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Grounded Conductor size in parallel runs at service

Grounded Conductor size in parallel runs at service

Grounded Conductor size in parallel runs at service

I have a 2000A service and am sizing the EGC and neutral wires from the transformer secondary to the service entrance. I have 6 parallel runs of 500MCM. NEC 2011 250.30(A)(3)(b) says the size of the grounded conductor in each raceway shall be based on the total circular mil area of the parallel derived ungrounded conductors in the raceway as indicated in 250.30(A)(3)(a). This (a) section seems to contradict (b). "...the grounded conductor shall not be smaller than 12.5% of the circular mil area of the largest set of derived ungrounded conductors." When they say 'set', to me, that means the set of conductors in one raceway rather than the total circular mils of all the conductors.

So I used table 250.66 to size my grounding conductor at 12.5% of the total mils and get 375 MCM (400). According to 250.30(A)(3)(a) the grounded conductor can not be smaller than the grounding electrode conductor so I sized this at 400MCM as well.

The note (a) for table 250.66 seems to indicate that I only need to size this based on the total mil area of conductors in one conduit not all of them together as do sections of 250.30(A)(3)(a). This just doesn't seem right to me since a fault in the raceway could be fed from as many points as we have parallel runs. Therefore it should be sized to carry the full fault current. I must be misinterpreting something in the NEC. Any clarification would be appreciated.

RE: Grounded Conductor size in parallel runs at service

Consider parallel runs from A to B.
At b, all the conductors contribute to a downstream fault. All the EGCs are common and all share the fault current.
Consider a fault midway on one feeder. The fault is fed by A but is limited by the impedance of one set of cables rather than the total conductors in parallel.
Yes, there is also a back feed from B. For simplicity neglect the voltage drops on the healthy conductors from A to B.
Again the current from B to the fault is limited by the impedance of a single set of conductors from B to the fault.
Therefore the same size EGC that serves a single set of conductors will be suitable for one set of conductors in parallel.

"Why not the best?"
Jimmy Carter

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