system ground size??

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Mike01

Mike01

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Grounding question ex.:
2000AA Pad mounted xfmr.
Substation located in building (remote) with 3200AF/3000AT main circuit breaker.

Phase conductors:
8-sets of 3#600kcmil+grd.

Ground to be sized based 250.66
If the conductors are larger than 1100kcmil copper and the system bonding jumper is not located at the source, the size is based on 12.5% of the area of the largest derived phase conductor. In this cas the largest derived phase conductor would be 600,000*8? = 4,800,000 so 12.5% would be 600,000 now do you divide that by 8 to get the size per set of conductors? It would be the equevlant of each raceway containing a conductor of 75,000cmills that would be a #1 but since a 1/0 is the smallest you can parallel than a 1/0 ground minimum would need to be installed with each set of phase conductors. Is this correct??

I have also seen medium voltage installation where the ground is a 600V ground is this acceptable??
 
Mike01 said:
Grounding question ex.:
2000AA Pad mounted xfmr.
Substation located in building (remote) with 3200AF/3000AT main circuit breaker.

Phase conductors:
8-sets of 3#600kcmil+grd.

Ground to be sized based 250.66
If the conductors are larger than 1100kcmil copper and the system bonding jumper is not located at the source, the size is based on 12.5% of the area of the largest derived phase conductor. In this case the largest derived phase conductor would be 600,000*8? = 4,800,000 so 12.5% would be 600,000 now do you divide that by 8 to get the size per set of conductors? Is this correct??QUOTE]
A full size gnd conductor in each conduit. See note 1 under the table NEC 2005.
Click to expand...
 
Bob,
This conductor is not covered by 250.122. I This conductor is covered by 250.30(A)(2) which sends you to 250.104(C). The bonding conductor in each raceway is sized based on the phase conductors in that raceway. This installation requires a 2/0 copper bonding conductor in each raceway.
Don
 
Corrected "System Bonding Jumper"

Corrected "System Bonding Jumper"

Let me try to restate my question:

250.28 Main bonding and system bonding jumpers:
(d) size: it states that if the supply conductors are over 1100kcmil copper the bonding jumper shall have an area that is not less than 12.5% of the area of the largest phase conductor.

The question if you have parallel sets say 8 sets of 600kcmil:
Is the size based on 12.5% of the 600kcmil cable or 12.5% of 600kcmil*8 the paralleled set? This would put you at 4800000cmills * 12.5% that would equal 600000 now if I am correct you would divide that by 8 (the number of sets) to arrive at the minimum conductor size required per conduit. Correct? (not less than 1/0) so in this case the minimum size required would be 75,000cmills the next largest size would be a #1 but the minimum allowed is a 1/0 so that would be sufficient per set for a total cmill area of 844,800cmills approx 40% larger than required. So 1/0 would be sufficient?
 
Mike,
In your first post you said that the sytem bonding jumper is not located at the source so the bonding conductor in the raceways is not a system bonding jumper. Those conductors are equipment bonding jumpers. See 250.30(A)(2) which tells you to look at 250.104(C). A 2/0 equipment bonding jumper is required in each raceway.
Don
 
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Thanks for the response how do you know where the contractor in the field is going to make his connection (in the xfmr or the first panelboard?) lets say that in the example above the derived conductors to be bonded within the transformer so the situation I explained above would have a system bonding jumper being installed form the transformer to the grounding lug / bar in the first panelbaord? So would that make what I stated above correct?
 
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