New Grounding Table for 2014 - Why?

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rbarnett's picture
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Joined: 2014-03-04

Table 250.102.(C)(1) is a new table for the 2014 NEC. Previously, we were required to use Table 250.66 for sizing the Grounding Electrode Conductor (GEC), Grounded Conductors, and Bonding Jumpers. Now, all Table 250.66 is used for is the Grounding Electrode Conductor. The new table is to be used for the others. The only difference I see is in the last line. Table 250.66 basically says if the Largest Ungrounded SE Conductor (or equivalent parallel area) is over 1,000kcmil, use a minimum size 3/0 Grounding Electrode Conductor.
The new Table 250.102.(C)(1) says if the Largest Ungrounded Conductor (or equivalent parallel area) is over 1,100kcmil then "see the notes." The notes then apply the 12-1/2 percent rule. That is; the Grounded Conductor can be no smaller than 12-1/2 percent of the largest ungrounded supply conductor. So, at 1,100kcmil, a 3/0 is slightly larger than 12-1/2 percent (Chapter 9, Table 8). If I parallel three (3) 500 kcmil conductors then I would have to move up to a 4/0 Grounded Conductor to meet the 12-1/2 percent requirement.
So, why don't we have to upsize the GEC in Table 250.66 beyond a 3/0? Must not this grounding electrode conductor also be able to carry available fault current imposed, even though it is into the earth? Hopefully, someone with some Code theory background can explain this one. 

Jimmie's picture
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Joined: 2014-05-29

The Grounding Electrode Conductor is not really designed to carry "Fault Current". The ground has a relatively higher impedance. Even if we were to assume a 480/277V Wye system, the voltage to ground is 277V.
277 divided by even 2 ohms is ......

gk1's picture
gk1
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Joined: 2014-06-17

[quote=Jimmie]The Grounding Electrode Conductor is not really designed to carry "Fault Current". The ground has a relatively higher impedance. Even if we were to assume a 480/277V Wye system, the voltage to ground is 277V.
277 divided by even 2 ohms is ......[/quote]

Comment: Above paragraph is not complete in view. Please, could you complete it? Thanks.

dkidd's picture
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Joined: 2013-11-13
al yudow's picture
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Joined: 2014-06-23

How does the Code before 1996 fit into the discussion of grounding the neutral on appliances?

loopen12's picture
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Joined: 2014-07-11

I have a question on grounding when working on outside commerical load side electrical service wire going from pull box to pull box with underground conduit PVC
if conductors are spliced in box made of qauzite (Fiber type material). Is it required to place ground rod in box and attach the ground wire to the rod?

This is an FDOT project. Any help is appreciated.

W A Werning's picture
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Joined: 2014-02-12

Hey, rbarnett:
Check 250.4(A)(1) and (B) for ungrounded systems. The requirements of 250.4 "outlines overall performance objectives for grounding conductors as applied to both grounded and ungrounded system".

I think it means that at any given moment in time there would never be more fault current imposed upon that particular grounding electrode conductor sized at 3/0, (the maximum). I believe this includes lightning strike(s) too.

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