I am running service to a small, 1 room office, at a barn in the country. The electric utility pole has a transformer (230 V, 1 phase) and 200 amp meter. On the pole I have a 200 amp circuit breaker. From the circuit breaker on the meter pole, I have 4/0 4/0 2/0 AL cable in 2" PVC conduit running 290' to the office, where I have another 200 amp breaker in the main breaker box. For equipment ground, I have a bare, #4 copper wire running in the ditch between the circuit breaker on the meter pole and the main breaker box at the office.

I have two questions:

1. Is the neutral conductor size (2/0 AL) satisfactory? Why can the neutral be a smaller cable than the hot legs, which are 4/0? The ditch is still open and, if necessary, I can run a single conductor of 4/0 in the ditch for neutral. Don't want to do this, but would like to know if and why 2/0 is satisfactory.

2. I have the equipment ground and neutral bonded at the breaker on the meter pole. They are currently not bonded anywhere "downstream" of the meter pole. Should I also bond the neutral and equipment ground at the office main breaker? This would result in the neutral and equipment ground being bonded at two places - once at the circuit breaker at the meter and again at the main disconnect at the office. What are the pro's and con's of bonding at two locations?

Thank you!

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Originally Posted by bertfisher

I have two questions:

1. Is the neutral conductor size (2/0 AL) satisfactory? Why can the neutral be a smaller cable than the hot legs, which are 4/0? The ditch is still open and, if necessary, I can run a single conductor of 4/0 in the ditch for neutral. Don't want to do this, but would like to know if and why 2/0 is satisfactory.

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Per NEC 220.61-Reduced neutrals are allowed if the service calculation and neutral current calculations are performed. Never allowed to be less than 70% of the line conductors. URD cables are available but only are to be used if the calcs were done to be sure the neutral is of sufficient size. If calcs were not done, then a full size neutral is required.

Since the neutral is the return for all 120 volt circuits, the neutral will carry a minimal mount of current, the amount of the unbalanced load. To illustrate, if you have two 20 amp circuits with a shared neutral, and each circuit is at full load, the neutral will have zero amps on it. If the same circuits one is at 20 and the other is at 16, the neutral will be carrying 4 amps.

The calcs must be done, and all the circuits need to be laid out and planned in the panel board to be sure of the unbalanced current that will flow in the neutral.

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Originally Posted by bertfisher

2. Should I also bond the neutral and equipment ground at the office main breaker?

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Per NEC 250.24 (A)(1) through (A)(5)-No, do not connect the neutral and equipment ground at any other point other than at the Main Service Disconnect. What you have done is perfectly correct.

The reason is a bit confusing to laypeople, and unfortunately some electricians. Primarily, the system equipment grounding needs to have as low impedance (AC resistance) back to the Main Neutral as this is the point the Neutral is grounded. This allows the Main circuit breaker and all other downstream breakers to operate quickly during a fault condition. There are other reasons beyond the scope of this forum.[/QUOTE]

I hope this helps.