Panel feeder ground size from xfmr

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Sparky5150

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I was just thrown a loop, what I thought was the correct ground size for a panel feeder from a xfmr may be incorrect.

According to the 2005 NEC handook the ground wire from a xfmr to a panel is called a systems grounding conductor and is sized per 250-66.
I've designed it as an equipment grounding conductor for 20+ years per 250-122!

What are your experiences????
 
Sparky5150 said:
I've designed it as an equipment grounding conductor for 20+ years per 250-122!
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You can not properly apply Table 250.122 to transformer secondary conductors.

When you look at Table 250.122 how do you size the EGC?

It is based on the size of the over current device.

However it is the over current device ahead of the circuit that is used for sizing.

The only OCP ahead of transformer secondaries are on the primary side and can not be used to determine the EGC size on the secondary side.
 
That is correct! The transformer is a separately derived system, and the ground conductor, and grounded conductor are the same at this point, and therefore are sized according to 250.66.
 
I agree with Bob. For transformer secondaries it's simple since the system grounding conductor to the panel, the neutral bonding jumper and the GEC are all the same size.
 
Sparky5150 said:
No one else willing to admit they sized it per 250-122 and the main breaker in the downstream panel?
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I have seen engineers design according to 250.122. Usually we just install the correct size conductor. If we see the engineer at some point we will tell him, but many times we never deal directly with the engineer.
 
I've also seen other engineers do that on plans I've reviewed. They also forget about the minimum of 12.5% of the phase conductors on large services.

Steve
 
Thanks Bob. I just deleted the post, thinking it didn't belong here.
Isn't that [250.28(D)] for grounded systems? I'm looking at bringing in a 3 phase 3 wire feed, 2000 Amps @ 480V. Two parallel services, actually, at 2000A max design capacity each. We're leaving X0 open on the transformer secondary, so no neutral exists there, per se.

250.66 seems a little easy going at the top end.

Edited 2014 Nov 02 - Somebody ought to give me a swift kick in the butt. The answer is that I've managed to confuse the grounding and bonding issues. The ungrounded service needs a GEC sized to provide transient protection, not an effective path for fault current. The EGC's are sized to provide the fault current path from the utilization equipment back to the source. The GEC only needs to be of sufficient size (minimal impedance) to be able to deal with transients, lightning strikes and the like which could be induced on either the service conductors or the building itself. I'll get back in the truck now, and I promise not to play with the radio.

Does anyone have an opinion of how low the Z should be on the GES of an ungrounded system spanning 200k ft^2? I also wonder whether it is permitted to use an element of the GES as a GEC. I doubt the latter, but invite comment.

Dan
 
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