Grounding Conductor for 4000 Amp Feed

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Rick Christopherson

Rick Christopherson

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Hi All,
I've been away for a while, and I am sure I was deeply missed, right? :D

Anyway, I am designing a stand-by inlet panel to get temporary power into facilities easily when they loose power. This is a modular system using parallel 4/0 cam locks ranging from 1 to 10 conductors per phase (400 to 4000 amps).

I just wanted to make sure there wasn't something I was missing, but when I looked through 250.66, it appears that a single 4/0 (actually 3/0) grounding conductor is all that is needed for anything up to the 4000 amp ceiling I am putting on this design. I haven't worked with systems this large before, so I was surprised that I can have a grounding conductor being less than 1/10th of the feeders. Is this correct, or is there something else I am missing?
 
Rick Christopherson said:
Anyway, I am designing a stand-by inlet panel to get temporary power into facilities easily when they loose power. This is a modular system using parallel 4/0 cam locks ranging from 1 to 10 conductors per phase (400 to 4000 amps).
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That sounds like you based your sizing on the cable manufacturers rating and not table 400.5(B).

I doubt cam locs are rated 90C {see 110.14(C)(1)} so at best the 4/0s run singly are rated 360 amps.

I just wanted to make sure there wasn't something I was missing, but when I looked through 250.66, it appears that a single 4/0 (actually 3/0) grounding conductor is all that is needed for anything up to the 4000 amp ceiling
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Your in the wrong table, you need to use table 250.122

Which for 4000 amps would require 500 Kcmil copper.
 
By the way, if you base your 4/0 ampacity at 360 amps each you may have voltage drop issues if the load is actully that high.

You may want to do some voltage drop calcultions before you lock yourself into just 10 runs for 4000 amps.
 
Rick Christopherson said:
Hi All,
I've been away for a while, and I am sure I was deeply missed, right? :D

Anyway, I am designing a stand-by inlet panel to get temporary power into facilities easily when they loose power. This is a modular system using parallel 4/0 cam locks ranging from 1 to 10 conductors per phase (400 to 4000 amps).

I just wanted to make sure there wasn't something I was missing, but when I looked through 250.66, it appears that a single 4/0 (actually 3/0) grounding conductor is all that is needed for anything up to the 4000 amp ceiling I am putting on this design. I haven't worked with systems this large before, so I was surprised that I can have a grounding conductor being less than 1/10th of the feeders. Is this correct, or is there something else I am missing?
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The inlet panel/enclosure is not the service means of disconnect. You will not be required to have the GEC at this location. The SDS will be where the GEC is located.

An EGC will be necessary. If the raceways are nonmetallic, then you will need to size the EGC as per 250.122(F) - remember, a full sized EGC installed in each nonmetallic raceway for parallel installations.


Note:
The GEC is not installed for reasons of fault current, read 250.4(A)(1)-(5), especially (1)&(2).
 
iwire said:
Your in the wrong table, you need to use table 250.122

Which for 4000 amps would require 500 Kcmil copper.
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Why do you suggest table 250.122 versus 250.66? I need to do some more reading, but I think 250.66 is the applicable table. I could be wrong of course, and that is the reason why I asked the question in the first place.

Update: It just occurred to me that there is no point in even worrying about this. I'll just be proactive and install a second ground cam on any panel over 1600 amps. The customer's electrician is the one to decide whether to use the second conductor at the time he connects the generator to the panel.

I had written responses to your other comments, but I ended up deleting them all when I had the above epiphany. In short, everything is 90C, I don't need to worry about voltage drop, and the customer is the one that would need to determine if any derating is required. My panel will be rated according to its design and the number of cam inlets installed (400 through 4000 amps) and the size of the applicable bus bar connecting the inlets together. (This is the next area I need to begin researching--designing custom bus bars.)
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You are designing these enclosures, and I presume selling them.

Is there a listing/labeling through a NRTL for the enclosures?

The reason I ask, is your comments do not seem as though you are too concerned about the applicable Standards that may be involved with designing such equipment.
 
iwire said:
I doubt cam locs are rated 90C {see 110.14(C)(1)} so at best the 4/0s run singly are rated 360 amps.
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i have a leviton cam connector sitting on my desk. it says 'use 90deg C cable' and then has a chart '4/0-400a 3/0-350a 2/0-300a 1/0 260a'
 
wireguru said:
i have a leviton cam connector sitting on my desk. it says 'use 90deg C cable' and then has a chart '4/0-400a 3/0-350a 2/0-300a 1/0 260a'
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Cool, I have never seen a 90 C rated device.

Not to sure I would want to handle a cam loc aproaching 200 F.

Are you sure we can use it at 90C or does it just require 90 C cable?

Not saying it does not ..... just asking.

We still have to follow the NEC regardless.
 
iwire said:
Cool, I have never seen a 90 C rated device.

Not to sure I would want to handle a cam loc aproaching 200 F.

Are you sure we can use it at 90C or does it just require 90 C cable?

Not saying it does not ..... just asking.

We still have to follow the NEC regardless.
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not sure. I do know the lighting guys consider 4/0 '400 amp cable' and use it as such. I have as well. I also think some event where everything is on for maybe 2 or 3 hours is a whole lot different than feeding an office building during a utility outage as far as how hot those connectors are going to get.
 
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