Equipment grounding size
Equipment grounding size
(OP)
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.






RE: Equipment grounding size
so the grounding cable has to be as indicated.
RE: Equipment grounding size
RE: Equipment grounding size
short-circuit[this current does not flow through the Grounding Resistor].
RE: Equipment grounding size
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.
RE: Equipment grounding size
RE: Equipment grounding size
"(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.
RE: Equipment grounding size
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.
RE: Equipment grounding size
RE: Equipment grounding size
RE: Equipment grounding size
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.
RE: Equipment grounding size