Grounding Parallel Feeders
Question: When running feeders in parallel, for example a 400 ampere motor control center, how do you size the ground (EGC) in the raceways? What if you are paralleling MC Cable?
Answer: If the parallel raceways are rigid steel conduit, intermediate metal conduit or electrical metallic tubing, an additional wire type EGC is not required.
If you plan to install a copper, aluminum or copper clad aluminum equipment grounding conductor, you would be required to install a 3 AWG copper or a 1 AWG aluminum or copper clad aluminum in each parallel path based on the 400 ampere overcurrent device.
If Type MC Cable is used, the manufacturer must be told the overcurrent device rating at the time of the order. This allows the manufacturer to provide the proper size equipment grounding conductor, since standard MC Cable is manufactured based on the maximum overcurrent device permitted to protect a single cable run.
References: 310.10(G), 300.5(I) Exception 1, 250.118, 250.122(A) and (F) and Table 250.122