(1) Gen - (2) 600a ATS after 1200a Main Breaker?

[SaElectrical]

I am looking at a job at a residence where they have any existing 1200a 240v 1ph service and want a generator. The Main disc. is outside and has a 1200a breaker, which feeds a MDP inside the house. Would it be code compliant to split the parallels between (2) 600a ATS's? or (3) 400a ATS's? Any info on this would be great thanks.

 

[mcnut]

What is the calculated load per NEC? What size generator are they installing?

 

[SaElectrical]

The load does not matter....I just want to know if I can take each parallel set of conductors and put them in separate ATS units before the MDP and after the Main disc/overcurrent protection.

 

[SaElectrical]

Its probably going to be a 100kw, I have not calculated it exactly yet. I am still trying to figure out how to wirelessly shed the A/C condensers. The are 11 units and they are spread all around the exterior of the house without an easy way to run control wiring.

 

[kwired]

The load does not matter....I just want to know if I can take each parallel set of conductors and put them in separate ATS units before the MDP and after the Main disc/overcurrent protection.

You can not separate them into two ATS units then bring them back together again into the same MDP, if that is what you are asking. You can separate them and feed two separate distribution panels.

 

[iceworm]

I am looking at a job at a residence where they have any existing 1200a 240v 1ph service and want a generator. The Main disc. is outside and has a 1200a breaker, which feeds a MDP inside the house. Would it be code compliant to split the parallels between (2) 600a ATS's? or (3) 400a ATS's? Any info on this would be great thanks.

First, a disclaimer: I don't do residential (well, except mine, and my brothers and dads, and my good buddy ....)

Everything about this looks wrong:
1200A single phase service - Is this maybe 20k sqft, $10M house? With the limited information we have, this is a small industrial grade project.

If I am understanding you correctly, you want to take a 1200A parallel feeder, split it between two 600A switches, the recombine back to a 1200A feeder to the MDP. First, it is poor design. And the code doesn't address poor design - just illegal design. At this level I wouldn't worry much about illegal design. I'd be a lot more worried about dodging poor design. Be brave. You are going to be the DIY engineer of record on this.

As kwired said, no. If you need a code reference, look at 310.10.H.1

I don't see anyone with a $10M house wanting anything marginal. I'd be looking at a 1200A, 3 pole, switch. Use one pole for a switched neutral. Install between the outside Main and the inside MDP. Conductors are likely quad 350kcmil CU. Generator will be a 400KW, 208V, 3ph, 12 lead generator, reconnected to 120/240, 1ph, grounded and connected as an SDS. Should rerate to 300KW.

Should hunt down the architect/engineer and break their pencil/sliderule for allowing the utility to override. 1200A single phase is near disgusting.

JMO

ice

 

[SaElectrical]

it's a 15000 sqft home with elaborate pool....

 

[charlie b]

The Main disc. is outside and has a 1200a breaker, which feeds a MDP inside the house.

This tells me that the conductors from the 1200a breaker and the MDP are likely to be parallel sets. For the sake of discussion, let us say it is four sets of 350 MCM per phase (i.e., 4 phase A conductors, 4 phase B conductors, and 4 neutral conductors, for a total of 12 wires, not counting the EGC). It seems like you are suggesting taking two sets of the 350's (i.e., 2 from phase A and 2 from phase B, along with 2 neutrals, for a total of 6 conductors, not counting the EGC), and running them to a 600 amp rated ATS. You would then take the remaining 6 conductors and run them to a different 600 amp rated ATS. From the two ATS's you would take the conductors back to the original 1200 amp MDP. Do I at least understand the installation you are trying to describe?

Presuming I have the right concept, I have to agree with what has been said already. This would be a violation of several code requirements. It also sets up the potential for an explosive failure, or an electrocution. I am serious: this is a big time bad idea. A catastrophe could take place if the utility power is lost, and if the generator starts up, and if one of the transfer switches fails to transfer. You would then be backfeeding the utility power lines from your generator, and that could electrocute a utility worker. On the other hand, if the utility power comes back fairly quickly, while the generator is feeding the MDP via only one of the transfer switches, then you would be placing the two power sources suddenly in parallel, with no controls over the synchronization of the sources. That would cause an explosion somewhere within the distribution system.

If you are trying to save the cost of a 1200 amp rated ATS, then please forget the whole idea.

 

[augie47]

What Charlie b says.....
a blueprint for a disaster on many fronts....

 

[kwired]

I thank everyone for backing up what I pointed out as a potential problem.

SaElectrical, you have replied but have not confirmed if what has been mentioned is what you are thinking about doing. If it is not what you are considering, then let us know just what it is that you are up to.

 

[iceworm]

... if the utility power comes back fairly quickly, while the generator is feeding the MDP via only one of the transfer switches, then you would be placing the two power sources suddenly in parallel, with no controls over the synchronization of the sources. That would cause an explosion somewhere within the distribution system. ...

charlie -
You're scaring the children. The most likely outcome is it will trip the gen OCP - not an explosion. Not that an explosion is impossible - just very unlikely.

ice

 

[Cow]

I agree with the others. They have a nice house, sell them the proper transfer switch.

 

[charlie b]

The most likely outcome is it will trip the gen OCP - not an explosion. Not that an explosion is impossible - just very unlikely.

I will stick with my assessment. If the two power sources are closed in together at the point in time at which their respective voltages are 180 degrees out of phase with each other, I don't think any breaker will be able to clear the condition before molten copper starts flying around the room. But whatever does happen, it will be double-plus ungood.

 

[iceworm]

I will stick with my assessment. If the two power sources are closed in together at the point in time at which their respective voltages are 180 degrees out of phase with each other, I don't think any breaker will be able to clear the condition before molten copper starts flying around the room. But whatever does happen, it will be double-plus ungood.

charlie -
Well, I don't do many 1ph fault calculations - okay, I have never done a single phase fault calc..

Say the utility is a 150kva, 5%. And the gen is 150kva, 20%. Reasonable considering the lack of data.

Continuing on: The two sources and the two impedances are in series. That calculates out to 5000A. This is not a QO or HOM panel - it's 1200A. the CBs will handle 5000A

My assessment is the CBs to just open and fairly quickly, say within a few cycles.

What is the science or model behind your assessment?

ice

 

[hillbilly1]

I have had some real time experience with generators backfeeding the utility without being synchronized, the breaker actually did trip, a couple of times. The customer had a 2000 amp 120/208 volt three phase manual transfer switch in which they were testing with an actual generator. They thought the breaker was bad, so they had the manufacture come out to replace it (brand new installation), the manufactures techs could not replace it because the generator breaker remained live even though the main was off. I went to investigate, and found the utility had been brought into the common tie instead of the utility input, the load was terminated to the utility input, so everything appeared fine until the generator breaker was closed with the generator connected and energized! Was not an easy fix, the conduits were stubbed up on the wrong side of the gear, and since that section was noty rated as service equipment, we could not cross the utility over to the correct section. We had to pick the entire gear up and move it over 3' and re-pipe the utility in.