portable generator transfer switches

[brian ploskunak]

I need to install a transfer switch on a 120/208V 400A service for the use of a future portable generator. After looking at exhibit 250.6 & 250.7 in the handbook, I am figuring that the use of a transfer switch that does not break the neutral would be best for portable generators since most portables are hooked up without a separate ground rod. Am I correct in this thinking?

 

[charlie b]

Re: portable generator transfer switches

No you're not right. A portable generator must have a ground rod, unless it meets the conditions of 250.34. If you are going to connect it to a transfer switch, then it won't meet those conditions. Therefore, it will need a ground rod.

But the presence of absence of a ground rod is not the determining factor for using a transfer switch that does, or does not, switch the neutral. That is a design choice. The important thing is that, once you have made the choice, you do the wiring correctly.

 

[iwire]

Re: portable generator transfer switches

Originally posted by charlie b:
If you are going to connect it to a transfer switch, then it won't meet those conditions. Therefore, it will need a ground rod.

Charlie you may not have seen them but many smaller residential transfer switches are cord an plug connected.

For that matter large commercial models can be cord and plug connected if they are optional standby units.

.....The generator supplies only equipment mounted on the generator, cord-and-plug-connected equipment through receptacles mounted on the generator,....

When the power fails you roll the generator out of the garage etc. then plug it in to a male 'inlet' mounted at a easy location.

So IMO the answer for Brian is no ground rod if the generator is cord and plug connected.

 

[brian ploskunak]

Re: portable generator transfer switches

Charlie, what are the exhibits 250.6 and 250.7 in the handbook refering to?

 

[charlie b]

Re: portable generator transfer switches

I don't own the handbook, so I can't address what those figures show.

 

[charlie b]

Re: portable generator transfer switches

Originally posted by iwire: Charlie you may not have seen them but many smaller residential transfer switches are cord an plug connected. . . . So IMO the answer for Brian is no ground rod if the generator is cord and plug connected.

You are right: I haven't seen them. But then, I haven't seen a portable generator in use. I would not expect a transfer switch to be plug and cord connected. I would also have thought that 250.34 was referring to plug and cord connected utilization equipment (e.g., lights, a fridge, the TV), and not to plug and cord connected transfer switches that serve panels. But looking at the wording, nothing prohibits such a connection.

But that was just a side issue. The question really has to do with choosing a transfer switch with, or without, the ability to switch the neutral. I would like to see a good list of design considerations that favor one over the other.

 

[iwire]

Re: portable generator transfer switches

Unless there is main GFP in place I see no reason to switch the neutral.

I would leave the generator 'un'-bonded and use a solid neutral switch.