1 Generator, 3 ATS 1200a Service

[SaElectrical]

Existing is a 1200a MD feeding a 1200a MDP, I plan to put (3) 400a ats in between. Should the ATS units have over-current protection in case 1 only transfers back and the whole house load would then pull through it?

 

[iwire]

Existing is a 1200a MD feeding a 1200a MDP, I plan to put (3) 400a ats in between. Should the ATS units have over-current protection in case 1 only transfers back and the whole house load would then pull through it?

I am not following you, are you planing on putting three 400 amp transfer switches in parallel to handle a 1,200 amp feeder?

 

[SaElectrical]

Yes

 

[iwire]

Yes

I could be wrong but I see that as unworkable unless the ATS communicate with each other and I am not sure the NEC allows it in the first place.

 

[SaElectrical]

The ATS's work fine when ganged, I have done it before. I have done it where they feed off of a CT and each individual switch feeds a panel, not having it all combine again at a single MDP. This is where I think I may need to put over-current protection in for each switch.

 

[iwire]

The ATS's work fine when ganged, I have done it before. I have done it where they feed off of a CT and each individual switch feeds a panel, not having it all combine again at a single MDP. This is where I think I may need to put over-current protection in for each switch.

From a code perspective I don't believe your conductors fall under the rules of parallel conductors. Once you insert the seperate ATS the conductors are no longer electrically joined at each end.

If you add overcurrent protection for each ATS the over current devices will be in parallel and that is a violtion.

I don't think you can do this without NEC violations.

As far as them working fine, how do you make sure they switch at exactly the same time?

The last one to open is going to be operting way beyond its ratings.

I think this is a bad plan.

 

[Smart $]

There's a lot of missing detail here that will likely prove this is not a compliant installation. The problem with just the basic (or less than basic) details you have provided thus far is there is little specific requirements as to the rating and connection of transfer switches.

However, I'll first assume this is an Optional Standby System (Article 702) because it has the most lax requirements of Articles 700, 701, and 702. I'll first point you to 702.5 and quote the first sentence...

Transfer equipment shall be suitable for the intended use and designed and installed so as to prevent the inadvertent interconnection of normal and alternate sources of supply in any operation of the transfer equipment.

IMO, this would mean that the three (3) ATS' must be mechanically interlocked such that they can only connect to the same source concurrently.

Another potential violation is that I doubt you can maintain the ATS listing and land 1200A worth of conductor on each ATS line connection. If you use a lesser rated conductor scenario, you will be into tap rules, and will have to provide tap conductor OCPD before the switches... and then if you do that, you'll not be able to combine on the load side because then you'd violate the parallel ocpd prohibition.

 

[charlie b]

I don't understand why you would want to do this in the first place. Are you simply trying to save money by buying three smaller ATSs instead of a single 1200A ATS? I would call that a poor financial decision. If you have one of the ATSs transfer back to normal before either of the other two disconnect from alternate, then the result will almost certainly be an explosion. I strongly suspect that the owner would look to you to pay for the repairs. How do you plan to prevent that from happening? By installing some type of mechanical interlock that the ATS manufacturer did not include in the listing? If I were the AHJ, I would throw the ever-unpopular 110.3(B) flag on that play.

 

[charlie b]

From a code perspective I don't believe your conductors fall under the rules of parallel conductors. Once you insert the seperate ATS the conductors are no longer electrically joined at each end.

I think that brings into the picture another code violation, albeit a rather subtle one. 310.10(H)(1) is what allows us to put conductors in parallel. It doesn't explicitly say this, but we all infer that by doing so we can declare the net ampacity to be the sum of the ampacities of the individual conductors. The suggested installation would put three 400 amp feeders effectively (though not legally, as Bob pointed out) in parallel. What, then, would allow you to say that the net ampacity of the arrangement is 1200 amps? You can't use 310.10(H)(1), and there is nothing else in the code that you could use. So you would have to use 1200 amps worth of wire in each feeder. That brings us back to the post by Smart $.

 

[steve66]

What if one transfers back to a live utility, and one is still on generator power?

This setup will destroy itself in no time, if it doesn't kill someone.

 

[SaElectrical]

All true......the generator is a generac 1ph 240v and they do not make a 1200a ATS that is 1ph. The largest ATS is 800a. If I use another manufacture's switch it will void the warranty on the generator.....

 

[Smart $]

All true......the generator is a generac 1ph 240v and they do not make a 1200a ATS that is 1ph. The largest ATS is 800a. If I use another manufacture's switch it will void the warranty on the generator.....

If all you have is a 240V 1PH generator, put one 1PH ATS downstream of the 1200MDP, in the feeder of the 1PH load...

 

[SaElectrical]

The generator is doing the whole load. The house has way too big of a service, I calculated it at a little of 400 amps. With load shedding a 48kw will do the whole house. I may just step their service down to 800a....

 

[GoldDigger]

The generator is doing the whole load. The house has way too big of a service, I calculated it at a little of 400 amps. With load shedding a 48kw will do the whole house. I may just step their service down to 800a....

Or you could split the whole house load into two panels and run separate 400A transfer switches from the generator to the two panels, neatly avoiding the problem. (I do not think that the ATS will need to be rated for a 1200A service even if the POCO service is nominally 1200A, as long as there is a 400A OCPD on each of the two branches.)

 

[don_resqcapt19]

...If I use another manufacture's switch it will void the warranty on the generator.....

Not according to federal law.

 

[George Stolz]

Or you could split the whole house load into two panels and run separate 400A transfer switches from the generator to the two panels, neatly avoiding the problem.

I'd go a step farther and reduce that down to a 400A transfer switch (or less) supplying a panel with "critical" loads. If you can't get lighting in all rooms, receptacles in the living room and bedrooms done with 100A you're not trying.

 

[Canton]

Existing is a 1200a MD feeding a 1200a MDP, I plan to put (3) 400a ats in between. Should the ATS units have over-current protection in case 1 only transfers back and the whole house load would then pull through it?

Wow......:jawdrop:.........I'd pay to see this live....from an explosion proof room....

 

[SaElectrical]

I'd go a step farther and reduce that down to a 400A transfer switch (or less) supplying a panel with "critical" loads. If you can't get lighting in all rooms, receptacles in the living room and bedrooms done with 100A you're not trying.

That would be possible if the panels weren't spread all over the finished home.....

 

[Smart $]

That would be possible if the panels weren't spread all over the finished home.....

How many panels?

Output breaker rating on generator?

 

[ceb58]

All true......the generator is a generac 1ph 240v and they do not make a 1200a ATS that is 1ph. The largest ATS is 800a. If I use another manufacture's switch it will void the warranty on the generator.....

You could put in a 3 phase and use A-C on the contactor. The switching coil only needs SP to switch and that is picked up off A-C.