Conduit as EGC

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fbhwt

fbhwt

Electrical Systems Inspector
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Spotsylvania,Virginia
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Electrical Systems Inspector
I've read 250.118, but have been told that above 60amps you must pull seperate EGC, I have looked but I can not find anything that says this. Anyone else know of this?
 
The 60 amp requirement is for LFMC. Other raceways have different requirements. For example for RMC, EMT and IMC the raceway is permitted as the EGC without ampacity restrictions.
 
fbhwt said:
I've read 250.118, but have been told that above 60amps you must pull seperate EGC, I have looked but I can not find anything that says this. Anyone else know of this?
Click to expand...

That could be something specific to a job but is not an NEC requirement.
Ask the oracle that shared this wisdom with you to show you the code section. Unless its your boss, the owner or your foreman of course, then just do what you are told. They may have sold your job that way.:grin:
 
re: Conduit as EGC

358.60 says that EMT can be used as an equipment grounding conductor with no ampacity restrictions.

I seem to remember though, that if it's a 480V (or above) circuit, you can't use the conduit as a ground. I'm not certain on that though.
 
buzzbar said:
358.60 says that EMT can be used as an equipment grounding conductor with no ampacity restrictions.

I seem to remember though, that if it's a 480V (or above) circuit, you can't use the conduit as a ground. I'm not certain on that though.
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I am not aware of any voltage restriction as to when EMT can be used as an equipment ground in the NEC.

Chris
 
A properly-assembled conduit system has a lower impedance than the required EGC for the conductors contained would have.
 
I have heard anectodal evidence that there's a length limit to using EMT as a ground, but I haven't searched much for anything to back that up.
 
480sparky said:
There's a length limit to ground wires in T250.122????
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No. :smile:

If you use only 250.122 to size your EGC there is a limit on how far you can go and still have an effective EGC.

That distance will normally be less then the effective length of steel conduit.
 
SEO said:
Bob would you happen to have the formula?
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No, I don't really know how to determine when we have an EGC that is in violation of 250.4(A)(5). :-?

All I do know is steel conduit can provide a lower impedance ground fault path then the conductor that would be installed in it.
 
Here is a link to a website designed for steel conduit applications. If you click on the download section you will see a free version of GEMI software that can be used to determine length of steel conduit for a fault curretn path.

Chris
 
There are grounding books that provide charts on the raceway size, and ocpd of contained conductors for the max length of metal raceways that may be used as a egc (not in the NEC). Also 250.122 (B)requires the egc to be increased in cma,. propotionally if the contained ungrounded conductors are increased for volatge drop.

As far as 250.4(A)(5) and (B)(4) are concerned a PE is better suited to explain this note. but it has to do with ampere squared seconds and a devices peak let thru. One table will give 1 amp for 5 seconds (I think) for every 42.25 cir mils of condcutor area (I think at 20 deg C) another Table uses 30cma (I think at 75 deg C) . Its a surviability (burn off) problem for the egc.
Again I don't have the book here but I'm sure someone does
 
There is a free design program called GEMI from the steel tube institute that determines how long an EGC can be without increasing its size in steel or PVC.Its based on research by the Univ of Georgia.
It seems to indicate that longer than 300ft the size of the EGC many need to be larger. See the note to table 250.122
That program used to be availble on Mikes site.
 
I've always wondered about these methods to determine max raceway lengths. I don't think they take in consideration of interconnected enclosures, locknut, and fittings.????

Do they??
 
cpal said:
There are grounding books that provide charts on the raceway size, and ocpd of contained conductors for the max length of metal raceways that may be used as a egc (not in the NEC). Also 250.122 (B)requires the egc to be increased in cma,. propotionally if the contained ungrounded conductors are increased for volatge drop.

As far as 250.4(A)(5) and (B)(4) are concerned a PE is better suited to explain this note. but it has to do with ampere squared seconds and a devices peak let thru. One table will give 1 amp for 5 seconds (I think) for every 42.25 cir mils of condcutor area (I think at 20 deg C) another Table uses 30cma (I think at 75 deg C) . Its a surviability (burn off) problem for the egc.
Again I don't have the book here but I'm sure someone does
Click to expand...
Charlie,
The maximum current that the EGC can "withstand" is not really the issue with a very long EGC, however that is an issue on circuits with a very high available fault current for the reason you stated. The problem is that a long conductor will limit the current flow and may not be an effective fault clearing path because it will limit the current flow to a level where the OCPD may not quickly clear the fault.
 
Thank you Don
I agree that the increase in size for long runs should compensate and provide a suffeicently low impedance path for faults in the range up to 500%of the cont rating of the ocpd

but on the other hand (A) (5) mentions the maximum ground fault and I think in such cases it may require even under extreme fault condictions I would thing the intent is to maintain the intergrety of the egc path??

What's your thoughts.
 
jrannis said:
That could be something specific to a job but is not an NEC requirement.
Ask the oracle that shared this wisdom with you to show you the code section. Unless its your boss, the owner or your foreman of course, then just do what you are told. They may have sold your job that way.:grin:
Click to expand...


Actually it is an NEC requirement for LFMC in trade sizes 3/4"-1 1/4". Take a look at 250.118(6)(c).
 
Let's not forget that, if the EGC needs to be upsized for VD, so will all of the conductors, and thus so will the conduit itself.
 
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