emergency/standby power

[rapf]

I have two buildings,each with 100A 208v 3phase 4 wire metered service supplied by power company.I want to use one emergency generator and two transfer switches with overlapping neutrals to supply power to any or both of the buildings if required. Do you see any technical problems,safety problems or nec code violation here as I am dealing with two services and not within one service. Note the two buildings are adjacent and the power derived from the same overhead HT lines.

 

[dpc]

I don't think you want to tie the two low voltage neutrals from two separate services together - they should remain separate. So you may need a four-pole transfer switch.

Also, you can't use a standard transfer switch as service entrance equipment since it doesn't provide any overcurrent protection. If you're talking about a 3-way fused disconnect switch, that might work. Or a circuit-breaker type transfer switch (Cutler-Hammer).

 

[advidana]

Check with your generator representative, he will design the system with the proper electrical protection for you. I also agree with dpc.

If you are going to use this as a bid document , you would be best to use two generators, one for each bldg or spend the money to get an electrical engineer to design a system with one generator because it will be complicated.

 

[rbulsara]

Nothing wrong with the concept you are proposing.

I believe 'overlapping N" is a 4 pole switch.

As for the service entrance, (not part of your question but respnding to some comments above), ATS for code required 'emergency' power can not be a service entrance by itself. You need to have a normal panel feeding a 'emergency panel' through the ATS. So that ATS itself does not become single point of failure. This ensures that the building will have two sources available, and at least one when the other is not available.

Putting entire building on generator by feeding the main board with ATS does not meet Code for emergency source. It is OK for 'stand by' application.

 

[peebee]

I generally agree with the above.

4-pole switches required, plus a ground electrode and neutral-ground bond at the generator.

"Emergency" = "Life Safety". Your ATS's can supply NEC Article 700 loads only, and cannot serve non-life-safety loads. The generator can serve both life-safety and non-life-safety loads, but separate additional ATS's would be required for non-life-safety loads.

Are these REALLY emergency loads? If you just wanna back up your fridge and TV, not provide emergency lighting, then you can ignore this issue, you're covered by NEC 702 (Optional Standby Systems) rather than 700 (Emergency Systems). Make sure you get this straight, and DON'T use the term "emegency" if it's not life safety ("generator source", "standby source", "standby generator", and "alternate source" might be handy terms to use).

Beware that NEC 700 would require your generator to have sufficient capacity to simultaneously serve all emergency loads from both buildings if this is truely an emergency system.