Neutral Ground Bond

[d]

I have a shelter that has an onboard generator as well as a shore input. The generator is hard wired to an off the shelf panel that is normally used for home installation. The generator and shore input are back fed through their own breaker. There is a switch on these breakers to prevent them both from being closed at the same time. The panel is a 240V single phase panel and the backfed breakers are 2 pole breakers.

There is also a neutral ground bond at the generator which is fine when running on generator power. When the shelter is on shore power, however, it is assumed that the shore source has a neutral ground bond somewhere up stream. I believe this is a problem? Any ideas on how I break the neutral ground bond at the generator when running on shore power easily so that an unskilled operator of the shelter can make the switch?

Another question is since the input power is 208V or 240V single phase and all loads are 208V or 240V single phase is the neutral conductor even necessary?

 

[augie47]

Since you are using a breaker as your "transfer switch" and since you generator has a neutral-ground bond, you might look at installing a switched neutral breaker.

 

[d]

Looked up switched neutral breaker offered by Square D for the QO panel we are using. Highest rated breaker is 50A. I need 60A. Any other ideas. Is it an issue if the neutral is not being used?

 

[RUWired]

Is it an issue if the neutral is not being used?

Not needing the neutral conductor from the generator could actually be to your benefit. Remove it and your parallel conductors to the panel will be gone.

Confirm the system bonding jumper is in the generator, confirm the main bonding jumper is out of the load center panel. Install an equipment bonding jumper from the generator to the load center panel sized to T250.66 and attach it to a seperate equipment ground bar if not already installed.

Rick

 

[erickench]

Instead of using two-pole breakers for the backfeed why not just use three-pole and then you'll be able to switch the neutral. As it stand's now the generator is not separately derived.

 

[hurk27]

If the neutral to ground bond is at the service, then all you need to do is remove the neutral to ground bond at the generator, nothing else, fault current will still flow through the service neutral bond.

If your not on the 2008, 250.32(B)(2) will allow a 3-wire feeder from the service if all the conditions are met, which will put the neutral to ground bond in this local panel, making the distance to the MBJ shorter, if there is a problem for fault current distance path?

 

[hurk27]

Instead of using two-pole breakers for the backfeed why not just use three-pole and then you'll be able to switch the neutral. As it stand's now the generator is not separately derived.

because the third pole in this panel will be a hot not a neutral, Square D makes a little interlock device for only QO 2 two pole breakers, these breakers are mounted side by side in a single phase panel, so using a three pole will not work, but there is a neutral switching type, but it only goes up to 50 amps.