I have a Hangar project that is required to have a separate and dedicated power supply from the facility transformer or ahead of the main service disconnecting means for providing power to Hangar bay doors. Additionally, I was told to provide a connection for a portable generator to serve the entire building. At the moment, I am of the opinion that NEC 701 applies to the Hangar bay doors and I will need to provide separate transfer equipment for the LRS and the rest of the building. Additionally, the transfer equipment for the LRS will have to be automatic per NEC 701.5. Do you agree? The facility will be served by pad-mounted transformer approximately 50 ft away and I am to provide secondary conductors to the transformer. All transfer switches will be located adjacent to the transformer. The estimated service for the Hangar bay doors is 100A and the estimated service for the rest of the facility is 1600A. What are your thoughts on the most practical approach for implementing these requirements (e.g., two service entrance rated, circuit breaker-based transfer switches (one manual and one automatic) connected to a generator connection cabinet)?
Services, Transfer Switch(es), and NEC 701 Question
- Thread starter NEC54
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I'm no engineer but, when I was a Power Production Technician in the Air Force, when we had to connect large mobile generators to entire buildings there would be a manual transfer switch or a power operated manually controlled transfer switch as one of the Service Disconnecting Means and a similar switch as the separate Service Disconnecting Means for the special load service when there was one. The most common special service was for the building's fire pump.
We didn't have to build those systems but we did have to connect Engine Alternator Sets to supply them. Our NCOs kept insisting that the transfer switch for the mobile generators had to be 4 pole because all of the mobile generators had bonded neutrals and needed to remain that way for the other things they were used for. The engineering types finally started singing to the same sheet of music and changed a lot of them out to 4 pole, double throw, center off, enclosed switches listed for use as service equipment.
On the stationary generators I quickly learned that there is no such person as a project manager that will voluntarily buy you anything that has the word optional in it's description. We always needed a neutral isolating kit or an additional pole for the transfer switch but since both were listed in the catalog as optional they were sure we did not need either one. I was grateful that working things out with project managers was way out of my rank's work capabilities. I got kudos for making isolation kits out of aircraft window plexiglass and brass hardware to get the generator in service while the Officers worked out who was right. The power production NCOs always won the argument but by the time we had another big stationary generator to commission it would be a new project manager and new officers so the whole dance had to be done again.
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Tom Horne
Last edited:
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
A 701 system can not be supplied from a portable genset. You would have to have a complete system permanently installed. I think the debate here is should this be a 701 system at all but rather a 702 system.
If both systems are 702 systems, then it appears that I would have two 702 feeders from the outside SE-rated ATS. Would I be allowed to connect both feeders to the transfer switch output terminals per 240.21(B)(5)? I assume this would be better than trying to tap into multiple sets of conductors ahead of the switchboard.
Thanks for all of your advice.
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