ONE GEN TWO BLDGS

Status
Not open for further replies.
P

PETER4

Member
Hello,

I'm connecting one generator to serve two separate buildings, we already have two ATS's 3poles, but when the generator it is not running the neutral and the EGC wires are going to be connected for both buildings, has anybody deal with the same case before? Do I have to go with a 4 pole Ats's?

Thanks for you inputs!
 
Re: ONE GEN TWO BLDGS

It seems as though creating a parallel path for neutral through the equipment grounding conductors is unavoidable, having two buildings fed from one generator, in my opinion.

Is there any chance of buying two generators?

Can you give us some specs on the services, the generator, etc?
 
Re: ONE GEN TWO BLDGS

Originally posted by georgestolz: It seems as though creating a parallel path for neutral through the equipment grounding conductors is unavoidable. . . .
Click to expand...
Creating a parallel path to where? Current seeks its own source. Current from one utility connection will go to a load in the one building, and return to the same source via the neutral in the same building. It does not matter that that neutral is connected (via the generator's neutral) to the neutral of the other building. There is no driving force to push current along that path.

Therefore, there is no driving force to push current along the "parallel path" (i.e., neutral return current in one building to the N-G Bond in that building, then via the grounding electrode systems of the two buildings to the N-G Bond in the other building).
 
Re: ONE GEN TWO BLDGS

Charlie, without more information, I guessed that it was possible that the two services were supplied by the same utility transformer. In that case, we got problems, I think. :)
 
Re: ONE GEN TWO BLDGS

I've got a question: I've always noticed what I presumed to be a grounded conductor running on power lines. Is that truly a grounded conductor, and is there a connection made to it in all utility transformer connections?

As in, if there were two different single-phase transformers, or two sets of three-phase transformers, would the load-side neutrals use the high-voltage neutral to complete a circuit? Or is there no direct connection?
 
Re: ONE GEN TWO BLDGS

Building No.1
Service : 120-208V/3Ph/60Hz
Max Peak Demand: 41.5 KVA - 116 Amps
ATS No. 1 = 200 Amps / 3 Poles
N-G Main Bonding jumper at Main Disconnect switch

Building No.2
Service : 120-208V/3Ph/60Hz
Max Peak Demand: 81.5 KVA - 227 Amps
ATS No. 2 = 400 Amps / 3 Poles
N-G Main Bonding jumper at Main Disconnect switch

New Generator:
200KW Diesel Stand-by 120-208/3Ph/60Hz
Two circuit mounted breakers, 200A & 400A
No N-G bonding.

Those two building are under single management and are feed from diferent utility transformers.
The two buildings are aprox 200ft apart.

I think that if the two bldgs were feed from the same utility transformer the neutral is also "share" for both buildings. Because the services are coming from different utility tansformers then.... I don't know, but I agree with Charlie b "There is no driving force to push current along that path" Do you see any code violation doing those electrical connections?

THANKS AGAIN!!
 
Re: ONE GEN TWO BLDGS

Well, I stuck my foot in a hole on this one. I'm not sure if the primary utility "neutrals" are interconnected. If they are, you got problems. If not, I think you're good. (I have a hunch they aren't.)

In response to the original question, if the neutral is bonded to the frame of the generator, then you need a four pole ATS to interrupt the neutral, or that is a parallel path. If it's not bonded, a three pole ATS is fine. So you appear to be ok there.

I'm not sure in your description what you're referring to with:
N-G Main Bonding jumper at Main Disconnect switch
Click to expand...
:confused:
 
Re: ONE GEN TWO BLDGS

The utility primary and secondary neutrals are bonded together. In many cases they are the same conductor.
Don
 
Re: ONE GEN TWO BLDGS

I think that if the two bldgs were feed from the same utility transformer the neutral is also "share" for both buildings. Because the services are coming from different utility tansformers then.... I don't know, but I agree with Charlie b "There is no driving force to push current along that path" Do you see any code violation doing those electrical connections?
Click to expand...
The paths are parallel and there will be current on all available paths. The current in each of the paths will be based on the impedance of the paths.
This is a violation of 250.24.(A)(5). The connection of the grounded conductors at the generator results in a second connection to ground on the load side of the service disconnect. The second connection to earth for building 1 is the main bonding jumper in building 2. The second connection in building 2 is the main bonding jumper in building 1.
Don
 
Re: ONE GEN TWO BLDGS

Answering peter4
If you bond the genset neutral to ground you will need the 4p ATS, so don't ground the gen. neutral. But do make sure that the frame is grounded!
Assuming (1)The gen. can carry full load for both buildings. (2) Your gen. falls under Art702. (3)These are commercial buildings.(4)No Fire Pump.
Unless all of your emergency and egress lighting is supported by 90min. batteries you will need a second ATS in each building to comply with Art700.6(D), that is, if it is your intention to use the genset for this purpose. And make sure that the emergency wiring system complies with Art700.9.
Remember, any one of the four ATS must be able to start the genset. I think you can get by with one Ann. Pnl. in each building but talk to your genset/ann.pnl. supplier about this.
I don't see a problem with one transformer serving two buildings, utilities do it all the time. Be careful with how you handle the metering, don't let the gen. output run thru the utility meter(s). One genset for two buildings is ok too. But talk this over with the AHJ before you apply for permit! It's unusual but doable, good luck.....what a fun job to have, I envy you.(Oops, I've broken the law... Ex.20v.17)
 
Re: ONE GEN TWO BLDGS

Originally posted by bill talbot:
(1)The gen. can carry full load for both buildings.
Click to expand...
This is not a requirement for 702 standby generators.

Transfer switches that 'pick up' entire buildings are not going to be article 700 Emergency systems.
 
Re: ONE GEN TWO BLDGS

I don't see how he can feed two buildings with two services, but this is out of my depth.

</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">There would be a second path for each building's neutral current through the EGC of the generator (because the neutrals of the utility are continuous on the primary side, according to Don).</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Even if sized to the neutral, the EGC paths couldn't be the same length (310.4)</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Even if they were the same length, 250.6(A) prohibits the configuration.</font>
<font size="2" face="Verdana, Helvetica, sans-serif">So, could he use 250.6(B) to fix the problem?

I've seen 250.6(B), but never understood when and how it could be applied. Is this such a case? :confused:
 
Re: ONE GEN TWO BLDGS

Originally posted by georgestolz:
I don't see how he can feed two buildings with two services, but this is out of my depth.
Click to expand...
I will go with one issue at a time.

He is not feeding the buildings with two services, he is feeding two buildings with two services (one each) and one SDS.

A generator is never an NEC 'service'.

By the way we are assuming each building has a 'service'. If seems likely that this is some sort of campus as he is using one generator for two buildings. In that case all the buildings may be served by feeders. The 'service' may be at the property line or located in a central building.
 
Re: ONE GEN TWO BLDGS

Reply to Peter4, again.

Now we know you have two transformers!....Let us think outside of the electrical box for a minute...... what if, in the future, the Owner of these two buildings decides to sell one building??? (Never say never). The single genset now becomes a problem for someone. Think about it....
Are the two transformers owned by the utility, or by the Owner because this is a commercial campus?.... Let us assume that they are owned by the utility. The utility will not care if one building gets sold off but what they do care about is being able to work safely on their transformers and some utilities do not trust ATS's to protect their crews because they cannot easily see an open condition in the ''normal'' feeder between the transformer and the ATS. OK, so you have SE rated ATS's, can the CB's be drawn out? You may want to have a chat with the utility rep. along with the AHJ as I suggested previously.

Bill Talbot
 
Re: ONE GEN TWO BLDGS

Originally posted by iwire:
He is not feeding the buildings with two services, he is feeding two buildings with two services (one each) and one SDS.
Click to expand...
Go with what I mean, not what I say. :D

In that case all the buildings may be served by feeders.
Click to expand...
True. That would explain the "N-G bonding" statement he made. While it's a given at a service, it's not so in a detached structure.

If that's the case, he'd still have parallel paths back to that main panel where they split, if EGC's were't pulled with the detached building feeders. I think.

But I also just went crosseyed. :D
 
Re: ONE GEN TWO BLDGS

It is so hard to give a straight answer to the questions without seeing a one line diagram of the lay out.

If these are all feeders IMO there would be no choice but to switch the neutrals.

If both buildings have services IMO the NEC will not prohibit a solid neutral transfer switch.
 
Re: ONE GEN TWO BLDGS

Bob,
If both buildings have services IMO the NEC will not prohibit a solid neutral transfer switch.
Click to expand...
I still see a violation of 250.24(A)(5) if the transfer switches do not switch the grounded conductor.
Don
 
Re: ONE GEN TWO BLDGS

Originally posted by don_resqcapt19:
Bob,
If both buildings have services IMO the NEC will not prohibit a solid neutral transfer switch.
Click to expand...
I still see a violation of 250.24(A)(5) if the transfer switches do not switch the grounded conductor.
Don
Click to expand...
Don I had assumed a service entrance transfer switch which in that case I did not see a violation.

Otherwise yes I agree with you about 250.24(A)(5).
 
Re: ONE GEN TWO BLDGS

Bob,
Don I had assumed a service entrance transfer switch which in that case I did not see a violation.
Click to expand...
Even if they are service rated transfer switches the grounded conductor from the transfer switch to the generator and then to the other transfer switch would create the violation.
Don

[ January 21, 2006, 10:16 PM: Message edited by: don_resqcapt19 ]
 
Status
Not open for further replies.
Top