When running on our 11KW standby generator (through whole house 200A automatic transfer switch), the a/c system (3.5 ton Carrier, about 18 yrs old) can cause the genset to brown out. I observed this once and shut off the a/c system. I've measured the run current of the a/c system at about 19A; I have no idea how much starting current is drawn. In theory, the genset should be able to supply 45A. Both systems run fine on their own, just not together (combined with other typical household power demands).

I'm looking for solution ideas, such as using a relay to enable the a/c system only when utility power is available, and then adding a toggle switch to allow me to switch the enable to generator power after I've reduced demand elsewhere in the house. What line would the relay be installed on, e.g. the thermostat transformer?

I'm also looking at new higher SEER a/c systems but even with a more efficient system I think I'd like to have the relay-type arrangement to prevent possible brown-out conditions when I'm not home and the generator comes on while other things (e.g. refridgerators) are running. A/C is a luxury during an emergency so I can live with the hassle of manually attempting to switch over to genset power from time to time.

Thanks in advance for your ideas.

First of all by law you have to have a automatic power choper to prevent your gen set voltage from going down stream and killing a lineman doing repair work. This is according to the NEC. There are also bonding and grounding code sections to deal with. If you cannot afford that I recommend that you do not connect anything into your electric panel.

Next get a hard start kit and put it on the A/C unit. This will stop the brown out when the unit tryes to start.

He said it has a transfer switch. Sometimes the lock rotor amp draw on large motors can cause a droop time (generator has not caught up to the load) LRA can be so strong and fast, it trips something. I don't suggest putting A/C's on generators. But a hard start kit (which is hard on the compresor) will help the problem.

A hard start kit (HSK) is only of interest if it reduces the starting current. Is there a rule of thumb on how much the current is reduced during starting? Some sources state an HSK will extend the life of the compressor, others say the opposite...

The problem I'm trying to solve is a rare occurrence. It's when we're on genset power + running a/c + I'm not home to manually shut it off. We're in Rochester, NY so we're not running a/c all that much. If an HSK is hard on the compressor, every time the compressor starts, it will be affected. The original relay idea would not affect compressor life. Another idea is to add a 5 minute delay (kit?) to prevent the compressor from restarting immediately after utility power goes out thereby avoiding a very high current draw.

Actually a hard start kit makes it easier on the compressor and makes the compressor last longer.

I read right past the auto transfer switch in the first post.

Your A/C unit is older and does cause a momentary draw that will get close to locked rotor as rated on the compressor name plate of the unit tag itself.
This is the reason for the brown out when the compressor starts. Once it starts it should be OK. If not you have another problem.

This is the statement I read in your post that spooked me. ((Both systems run fine on their own, just not together (combined with other typical household power demands).))
Power from both sources should never be active or available at the same time unless momentarily controlled by the automatic change over switch or for testing purposes.

The idea for a 5 minute time delay on the A/C will work and not cause a problem. They are very easy to wire in the 24 volt control circuit but they depend on a constant source of 24 volts where the hard start does not depend upon a constant source of power because they activate upon power applied.

Over the past 10 years we have lost 2 linemen in this area due to home made gen set transfer switches. Usually they are being manually controlled which is illegal in most states instead of the automatic voltage sensing UL approved units.

If you fail to see where I am coming from please read the National Electric Code for yourself. It has an entire section on this subject.
Electrical Safety Inspector (State of Ohio) # 1142 Active since 1982

The transfer switch is fully automatic and completely isolates one power source from the other. All the gear (genset, transfer switch, controls, etc) is high quality Kohler equipment and everything was professionally installed. There should be no doubts on safety or NEC compliance. This was done by the book and is as safe as it gets.

Sorry if the "both systems together" was confusing - I simply meant when the genset and the a/c are running together.

Sounds like the latest a/c systems are likely to stay away from LRA current draws, which would be of interest for my problem.

Also, since no one is commenting on the relay idea, I'll assume it's somewhat unconventional. It was an approach one of our larger HVAC contractors suggested.

Just saw this , and the relay is a good idea, and is done just for this purpose of dropping out the AC when genset power is on.

Better to ask for a definite purpose contactor, size the same as the one inside the compressor control cabinet, but very important, must use Normally Closed contacts.

Use a 240 volt coil, which will connect to the generator power lugs at the ATS.This will monitor the gen power, and will energize the contactor coil when the gen power is on. The contacts will open and keep the AC off.

Once the utility power returns, the gen power will go off and let the contacts close at rest, and AC can be resumed.

Fuse the coil circuit, can use in line fuse holders with a 1 amp fuse.

The generators I've repaired in the past start, warm-up, then the transfer switch opens the utility side of the power and over to the generator side. This is an issue with droop time on the generator, I think. A time delay may help, since everything on the 8 or 12 circuits you have, are coming on and also increasing the droop time. I suggested the hard start kit because I think a storage device of some kind would help alleviate the LRA demand. I would also ask in electrical, as I am not an electrician. My associate degree is in small electronics not industrial. I have also heard engineers tell me that hard start kits will do no damage and others say it will, who knows?

Many thanks for the input on the delay (from tkrussell) and delay (from NorthernHeat and hvac1000). The combination of the two will help me avoid problems during long and very short periods on the generator. While I'm a newbie on this forum, I'm impressed with the quick advice. Thanks again.

To recap the relay idea:
A relay (with Normally Closed contacts) on the compressor would open when generator power is present on the relay coil and close when the genset shuts down. This would automatically prevent the a/c system from running when on generator power. Problem solved.

A couple more questions:

1) Why use a large relay to directly switch the compressor instead of a small relay to switch the compressor call line coming from the thermostat? Is one approach safer than the other?

2) The generator exercises itself once a week. It comes on for 20 minutes but the automatic transfer switch (ATS) doesn't switch from utility to genset power. With the relay as described above, the a/c system would shut down any time the generator was supplying power, even if the house was still being supplied by utility power. If I didn't want the compressor to shutdown during the exercise event, could another NC relay be used to open the 1st NC relay coil circuit when utility power is present? This should allow the a/c system to keep running while the genset is exercising and the house is still on utility power. Instead of two separate relays, is there such a thing as a combo relay?

I know I'm making this more complicated and I'm not making any of this up - I really want to understand the realistic options. I appreciate all the advice so far.

1) Why use a large relay to directly switch the compressor instead of a small relay to switch the compressor call line coming from the thermostat? Is one approach safer than the other?

Actually that is a good idea, just break the control circuit that controls the AC compressor, allows use of a small relay with contacts rated for 24 volts volts, not the large 30 or 40 amp contactor.

2) The generator exercises itself once a week. It comes on for 20 minutes but the automatic transfer switch (ATS) doesn't switch from utility to genset power. With the relay as described above, the a/c system would shut down any time the generator was supplying power, even if the house was still being supplied by utility power. If I didn't want the compressor to shutdown during the exercise event, could another NC relay be used to open the 1st NC relay coil circuit when utility power is present? This should allow the a/c system to keep running while the genset is exercising and the house is still on utility power. Instead of two separate relays, is there such a thing as a combo relay?

Another good observation and idea. OK, let's take control voltage from the coil of the ATS that is energized when the ATS is switched to gen power. This way the AC can be shut down only when there is a true power failure, sine the ATS does not switch during the weekly test.

Those are my solutions, hope they help.

Yes, tkrussel, your solutions are very helpful. Thanks!

Lot of good info and advice above.

That may not be a bad idea, but instead of interupting compressor contactor control,
How about interrupt power to themostat, other way, Air handler fan would still turn on.
Then you could use a normally open contactor, or relay. Coil would be connected to utility power(fused)

Lot of good info and advice above.

That may not be a bad idea, but instead of interupting compressor contactor control,
How about interrupt power to themostat, other way, Air handler fan would still turn on.

Yes, we wound up agreeing that interrupting the control line from the thermostat to the compressor would be a good way to go.