Equipment grounding size 4

[mikeengurs]

I'm feeding a 1200A MV breaker directly from the secondary side of a 69K/4.16K transformer. I'm using 6 1/C #500 MCM (2 conductors per phase). Table 250.122 in NEC-2011 indicates a ground size of 3/0 if overcurrent device protecting cables is 1200A. In this case, there is no overcurrent device ahead of MV breaker (fed directly from transformer). What grouding conductor size should be used? This is a high resistance grounded system.

 

[7anoter4]

My opinion is that you protect the grounding cable by limiting the current will flow through the live cable and the equipment downstream the cable
so the grounding cable has to be as indicated.

 

[mikeengurs]

they have a 50:5A ground CT, so the ground current can't be that large. So there is no code requirement for this?

 

[7anoter4]

The phase- to- ground short-circuit is reduced indeed but a part of the grounding conductor could conduit a phase- to- ground- to- phase [double grounded ]
short-circuit[this current does not flow through the Grounding Resistor].

 

[dpc]

they have a 50:5A ground CT, so the ground current can't be that large.

That's not a good assumption for ground CTs. (in general)

I do not believe there is anything in the NEC that allows the size of the equipment grounding conductor to be reduced when an impedance grounded neutral is used.

 

[mikeengurs]

Good point 7anoter4. dpc, do you know which NEC requirement is applicable with this? 250.122 talks about using the upstream overcurrent protector as guidance, but this is not present here.

 

[resqcapt19]

The following is from 250.30(A)(2) in the 2011 code.
"(2) Supply-Side Bonding Jumper. If the source of a separately derived system and the first disconnecting means are located in separate enclosures, a supply-side bonding jumper shall be installed with the circuit conductors from the source enclosure to the first disconnecting means. A supply-side bonding jumper shall not be required to be larger than the derived ungrounded conductors. The supply side bonding jumper shall be permitted to be of nonflexible metal raceway type or of the wire or bus type as follows:
(a) A supply-side bonding jumper of the wire type shall comply with 250.102(C), based on the size of the derived ungrounded conductors.
(b) A supply-side bonding jumper of the bus type shall have a cross-sectional area not smaller than a supply-side bonding jumper of the wire type as determined in 250.102(C)."

"250.102(C) Size — Supply-Side Bonding Jumper.
(1) Size for Supply Conductors in a Single Raceway or Cable. The supply-side bonding jumper shall not be smaller than the sizes shown in Table 250.66 for grounding electrode conductors. Where the ungrounded supply conductors are larger than 1100 kcmil copper or 1750 kcmil aluminum, the supply-side bonding jumper shall have an area not less than 121⁄2 percent of the area of the largest set of ungrounded supply conductors.
(2) Size for Parallel Conductor Installations. Where the ungrounded supply conductors are paralleled in two or more raceways or cables, and an individual supply-side bonding jumper is used for bonding these raceways or cables, the size of the supply-side bonding jumper for each raceway or cable shall be selected from Table 250.66 based on the size of the ungrounded supply conductors in each raceway or cable. A single supply-side bonding jumper installed for bonding two or more raceways or cables shall be sized in accordance with 250.102(C)(1)."

I believe these new sections address the question.

 

[rcw retired EE]

Cables from transformer to first overcurrent device can be treated as service conductors and grounding conductors sized per Table 250.66. A single 500 kcmil cable requires a #1/0 awg (105.6 kcmil) Grounding Electrode Conductor (GEC). Two 500 kcmil per phase is equivalent to 1000 kcmil which requires a #2/0 awg (133.1 kcmil)per the Table.

If the wires are in two conduits a strict adherence to the code requires a #2/0 awg ground wire in each conduit. See NEC 250.122 (F)(1).

As the others have pointed out, the ground/bond wire could be subjected to phase-to-phase fault current during a double phase to ground fault. Usually, the sizing in Table 250.66 or 250.122
gives a cable which will not melt during a fault. But it wouldn't hurt to check the cable withstand against your protection curves.

 

[mikeengurs]

Thanks everyone. Since I have 2 parallel #500 MCM, I can have 2-1/0 for each set correct?

 

[dpc]

I haven't checked your sizing, but in general, you have to run a full-sized ground wire in each conduit.

 

[resqcapt19]

dpc,
The rule that says you have to have a full size grounding conductor in each raceway or cable of a parallel run applies to EGCs.

In my opinion, we do not have EGCs in the runs that are being talked about in this thread. The grounding conductors in this thread are "supply side bonding jumpers" and are sized per T250.66 and the size of the ungrounded conductors in each raceway. Note that the issue of sizing these grounding conductors was clarified in the 2011 code.

 

[mikeengurs]

Thanks everyone. As allways, this site is very helpful.