Vehicle Generator Neutral Bonding?

[enigma]

Hi All,

I am working with a generator set that is installed in a truck for auxiliary power. The generator supplies loads only on board the truck and is connected through a transfer switch to the loads. The other side of the transfer switch allows "shore power" to feed the loads as well. The transfer switch switches both the hot and neutral sides.

I'm trying to figure out what the NEC stipulates for both grounding and bonding. Is the generator required to be grounded to earth? Is the neutral required to be bonded to the generator frame at the generator?

Thanks!

 

[coulter]

enigma -

Look at 250.24.B (2005). That should be your guide. When you are connected to shore power, the shore power will provide your frame earth connection.

carl

 

[enigma]

enigma -

Look at 250.24.B (2005). That should be your guide. When you are connected to shore power, the shore power will provide your frame earth connection.

carl

That is just saying that the the neutral must be bonded to the metal frame of the genset, correct?

 

[coulter]

It doesn't address bonding the gen N-G (unless I missed something - which is entirely possible). It only addresses bonding the gen frame to the vehicle frame and not connecting the vehicle frame to earth ground.

Gen N-G bonding is a diferent section. Try 250.30

carl

 

[tom baker]

Yes but:
90.2(B) Not Covered. This Code does not cover the following:
(1) Installations in ships, watercraft other than floating buildings, railway rolling stock, aircraft, or automotive vehicles other than mobile homes and recreational vehicles
Thats not much help.
But in Article 551 RVs, the generators there don't follow the rules in 250.
The challenging part is where is the system bonding jumper?
If used standalone, a SBJ is required. When connected to the utility supply, a SBJ must not be installed.

 

[coulter]

Whoops - nobody, including me, noticed my first code reference should have been 250.34.B:-?

enigma -
Hopefully this section will make more sense.

carl

 

[coulter]

... This Code does not cover the following: ...Installations in ...automotive vehicles ...
But in Article 551 RVs, the generators there don't follow the rules in 250.

Tom -
Excellent reference. However, I've always tried to not let the NEC stand between me and a good, well designed, reliable, safe, job.

And, the heading on 250.34, "Portable and Vehicle-Mounted Generators" would appear to apply.

...The challenging part is where is the system bonding jumper?
If used standalone, a SBJ is required. When connected to the utility supply, a SBJ must not be installed.

Shouldn't be tough at all. Just have to pay attention to the physics and a few regulatory ssues.

carl

 

[coulter]

enigma -
I sure didn't do you any favors with my fat-fingered typing on my first post.

Here is my recomendation:
1. Assuming 1ph, 120/240
2. References: (2005)
a. 250.34.B
b. 250.30

Consider the generator an SDS. Bond the Gen N-G. Bond the gen frame to the vehicle frame. Come out of the gen to a 3W transfer switch. Switch both hots and the neutral.

The other input to the transfer switch is the shore power connection. This connection will need to be 4W - two hot and the neutral go to the transfer switch, shore power grounding conductor to the vehicle frame.

Come out of the transfer switch to the equipment power panel with 4W - two hots, neutral, and grounding conductor. Bond all of the utilization equipment to the frame.


Alternate connection: (possibily simpler, easier)
Don't install a transfer switch.

Install a 4W male cord and plug to the equipment power panel input- two hots, neutral, and grounding conductor. Grounding conductor connects to vehicle frame.

Generator gets an N-G Bond and a 4W receptacle. Bond gen frame to vehicle frame

Shore power extension cord is 4W - two hots, neutral, grounding conductor. Again, the shore power has to supply a grounding conductor.

Plug the equipment panel into either the gen or the shore power.

Either way you do this:

The gen is an SDS. It has an N-G bond and the gen frame is connected to vehicle frame.

There must be a 3W transfer system (must switch the neutral).

When you are connected to shore power, the shore power connection supplies an equipment grounding conductor.

I hope this is clearer that my last try.

carl