
Right, there is no 250.122(G). What you are discussing is the note to Table 250.122.
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And that note directs to a section without criteria for what exactly is an effective fault path.

You can not tell me I fail if I install per 250.122 simply because in your opinion I have not created an effective fault path.

You would have to be able to say that the NEC requires the OCP to operate in X cycles and you can not because the NEC does not. :)

If I was a cop :D and I pull you over for speeding would you not expect me to tell you what the speed limit is?

If my answer was the posted speed limit is 50 you where traveling 50 but I think you are traveling to fast for the present conditions would you accept that ticket?
 
Re: equipment grounding

I understand your analogy, but, to take it further...The speed limit isn't posted in this case. The speed limit is "safe". Unfortunatley, "sfe" is not a prescriptive rule, it is a performance based requirement. What is "safe"? Only the cop/AHJ knows. :(
 
Re: equipment grounding

Originally posted by ryan_618:
Only the cop/AHJ knows. :)

For what it's worth MA does have vague traffic rules, I got a ticket for an "Unsafe lane Change" I fought it and won. Which I considered pretty good as the lane change resulted in an accident. :D
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Re: equipment grounding

Originally posted by ryan_618:
Originally posted by iwire:
Why not have a section in the NEC that defines an effective ground fault path? :)
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It is already defined in 250.2. :)
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That is not what I mean at all.

I am talking about a section that spells out what that means.

As in

"An Effective ground fault path is one that causes the OCP to operate in X cycles."

Or

"An effective ground fault path is one that causes the OCP to operate before any conductors exceed their temperature ratings"

Or

"An effective Ground fault path is one with an overall impedance less than..."

You see what I am getting at, the NEC should put some numbers behind 240.5(A)(5). :)

Put some clear requirements down and I will be glad to follow them.
:)
Leave them vague and up to each inspectors whim of the moment and I will kick and fight all the way till I pass. :cool:
 
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