Generator Install

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steve66

steve66

Senior Member
Location
Illinois
Occupation
Engineer
A client has a 10KW portable generator that he would like to use as backup power for a new house. The generator has a 240V, 40A NEMA 6-50 receptacle on it.

When I look at a 6-50 receptacle, it has two current carrying contacts, and a ground (per exhibit 406.2 in the handbook). Can I use the ground terminal as a neutral to get (2) 120V phases?

Thanks in advance;
Steve
 
Re: Generator Install

Here is how I think this should work:

1. Provide NEMA 6-50 plug and 3 wire SJOW cord to connect to the generator.

2. Terminate the cord in a Jbox. Here I would bond the neutral wire to the neutral and ground wires going to the transfer switch.

3. The transfer switch would be a 3 pole model, and the ground wire would be solidly connected to the service ground.

Does this sound right? Do I need an additional ground rod at the Jbox where I bond the N and G?

Any help is welcome:

Steve
 
Re: Generator Install

Steve, the final connection depends on whether the generator neutral is bonded to it's frame, or not.

If it is not, Diagram 1 is recommended.

If the neutral-to-frame bond exists, it will need to be configured as an SDS, Diagram 2.

In either case, an equipment bonding conductor will be required. (If it is cord-connected, 14-50 or L14-50)

Ed

Gen4.gif


Gen5.gif


[ July 19, 2004, 12:09 PM: Message edited by: Ed MacLaren ]
 
Re: Generator Install

Ed:

Thanks for the quick reply, but I can't do either of these exactly. The generator has a 3 prong receptacle, so I only have 3 connections: 2 lines and 1 connection that has to serve as the neutral and ground. I also can't clamp a ground wire onto the frame since it is portable.

Steve
 
Re: Generator Install

I only have 3 connections: 2 lines and 1 connection that has to serve as the neutral and ground
Click to expand...
This generator was very likely intended for portable (cord-connected) loads only,

To safely wire it into a premise wiring system, by way of a transfer switch, the receptacle would have to be replaced with a 4-wire type.

Ed

[ July 19, 2004, 12:46 PM: Message edited by: Ed MacLaren ]
 
Re: Generator Install

Ed:

I did wonder about that, but how many cord and plug appliances would fit a NEMA 6-50 outlet? Especially in a residential application?

Steve
 
Re: Generator Install

how many cord and plug appliances would fit a NEMA 6-50 outlet?
Click to expand...
That was the point I was trying to make in my last post.

If that was the only receptacle installed by the manufacturer, this generator was not intended to supply "residential" loads.

Ed
 
Re: Generator Install

I agree with Ed, but would like to ask:

Do you have an installation manual from the manufacturer?
Did you contact the manufacturer?
What brand is it so we can look it up?
 
Re: Generator Install

What is the difference between a back up generator and a sub panel 4 wire all the way but is it UL listed to do this??A portable generator is just that portable and a back up generator for a home is built and listed for this.If not mistaken the big orange has a gen set for just this application.Not sure if it is 4 wired or not.Will have to stop by tommorow to see.
 
Re: Generator Install

I don't have a manual, but I will get one. Here is what I found on the web:

John Deere Generator

It is made to be ran from a garden tractor. It also has a couple of NEMA 5-20 outlets, but I can't get 10KW out of those.

Steve
 
Re: Generator Install

It has been my experience that many of these "portable" generators were intended for use only with loads that are cord connected to the generator's manufacturer-supplied receptacles, for example, for RVs or at construction sites.

Here (Canada), the generators that are intended for connection to a residential premises wiring system come with a 4-wire twistlock receptacle and are required to have a label indicating whether or not the generator winding neutral is bonded to it's frame.

Ed
 
Re: Generator Install

Something else I noticed is they say it has a voltmeter for adjusting "engine speed". If you have to adjust the engine speed to get the right voltage, I don't think I would want to connect it to my house.

Steve
 
Re: Generator Install

Is Neutral isolated in both diagrams at transfer switches?

[ July 20, 2004, 01:46 PM: Message edited by: trip ]
 
Re: Generator Install

Our AHJ's require portable generators (120/240) to be rewired with a 4 conductor cord.

most portable gensets have a neutral to frame connection. CMP-5 recoginzes this is a violation and they don't feel its all that important.
 
Re: Generator Install

Be sure to check the output voltage on the outlet from the ground to the 2 hot wires. A typical hookup for a small generator is to connect the two hots to the outlet and connect the chassis ground to the 3rd terminal. Ground, not neutral! - therefore no neutral or center tap is connected to the alternator windings and the outlet and thus the system voltage is 240 only, not 120/240.
Warren Buchter
Generac Power Systems
 
Re: Generator Install

I have a schematic for the generator. There are two windings. Each feeds a 120V outlet, and then the two together feed the 240V NEMA 6-50 outlet.

The ground terminal on the NEMA 6-50 outlet is connected to one side of each winding, and bonded to the generator case. The manual says there is a ground strap for a earth ground.

Can I use the ground terminal on the outlet as a neutral to split the 240V into (2) 120V phases?

Steve
 
Re: Generator Install

Can I use the ground terminal on the outlet as a neutral to split the 240V into (2) 120V phases?
Click to expand...
It appears from the description in your last post that the receptacle "ground" is, in effect, a winding center tap, and it is also bonded to the case.

If this is true, the short answer to your question is - yes.

But, for safe operation, there should be an equipment grounding conductor between the generator and 3-pole transfer switch, requiring a four-wire feeder, and the generator center-tap/neutral should be connected to an acceptable grounding electrode, as shown in my Diagram 2 above.

[ July 21, 2004, 12:50 PM: Message edited by: Ed MacLaren ]
 
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