Parallel feeder tap, what size eqipment ground wire?

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Mezani

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Washington
Per 250.122 (G) Feeder Taps. Equipment grounding conductors run
with feeder taps shall not be smaller than shown in Table
250.122 based on the rating of the overcurrent device ahead
of the feeder but shall not be required to be larger than the
tap conductors.
I have a 4000 amp feeder and am installing a 500 amp 10' tap with parallel 300 Kcmil CU conductors. The CU ground wire required is 500 Kcmil.
The first question is what is the tap conductor? Is it 300 Kcmil or the aggregate of the two, 600 Kcmil? Should the ground wire be 300 Kcmil or the 500 Kcmil?

Thanks for your help.
 

augie47

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That is a most interesting question.
I looked at it two ways. You did not state how the conductors were installed (parallel raceways, wireway, etc) so I first thought of it as multiple conductors in a wireway. If you installed one single conductor per phase (say a 900 kcmil), the requirement would be for a 500 kcmil. In the event of a fault, the require met to trip the OCP would be the same for one conductor or parallel conductors so I would say you need a 500 kcmil.
Any reduction in size based on (G) would be due to the phase conductor (singular) only being able to carry a finite amperage, but in your case you have more than one conductor so that limit is not there.

To me it became more interesting when I visualized parallel runs in separate conduits. You would have 300 kcmil phase conductors with a 500 kcmil EGC. Unusual, but I see nothing that changes that requirement.

Hopefully we will get additional comments.
 

charlie b

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The "tap conductors" are 600's, not 300's. So the EGC associated with the tap need not be larger than 600. Since the EGC of the conductor being tapped is 500, then that is the minimum size. Since the minimum that is required is 500, and the maximum that could be required is 600, I would go with 500. It does not matter if you are using a single conduit or a set of two conduits, you still need the 500. That would mean you would have an EGC of 500 MCM in the same conduit as 300 MCM phase conductors, but so be it.
 
Location
Virginia
Just a question Charlie...With regard to running the parallel taps in separate conduits, couldn't you refer to 250.122(F) for each set of taps, allowing you to run a 300kcmil instead of the 500kcmil?
 

charlie b

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Quite the contrary. 250.122(F) tells me that the EGC that goes with the tap conductors has to be sized per the rating of the upstream OCPD. The EGC in each parallel conduit is not sized on the basis of the phase conductors in the same conduit.
 
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I guess to try to add to my argument, that 250.122(F) goes along with (G) statement that ecg shall not be required to be larger than the tap conductors. That in each parallel run, the ecg could be the same as each ungrounded tap conductor. Maybe I am making this more difficult than it needs to be? Back when I was in the Navy, we called that "Nuking it".
 
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infinity

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I guess to try to add to my argument, that 250.122(F) goes along with (G) statement that ecg shall not be required to be larger than the tap conductors. I maybe making this more difficult than it needs to be? Back when I was in the Navy, we called that "Nuking it".

What size are the tap conductors in this example? I say 2 * 300 kcmil or 600 kcmil.
 

infinity

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You kinda edited out the part of my question where I said "That in each parallel run, the ecg could be the same as each ungrounded tap conductor. "


Don't know how it got edited since I used the reply with quote button. :?

Back to your question the EGC is required to be sized based on the OCPD ahead of the tap. It has nothing to do with the size of the parallel conductors in individual raceways. The tap conductors in this case are 600 kcmil. If not, then they wouldn't be permitted under the 10' tap rule which requires a tap conductor ampacity of 10% or more.
 

iwire

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That would mean you would have an EGC of 500 MCM in the same conduit as 300 MCM phase conductors, but so be it.[/SIZE][/FONT]

I disagree with that premise.

In no case would the EGC be required to be larger than the circuit conductors in the same raceway or cable.

The tap conductor 'size' is not the total of the group it is the size of one single conductor.

Three 300 kcmils together are not a '900 kcmil conductor(s)'
 
Location
Virginia
I think understand what you and Charlie are saying after thinking about it.

If I were to lose a egc in any of the parallel runs, my remaining egc wouldn't be large enough to handle the fault current to trip my ocpd. So they should be sized by the supply ocpd only?
 
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charlie b

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I disagree with that premise.
And I disagree right back at you.

In no case would the EGC be required to be larger than the circuit conductors in the same raceway or cable.
That is not what 250.122(G) says. It says the EGC need not be larger than the tap conductors. The tap is 600 MCM, whether you put two sets conductors in the same raceway or run them in parallel raceways.
The tap conductor 'size' is not the total of the group it is the size of one single conductor.
A 300 MCM tap would violate the tap rules, as Infinity has already pointed out.

 

charlie b

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If I made the 500 amp tap by using four parallel conduits, each containing four 1/0 conductors for the phase and neutral wires, would you accept a 1/0 EGC in each conduit? You could have 4000 amps worth of power source feeding a fault, with only a 1/0 to carry the fault current back to the tap point (after which you have a 500 MCM to take it the rest of the way). I think that could melt the EGC before the fault is cleared.
 

augie47

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The 2008 Handbook does a decent job of showing why the EGC must be full size based on the OCP for parallel conduit runs in Figure 250.50
 

RB1

Senior Member
Charlie,

In your scenario I think the No. 1/0 would do just as good a job sinking the fault current as the No. 1/0 did sourcing the fault current.
 

iwire

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Massachusetts
If I made the 500 amp tap by using four parallel conduits, each containing four 1/0 conductors for the phase and neutral wires, would you accept a 1/0 EGC in each conduit?

Absolutely and that is IMO what the NEC expects.

And the ground fault would still be on four 1/0s not one.
 

iwire

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Location
Massachusetts
That is not what 250.122(G) says. It says the EGC need not be larger than the tap conductors. The tap is 600 MCM, whether you put two sets conductors in the same raceway or run them in parallel raceways.

OK if the tap was 600 the EGC would never have to be larger than 600.
 
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