There are two medium voltage services from the utility to the customer. These are used to feed two customer-owned padmounted 480/277V transformers. These transformers feed an outdoor service entrance rated, 400A, 3-pole transfer switch. The transformers are located on a common pad and have a common ground ring. The transfer switch is strut mounted with a ground rod triad. The conduits from the transformers to the transfer switch are direct buried PVC. The customer owned transformers are considered SDS.
The plan was to make a N-G connection at each transformer and a N-G connection at the transfer switch with no supply side bonding jumpers in the conduits (phase and neutral conductors only). This seems to be allowed by 250.30(A)(1)Exception 2. Is my thinking correct here? The exception mentions parallel paths. The concern became the neutral from both systems are connected at the transfer switch and at the common ground ring at the transformers. In a fault condition current could return on both neutrals to the one transformer. Is the connection to a ground ring considered a parallel path?
In further thinking we don't need the neutral at the transfer switch. There are no neutral loads. Are we required to take the neutral to the transfer switch from an SDS transformer? I know for a service transformer yes, but I can't find a requireement for an SDS transformer. Can a N-G connection be made at each transformer with 3 phase and 1 supply side bonding jumper (no neutral) to the transfer switch? Because there are no neutral loads it seems the same as the original scenario except a different color on the wire.
Any thougs are appreciated, my head now hurts.
The plan was to make a N-G connection at each transformer and a N-G connection at the transfer switch with no supply side bonding jumpers in the conduits (phase and neutral conductors only). This seems to be allowed by 250.30(A)(1)Exception 2. Is my thinking correct here? The exception mentions parallel paths. The concern became the neutral from both systems are connected at the transfer switch and at the common ground ring at the transformers. In a fault condition current could return on both neutrals to the one transformer. Is the connection to a ground ring considered a parallel path?
In further thinking we don't need the neutral at the transfer switch. There are no neutral loads. Are we required to take the neutral to the transfer switch from an SDS transformer? I know for a service transformer yes, but I can't find a requireement for an SDS transformer. Can a N-G connection be made at each transformer with 3 phase and 1 supply side bonding jumper (no neutral) to the transfer switch? Because there are no neutral loads it seems the same as the original scenario except a different color on the wire.
Any thougs are appreciated, my head now hurts.
