1 Generator: 2 Transfer switches

[kbsparky]

We are installing a new transfer switch on an installation that currently has a generator and transfer switch in place.

We want to configure this so that either transfer switch will operate the generator.

The current ATS only has 2 control wires connected to the generator control box. All gauges, meters, etc mounted on the generator control box.

I figure this should be a simple switch leg.

So the question is: Can we simply wire in the alternate ATS control wires in parallel with the existing one, so either one will actuate the gen set? OR would we need to install an isolating relay to do the job?

Additional info: Each ATS is rated 400 Amps. Gen set is rated 107 kVA. Utility transformer is only 75 kVA.

120/240 single phase services.

Comments?

 

[charlietuna]

You will need an isolation relay controlled by either transfer switch control panel to start the generator.

 

[markstg]

You will need an isolation relay controlled by either transfer switch control panel to start the generator.

Why the interposing relay? The ATS Gen Start contact is a dry contact with the voltage coming from the Gen Set Batteries. Wiring these 2 contacts in parallel would work. With the interposing relay, the relay would need to to be a 12DC coil and get its voltage from the Gen Set, and the 2 ATS contacts wired in parallel to the Interposing relay coil. A contact of the IR would be then wired to the Gen Set start terminals. Why bother with the Interposing relay?

 

[hillbilly1]

Why the interposing relay? The ATS Gen Start contact is a dry contact with the voltage coming from the Gen Set Batteries. Wiring these 2 contacts in parallel would work. With the interposing relay, the relay would need to to be a 12DC coil and get its voltage from the Gen Set, and the 2 ATS contacts wired in parallel to the Interposing relay coil. A contact of the IR would be then wired to the Gen Set start terminals. Why bother with the Interposing relay?

I agree, most generators work this way, except genercrap, the voltage sense, and transfer control is built in to the generator on the smaller units, but with Kohler and Cat, you can parallel as many as you want. Onan is a three wire system, the extra wire I think is for powering the electronics in the t switch.

 

[Cold Fusion]

You really need to get the prints - new ATS, existing ATS, and generator. What you are trying to do is likely considered non-standard by at least one of the equipment mfgs. I wouldn't recommend guessing. And there's no reason to believe that any two mfgs do anything the same.

Then again, I don't do much work on small, self contained generators/ATSs and maybe they are all the same with few exceptions (as hillbilly said)

cf

 

[charlietuna]

I agree with hillbilly--there is a chance they would work if put in parallel -- and the best reasoning would be to place the responsibility in the hands of the new transfer switch manufacturer and ask for drawings. The biggest problem wiring them in parallel is troubleshooting ! And this is not an uncommon problem with transfer switch controls--try to determine which transfer switch is calling for the generator to run. If wired in parallel you will read a closure on either set of wires.

 

[vinster888]

i'm with charlietuna on the relay

 

[hillbilly1]

We do dual transferswitches for Home Depot's and Lowes all the time, one switch does life safety, the other is conveince loads such as registers and the like. We just parallel the the start contacts in the switches, both stores also use a dry contact from energy management to remotely start the generator.

 

[brian john]

99.9% of the ATS we work on do not use interposing relays, they parallel the contacts no issues.

 

[kbsparky]

According to the information I have on hand, the transfer switches utilize a set of normally open contacts that close when the power fails.

Wiring these in parallel should produce satisfactory results.

The existing ATS is a Cutler-Hammer ATVALDA20400W
The new ATS is a Cutler-Hammer ATV3LDA20400W

Both appear to have the N.O. Contacts for engine control.

 

[iwire]

We have done this many times, just parallel the start contacts and move on.

 

[kbsparky]

Sounds like a plan, Bob. I did call the Eaton support folks and they confirmed that this is the case. Basically, "dry" contacts can be wired in parallel so that either transfer switch can initiate the gen set as needed.

The other thing we are doing on this project is tying into the utility's peak demand system, so that during periods of high peak usage, we can go to standby power, shaving demand off of the grid.

I think we get a better rate from the utility for agreeing to do this.

The option on the ATS is called "go to source 2" which forces the transfer switch to standby power even though the utility power is present and available.

For that to work properly, we will have to install an additional isolation relay to control both transfer switches.