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Need to know what size service is going to be installed at the end of the run.
On large service entrance services the neutral can be reduced. The amperage of the service and the voltage of the service and the insulation factor of the 500 MCM conductors need to be stated to answer this question correctly.
By rule of thumb, the neutral conductor is half the size of main conductor.
The grounding conductor of a service distribution is sized by the ampacity of the service. By just stating the wire size, this does not give this information, as many variables enter into what wires are used depending on length, ambient temperature and insulation factor to name a few.
Notes: 1. Where multiple sets of service-entrance conductors are used as permitted in Section 230-40, Exception No. 2, the equivalent size of the largest service-entrance conductor shall be determined by the largest sum of the areas of the corresponding conductors of each set. 2. Where there are no service-entrance conductors, the grounding electrode conductor size shall be determined by the equivalent size of the largest service-entrance conductor required for the load to be served. 1 This table also applies to the derived conductors of separately derived ac systems. 2 See installation restrictions in Section 250-64(a).
500
4/0 copper phases and neutral
Need to know what size service is going to be installed at the end of the run.
The size of wire for service entrance conductors for a 400 amp 3 phase service could be 500 MCM copper. This is very large size wire and difficult to bend and shape, to connect to terminals in the main disconnect and distribution panel. This problem is solved by using parallel conductors or on higher ampacities triple conductors. Using parallel conductors the wire size would be three ought (3/0) copper.
From the question it is hard to establish whether the service size is 350 amps or the service conductors are 350 MCM in parallel. If the service is 350 amps, the grounding conductor is #3 bare copper wire. Parallel 350 MCM conductors will allow for a 600 amp service. The grounding conductor for a 600 amp service is a #1 bare copper wire.
It depends on your definition of efficiency. 480 can certainly push more power than 208 through the same size conductors, but it would not be efficient to wire a motor for 480 when 208 was all that was needed.
Single phase requires 2 wires - a hot, and a return (both wires of the same size. Three phase requires three (same size), and if it is grounded will require a forth (doesn't normally carry current so it can be small). Two phase requires three wires (two phase conductors of one size, and a return that can handle double the current of the phase conductors), or four wires (same size for all, effectively two separate single phase supplies). It isn't economical is the bottom line. It costs ~the same (in wire) for two phase as three, and you get 50% more power delivering capability with three phase.
Assuming this falls under the National Electrical Code (NEC) the size for the service ground is based on the conductor size of the Ungrounded (Hot) conductors. For 150A the size of XHHW Copper would be 1/0 for the Hot conductors and #6 AWG copper for the ground. Aluminum would be 3/0 for the Hot conductors and #4 AWG aluminum for the ground.
They can be either separated or combined. What governs the choice is the size of the service and the conductors feeding the service and the ability to physically connect metering equipment to larger size conduits. When paralleling conduits make sure all the phase wires are in their individual conduits to cancel the field effect of the wires. Eg. conduit 1 - L1 + L2 + N , conduit 2 - L1 + L2 + N. Likewise with three phase, conduit 1 - L1, L2, L3, conduit 2 L1, L2, L3.
On large service entrance services the neutral can be reduced. The amperage of the service and the voltage of the service and the insulation factor of the 500 MCM conductors need to be stated to answer this question correctly.
By rule of thumb, the neutral conductor is half the size of main conductor.
The grounding conductor of a service distribution is sized by the ampacity of the service. By just stating the wire size, this does not give this information, as many variables enter into what wires are used depending on length, ambient temperature and insulation factor to name a few.