Greetings All;
I'm wondering about the affect on compliance with Article 240.21(C)(4) when installing a Transfer switch between a transformer and a remote electrical building (MCC inside building).
Case 1:
The station has a customer owned transformer located approx 1000 feet in from the road, where the utility meter and service disconnect switch are located. There is an electrical building with an MCC inside located another 500’ in from the transformer. As per NEC Article 240.21(C)(4): “Outside Secondary Conductors of Unlimited Length”:
Secondary conductors located outside a building or structure can be of unlimited length, without overcurrent protection at the point they receive their supply, if they’re installed as follows:
What then If we install a Transfer switch near to the transformer and wired between the secondary terminals of the transformer and the incoming feeder circuit breaker in the MCC in the electrical building ? :
The actual transformer secondary conductors terminate at the input terminals of the transfer switch, not at a single overcurrent protection device that limits the load to the ampacity of the outside secondary conductors. The output terminals of the transfer switch then terminate at the incoming feeder circuit breaker at the MCC in the electrical building.
My question then is if a transfer switch is installed (as described above) between a transformer and an incoming feeder circuit breaker in an MCC located in the electrical building, does this comply with Article 240.21(C) ? or do we need to install a disconnect switch or circuit breaker between the transformer and the transfer switch ?
Thanks for your comments.
I'm wondering about the affect on compliance with Article 240.21(C)(4) when installing a Transfer switch between a transformer and a remote electrical building (MCC inside building).
Case 1:
The station has a customer owned transformer located approx 1000 feet in from the road, where the utility meter and service disconnect switch are located. There is an electrical building with an MCC inside located another 500’ in from the transformer. As per NEC Article 240.21(C)(4): “Outside Secondary Conductors of Unlimited Length”:
Secondary conductors located outside a building or structure can be of unlimited length, without overcurrent protection at the point they receive their supply, if they’re installed as follows:
- The conductors are protected from physical damage within a raceway or manner approved by the authority having jurisdiction.
- The conductors terminate at a single overcurrent protection device that limits the load to the ampacity of the outside secondary conductors.
- The terminating overcurrent protection device for the outside secondary conductors is part of the building feeder disconnect.
- The building feeder disconnecting means is readily accessible either outside the building or nearest the point of entrance of the outside secondary conductors.
- The transformer secondary conductors are run in galvanized rigid conduit from transformer to the electrical building, approx 500’ away.
- The transformer secondary conductors terminate in an incoming feeder circuit breaker which is the input of an MCC in the electrical building.
- The terminating overcurrent protection device for the outside secondary conductors is a circuit breaker which is the disconnecting means for the electrical building.
- The building feeder disconnecting means (incoming feeder circuit breaker in this case) is readily accessible and is located inside the electrical building at the wall where the transformer secondary conductors enter the building.
What then If we install a Transfer switch near to the transformer and wired between the secondary terminals of the transformer and the incoming feeder circuit breaker in the MCC in the electrical building ? :
- The transformer secondary conductors are the same size as the conductors from the output terminals of the transfer switch to the incoming feeder circuit breaker in the electrical building/MCC.
- A generator, with it’s own circuit breaker (same or lower rating for loads being protected by the generator) is installed and wired to the backup input terminals of the Transfer switch.
- (1) The transformer secondary conductors from the transformer to the transfer switch, and from the transfer switch to the electrical building are run in galvanized rigid conduit.
- (4) The building feeder disconnecting means (incoming feeder circuit breaker in this case) is readily accessible and is located inside the electrical building at the wall where the transformer secondary conductors enter the building.
The actual transformer secondary conductors terminate at the input terminals of the transfer switch, not at a single overcurrent protection device that limits the load to the ampacity of the outside secondary conductors. The output terminals of the transfer switch then terminate at the incoming feeder circuit breaker at the MCC in the electrical building.
My question then is if a transfer switch is installed (as described above) between a transformer and an incoming feeder circuit breaker in an MCC located in the electrical building, does this comply with Article 240.21(C) ? or do we need to install a disconnect switch or circuit breaker between the transformer and the transfer switch ?
Thanks for your comments.