Two generators on the same service

[Leos98]

Hello,

We are facing the following situation. Our customer wants a generator to back up his whole service, it is a 400A, single phase 120/240V service. So we are installing a 36KW and a 400A service rated ATS, no problem there. However, the property has a backup generator which does partial loads. It is an existing 15Kw generac genset which is backing a 100A panel via a 100A 'not service rated' ATS. We would like to know if we can leave the existing generator and set it 100A ATS as a MTS and control the existing 15Kw genset manually. What this achieves is that in any circumstances in which the man generator fails, the client can have the same backup he has now. Any thoughts? Do you see a problem with this? I have been researching but with little luck.

Thanks,

Leo

 

[kwired]

I see no problem, the transfer switch for the smaller unit isolates that load from other loads when operated.

 

[augie47]

If your times are adjustable, leave the 100 ATS as is with a longer pick-up time than the 36kw ATS. If all goes as planned the 15KW would not start unless the 36KW fails to do so.

 

[hillbilly1]

I have done this several times, as Augie said, just set the second set for a longer delay, or the first set for a shorter delay. Even if the delays are the same, it will not matter, the second set will just get exercised under load if it happens to transfer first. If it is life safety, you may not be able to lengthen the delay anyway.

 

[iwire]

Yeah, I agree with gus and hillbilly.

Many of our customers have a larger standby generator backed up by a smaller emergency generator.

 

[Leos98]

Thanks for all of the responses. Setting the delay on the 100A ATS to a longer value than the new 400A ATS is what I thougth and did not see anything wrong with the setup. However, I spoke with the inspector yesterday and he seems to think that the only way of doing it is by having the two ATSs connected so that the 400A ATS works as a master and the 100A ATS works as slave. Has anybody encounter this? any code or manufacturer references to this type of setup?

Thanks,

Leo

 

[hillbilly1]

Thanks for all of the responses. Setting the delay on the 100A ATS to a longer value than the new 400A ATS is what I thougth and did not see anything wrong with the setup. However, I spoke with the inspector yesterday and he seems to think that the only way of doing it is by having the two ATSs connected so that the 400A ATS works as a master and the 100A ATS works as slave. Has anybody encounter this? any code or manufacturer references to this type of setup?

Thanks,

Leo

It already would be, the utility feed to the smaller generator would be powered by either the utility or the larger generator, failure of both would cause the smaller to crank and transfer. Unless......the feed to the smaller transfer switch is tapped ahead of the main and larger transfer switch, then it would act independently.

 

[Leos98]

The attached picture shows what we would like to do.

 

[hillbilly1]

The attached picture shows what we would like to do.

That would work fine, I'm assuming the load calc allows the 36 kw to run the entire load. Maybe the inspector is assuming that the 15 kw would be part of load shedding if it is not large enough?

 

[hurk27]

I think your going to find that most of these generators will not have any delay settings that are field changeable (at least Generac wont on the smaller ones) the 36kw might, but I haven't seen any, most will start and transfer within one minute or less, depending upon how old the 15kw is some of the older Generacs had a 1 minute delay then it started, then a 1 minute warm up before it transfers the load which would work in your favor as the newer 36kw would start in 10 seconds after power failure then transfer in 30 seconds, the reason they did this in the older units was to keep from cycling the generator in cases where the power would try to come back on as the utility would try to close new feeds into the fault, here we call it a three strikes your out as the utility will try three time to close a new feed into a fault before it stays off, if the generator doesn't have a delay it will try to start between each closer cycle if there is enough voltage, but I have noticed that the newer Generacs have a much shorter delay now so that might work in your favor.

But even if both do have the same delay the only problem I can see is the amount of gas flow the gas service might have if these are NG generators, if both generators are trying to start at the same time the generators might starve for fuel, a 20 KW requires between 250-350k cubic feet of fuel per hour, so not sure what the requirement for the 36kw would be, newer NG meter/regulators have a 250cf per hour rating but some older ones were 150cf so you going to have to make sure the gas service is large enough, also make sure the pressure is correct, I did a 45kw that required 14"wc but standard is 5"wc. or .5psi so you have some home work to do on this part.

Also make sure the 36kw will handle the load it is going to transfer into, if the heating and water heater is gas it will be a big help, but 150 amps is not a lot of current for a 400 amp service if this is commercial then make sure it will handle it, I have seen a few Generacs burn up the alternators and never trip the breaker because the load exceeded the alternator rating but not the breaker rating, most of these generators will have a breaker that is larger then the output of the generator, a 36kw will have 175 amp breaker but will only supply 150 amps continuous, so if the load is over the 150 amps but below the trip curve of the breaker at a point that it would over heat the windings in the alternator it is very possible to burn it up.

The other problem is because the service rated ATS will have a main breaker this becomes your service point and the grounding electrodes will have to be moved to this ATS, as well as neutrals and grounding will have to be separated in the 400 amp existing panel, make sure you can isolate the neutral bar and install separate grounding bars as now this 400 amp panel becomes a sub panel, this will also make 3-wire range and dryer feeds no longer allowed as they were never allowed in a non-service panel, this is a got ya that surprised a few.

Sorry for being long winded but I have seen to many of the above mistakes made by many (including myself):ashamed1:

 

[tkb]

I don't see a problem with your transfer scheme.
When there is an outage both generators will start and once the main ATS transfers the smaller ATS will go back to the normal source and shut down the small generator.
If the larger generator fails them the smaller one will feed its loads.

I wouldn't change the pickup times. The worst thing will be that it exercises the smaller generator during an outage.

 

[Leos98]

But even if both do have the same delay the only problem I can see is the amount of gas flow the gas service might have if these are NG generators, if both generators are trying to start at the same time the generators might starve for fuel, a 20 KW requires between 250-350k cubic feet of fuel per hour, so not sure what the requirement for the 36kw would be, newer NG meter/regulators have a 250cf per hour rating but some older ones were 150cf so you going to have to make sure the gas service is large enough, also make sure the pressure is correct, I did a 45kw that required 14"wc but standard is 5"wc. or .5psi so you have some home work to do on this part.

Thanks for your response, I should had mentioned before that the two generators are LP. The 15Kw has a 250 gallon tank that also supplies the rest of the house (stove, fire place, BBQ). We are installing a new 1000 gallon underground LP tank dedicated to the new generator, during Sandy, the LP company was not able to get to that house for 6 days.

The other problem is because the service rated ATS will have a main breaker this becomes your service point and the grounding electrodes will have to be moved to this ATS, as well as neutrals and grounding will have to be separated in the 400 amp existing panel, make sure you can isolate the neutral bar and install separate grounding bars as now this 400 amp panel becomes a sub panel, this will also make 3-wire range and dryer feeds no longer allowed as they were never allowed in a non-service panel, this is a got ya that surprised a few.

Correct, already in the plan.

Thanks,

Leo