Generator To Existing Service

[Kidd]

Hi all,

I'm working a project that came across a unique (at least to me) situation.

Usually when we design a generator to an existing service, there is a service disconnect and if there isn't one, we add one to the line side of the meter. From there we go into the ATS then to the Main distribution panel.

For this project, the service conductors go from the pad mount transformer underground to an MDP (with a main breaker). The client is asking us to remove the connections from the main breaker to MDP's bussing and intercept the line side of the main breaker to feed the ATS from there. Then backfeed the same MDP panel from the ATS through a new breaker on MDP. I've been digging through the code and can only find section 702.12(E) that looks like this might be a violation of. Can anyone tell me if they've seem something similar or if this would be up to code? I'm assuming it's not, but if it is then I'm assuming we would also have to split the neutral bars and ground bars in MDP to keep them separate from the service conductors and the rest of the panel and the main breaker and back feed breaker would have to be interlocked.

I'm attaching a sketch to help visualize what I was trying to describe.

Thanks!

 

[don_resqcapt19]

I don't see any issues other that when you intercept the load side of the existing service equipment, you will have to run the service neutral along with the ungrounded service conductors to the ATS. Also you need to make sure that the neutral is not bonded at the generator or at the ATS. The service bonding will still be in the MDP.
I don't see a 702.12(E) so not sure what code section you are trying to reference..

 

[augie47]

Might be some physical limitations on your MDP. What size is the main ? and what size back feed breaker do you propose ?

 

[Kidd]

I don't see any issues other that when you intercept the load side of the existing service equipment, you will have to run the service neutral along with the ungrounded service conductors to the ATS. Also you need to make sure that the neutral is not bonded at the generator or at the ATS. The service bonding will still be in the MDP.
I don't see a 702.12(E) so not sure what code section you are trying to reference..

Thanks for the response!

Sorry! I meant 701.12(E). I just went back and double checked that. Also, I thought this might be a violation of 408.3(A)(3), but since the conductors do terminate on the main breaker then I don't think it violates that either. I don't think I've found anything, code wise, that says we can't do it this way.

As for the neutral, I figured the MDP would have (1) ground bus and (1) neutral bus for the service and (1) ground bus and (1) neutral bus for the "panel"/ branch circuit portion of MDP. I tried to sketch out what I meant with that, but the main bonding would be at the MDP.

Thanks!

 

[Kidd]

Might be some physical limitations on your MDP. What size is the main ? and what size back feed breaker do you propose ?

It's an 800 amp main, but there is enough space to add an 800 amp breaker to it as well for the back feed. I think we're good on that.

The physical aspect of this that I was worried about, was being sure to keep the line side of the main breaker disconnected/separated from the bussing and not capable of touching the bussing for the rest of the MDP panel.

Thanks!

 

[don_resqcapt19]

First is this a "legally required standby system"? If not, nothing in Article 701 apples. As far as 701.12(E)(E) Connection Ahead of Service, Disconnecting Means, that never applies where you have a generator. That is a permission to use a line side tap of the service conductors as the power source for the legally required standby system.

The only issue I could see is that the AHJ would consider the removal of the bus or conductors between the service disconnect breaker and the MDP bus as a listing issue.

 

[jap]

It's an 800 amp main, but there is enough space to add an 800 amp breaker to it as well for the back feed. I think we're good on that.

The physical aspect of this that I was worried about, was being sure to keep the line side of the main breaker disconnected/separated from the bussing and not capable of touching the bussing for the rest of the MDP panel.

Thanks!

I may be looking at this wrong, but, it doesn't look by your drawing that you are intercepting the "LINE" side of the main breaker.

By the drawing you are removing the "Load" side connection of the main breaker to the panels bussing, taking the "LOAD" side of the Main Breaker to the the ATS, then back to the MDP.

Never seen the load side connection from the main breaker to the panels bussing removed in this way.
Seems odd.

Regardless, If it is allowed, I don't see why you're needing a backfed breaker in MDP?.

This would be a feeder to the ATS, not service conductors, and, those conductors would be protected by the 800a main breaker.

Seems one could go back to Main Lugs on the panels bussing since you never took the OCPD out of play.

Oh well, I may be seeing this all wrong.

JAP>

 

[jap]

If you intercept the "LINE" side in the MDP to get to the ATS, then you're back to dealing with Service Conductors.

I would venture a guess that the Service Conductors could not pass through Panel MDP to get to the ATS in this fashion since Panel MDP would then not be the Service Disconnecting point.

JAP>

 

[Kidd]

First is this a "legally required standby system"? If not, nothing in Article 701 apples. As far as 701.12(E)(E) Connection Ahead of Service, Disconnecting Means, that never applies where you have a generator. That is a permission to use a line side tap of the service conductors as the power source for the legally required standby system.

The only issue I could see is that the AHJ would consider the removal of the bus or conductors between the service disconnect breaker and the MDP bus as a listing issue.

It's an optional back up system, so I figured 701 technically would not really apply but it seemed that the way I was interpreting 701.12(E)(E) might have given some guidance as to why this install would not be ok. It also seems that my interpretation of 701.12(E)(E) was wrong anyways haha, so I guess it didn't even matter.

After talking to the contractor, he's planning on installing a feed thru lug kit that would physically prevent the load side of the breaker to touch the panel's bussing, so he wouldn't be removing any bussing.

 

[Kidd]

I may be looking at this wrong, but, it doesn't look by your drawing that you are intercepting the "LINE" side of the main breaker.

By the drawing you are removing the "Load" side connection of the main breaker to the panels bussing, taking the "LOAD" side of the Main Breaker to the the ATS, then back to the MDP.

Never seen the load side connection from the main breaker to the panels bussing removed in this way.
Seems odd.

Regardless, If it is allowed, I don't see why you're needing a backfed breaker in MDP?.

This would be a feeder to the ATS, not service conductors, and, those conductors would be protected by the 800a main breaker.

Seems one could go back to Main Lugs on the panels bussing since you never took the OCPD out of play.

Oh well, I may be seeing this all wrong.

JAP>

You are correct! I meant the load side of the breaker, not the line side. We're taking the load side of the main breaker to the ATS and then feeding the MDP from the ATS.

I've never seen it done this way either, but I can't find anything that says it's a code violation. The only reason they're trying to do it this way is because the MDP is existing and the feeds from the utility transformer to the MDP are coming in underground so everything is set in place already.

I believe you are also correct that we could go back to the main lugs on the MDP, but they prefer to have a "main breaker" on MDP.

Thanks guys!

 

[LarryFine]

I don't see any issues other that when you intercept the load side of the existing service equipment, you will have to run the service neutral along with the ungrounded service conductors to the ATS.

Why is that? When I added an ATS to an existing residential service with an outside main disco, I only ran a single neutral conductor from the ATS to the disco that was equal the the generator feeder's neutral. I also removed the bond in the ATS.

 

[jap]

You are correct! I meant the load side of the breaker, not the line side. We're taking the load side of the main breaker to the ATS and then feeding the MDP from the ATS.

I've never seen it done this way either, but I can't find anything that says it's a code violation. The only reason they're trying to do it this way is because the MDP is existing and the feeds from the utility transformer to the MDP are coming in underground so everything is set in place already.

I believe you are also correct that we could go back to the main lugs on the MDP, but they prefer to have a "main breaker" on MDP.

Thanks guys!

To me, this would be very deceiving.

What happens if you shut the 800 amp main off to work inside the panel and the power goes out?

In this scenario, shutting off the 800 amp Main Breaker in the panel would not disconnect any power when the panel was being fed from the generator.

The 800 amp main could be shut off yet the buss bars still be energized when on generator power.

Something about this seems all wrong.

JAP>

 

[Kidd]

To me, this would be very deceiving.

What happens if you shut the 800 amp main off to work inside the panel and the power goes out?

In this scenario, shutting off the 800 amp Main Breaker in the panel would not disconnect any power when the panel was being fed from the generator.

The 800 amp main could be shut off yet the buss bars still be energized when on generator power.

Something about this seems all wrong.

JAP>

Good point. That could be why they're requesting to add a new "main" breaker for the MDP and the old "main" just be labeled as a breaker for the ATS. Although I can't remember exactly, but I believe the ATS had a breaker on it as well. The ATS is going to sit right next to MDP.

What you're saying could be an issue, I think with proper labeling though, it could work.