Residential Emergency Generator

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Prucinski

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Does NEC provide a "code" for the hook-up of a single generator ( 67 KW) to two separate residences from the same utility transformer (from the street)?

I envision that each would have a separate transfer but only one would be ?sensing? as would be the case if you had two separate panels on the same house.

Thanks
 
This thread had been temporarily moved to an area accessible only to the Moderators. This action was taken because the nature of the question and the occupation shown in the Original Poster's profile raised a concern that this may have been an attempt by a Do It Yourself person to obtain "how-to" information.

In an exchange of Private Messages, the Original Poster has provided assurances that he is a licensed general contractor, that the installation work would be performed by licensed electricians, and that the question had to do with a business issue. Therefore, the thread is being returned to the active area. I offer an apology to the OP for the inconvenience and the delay.
 
Can you elaborate on the setup a bit more? Where is the transformer in relation to the generator?

Who pays for the fuel for the generator, if it serves two houses?

As it stands, it's tricky to see how the NEC applies to this without some background.
 
George, now that you ask, the first thing that comes to mind is that the neutrals must be switched, as they shouldn't be interconnected, thus rendering them SDS's.
 
The first thing that comes to mind for me is that the NEC is on shaky ground at all, if these residences are supplied from a power company. Half the equipment sounds like it's on the other side of the line, IMO.
 
If both residences had service rated automatic transfer switches, and only one transfer switch was arranged to actually start the generator, that setup could be made compliant I believe.
 
Emergency Generator is one of those misused terms in the NEC. What you are installing is a Optional Standby System to watch the superbowl when the power goes out.
With an OSS you can do pretty much what you want, even run a 200 A ATS from a 5 KW generator.
However you need to comply with grounding rules for generator and disconnecting means for multiple services, including the signs that no one installs. Well maybe George does.
 
Same transformer, and two different properties supplied from one transformer is fine. Using one generator for both properties... I do not believe that would work with the Utility company or with the Building departments. I am curious as to what the result will be.
 
We installed an ATS for a customer who was using the generator from the "neighbor" --->

The customer was a radio station transmitter site, and their "neighbor" was a Verizon cell tower site.

They were both supplied by the same utility transformer, with both electric meters mounted together on the same pole. Those meters were over 100 yards away from their respective buildings.

The gen set was a diesel fueled monstrocity that had sufficient oomph for both loads. It's status was determined by the power availability from the Verizon side only.

This setup works great.

As for switching off the neutrals, forget it. We are dealing with 2 customers who have rather large grounding grids for lightning protection: connected to the transmitter tower (they both share the same tower), and an extensive network of ground rods, and flat grounding tape(for lack of a better term) buried on the premises. All connected to the system neutrals.
 
I just don't see a problem with this proposed installation at all. Each home owner can pull power from the generator as needed. It can be a whole-house set-up where the generator supplies the entire service or a sub-panel arrangement on each or both. The automatic transfer switches should both have a set of start contacts that close when power at the utility lugs of either switch is lost. So by wiring the start contacts in parallel to the generator start signal circuit, either switch can start the generator as needed. I would think that switched neutrals in the ATS's would be required in this case so that parallel neutral paths are avoided. The only problem I can think of is "who pays for the fuel?"
 
"who pays for the fuel?"
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IMHO in most cases if one of them is supplying the generator the other will gladly pay the fuel costs. Or they could conceivably split all costs right down the middle.:smile:
 
I don't see a problem with this either, as long as the installer knows what they are doing.

I think Larry is right about switching the neutral. You don't want current flowing from one house to the other through the grounded conductor going to the transformer. (In fact, separate transformers might make this simpler.)

If you were my neighbor, I might try to convince you that sharing the generator is required. It's right there in Chapter 12, Part III, 12.121..... :smile:
 
georgestolz said:
Can you elaborate on the setup a bit more? Where is the transformer in relation to the generator?

Who pays for the fuel for the generator, if it serves two houses?

As it stands, it's tricky to see how the NEC applies to this without some background.
Click to expand...

The 67kw generator would be sitting between the two residence but on the new house lot.

The new house will have (2) 200 amp breaker panels coming off the meter. This will require two transfer switches (one for each panel). The house next door (25 feet from the generator location) has a 200 amp service also. Thus, yet another transfer switch here.

Both feed from the same utility transformer. Thus, if power from the public utility goes out, both houses would loose power at the same time.

Thus, we would be dealing with 3 transfer switches, but only on would be ?sensing?.

The generator would be in plan view of both meter cans.

I?m told that both parties will share the cost 50/50, including fuel, maintenance and the such.
 
Well, whether the neutral is switched or not (seperately derived system or not) neutral current will flow between buildings because the building's neutrals will be connected (either by the equipment grounding conductors or the neutral).

It could be construed as a 250.6 and 310.4 violation; but interconnected metallic municipal water systems do the same thing. I'm not sure I favor one method over another in this case.

I'd say the best design would have the EGC to the generator as being the same size as the service neutrals from the utility, but I don't think it would be required. I look forward to seeing more opinions on this setup, for sure. :)
 
Sounds like you need three service entrance rated 200A 250V 3P ATS's. You will also need three circuit breakers on the generator or a distribution panel at the generator to distribute power to the three switches. If it were me, I would just pull a start circuit to all of the switches. Most ATS's have start contacts in them anyway so why not? If one neighbor should happen to lose power due to a damaged service drop cable or something similar, the system would function for just that one switch. Also, when utility power returns after an outage, the generator would continue to run until the last switch has transferred back to utility. Just a suggestion :)
Good luck with the install!
(Man, I love generators and transfer switches!!! They are the most exciting part of my job!)
 
George
I agree with you 100% This is similar to the argument that current can travel from house neutral to house neutral through the ground creating a parallel path. But in this case were are using actual grounding conductor paths to create this situation. I had not thought about upsizing the grounding conductors voluntarily. Good idea! I feel reasonably sure the current traveling through the grounding system would be small in most cases. But due to so many "unknowns" I also feel upsizing the grounding conductors would be a good idea.
 
ramdiesel3500 said:
George
I agree with you 100% This is similar to the argument that current can travel from house neutral to house neutral through the ground creating a parallel path. But in this case were are using actual grounding conductor paths to create this situation.
Click to expand...
I was not including the earth in the circuit above - From house neutral to house neutral via a continuous metal water pipe between buildings, was the path I was thinking of.

The earth itself doesn't concern me, IMO. (Obviously, I'm half suspecting I misunderstood what you just said. ;) )

I feel reasonably sure the current traveling through the grounding system would be small in most cases.
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I agree, which is why I suspect I'm going to get ripped for the suggestion at some point. :D

Edit: You know what would be really nifty in this case? An EGC with the service conductors, neutral and EGC bonded at the utility transformer, and unbonded services. Not legal, but it would be nifty. ;)
 
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georgestolz said:
Edit: You know what would be really nifty in this case? An EGC with the service conductors, neutral and EGC bonded at the utility transformer, and unbonded services. Not legal, but it would be nifty. ;)
Click to expand...

Yep! That would solve the problem! ;)

This site is great! George, you and the other regulars on this site are a huge wealth of knowlege and information. Thanks! I learn something new every day! :)
 
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