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-   -   AC breaker trips on generator (http://www.diychatroom.com/f18/ac-breaker-trips-generator-149452/)

wiz561 07-07-2012 10:48 AM

AC breaker trips on generator
 
Hi!

I have a generator that backfeeds into the electrical panel. It's a 240v 30 amp connection. When I'm running on the generator and if the AC goes on, it trips the AC breaker.

I would assume that if the generator couldn't handle it, it would trip one of the breakers on the generator and not on the AC end. Also, if the generator was 'overloaded', I'm assuming the engine would just be taxed and die out.

On the 'grid', the AC works just fine and doesn't trip. I was wondering if anybody has run into this before and has any suggestions on what to look into.

Thanks!

JACK HOTTEL 07-07-2012 11:57 AM

When the AC starts it draws a very high current for a short time (see Locked Rotor Amps(LRA) on the data plate. The grid can provide this with little voltage drop. The LRA is higher than the circuit breaker rating, but the time is short enough not to trip the breaker.
The generator cannot provide the LRA so the voltage falls. In your case there may be enough current to start the unit but the process takes longer. The current persists above the breaker rating long enough to trip the breaker.
Sometimes a hard start kit added to the AC will solve the problem.

wiz561 07-07-2012 11:59 AM

Thanks for the response. I was thinking about adding a hard start kit to the AC, but I would be surprised if the generator would be able to handle the AC even with the kit. Nonetheless, it might be something good to add because what I've read, it is better for the compressor since there's not drop in voltage...

Thanks!

kontoose 07-07-2012 06:12 PM

When the A/C tries to start, the inrush load causes a voltage drop in the generator, which extends the starting cycle of the compressor. This longer start time trips the breaker due to inrush current posing an overload.

Speedy Petey 07-07-2012 06:38 PM

How big is the generator, and how big is the A/C unit?

How are you backfeeding the panel?

biggles 07-08-2012 07:21 AM

keep in mind the compressor being single phase 230V it needs time delay to equalize from an effort to start due to the pressures within the system.do you shut the AC down before the switch over and how long before the restart on generator power.hard start kits push the windings to overcome unbalanced pressures could lead to compressor burn outs...lets hear back on the times between stop and starts.

wiz561 07-08-2012 08:08 AM

Thanks all for the responses....

I have a 7000/9000 watt generator and a 5 ton ac unit. I read the running amps using a fluke and it was about 18.5. I have it backfed into the panel with 3 - 8 gauge wires into a 30-amp 2pole breaker and an interlock for safety. The neutral and ground are unbounded at the generator. I have also flipped the breaker for the ac before starting the generator so there wouldn't be a rush.

I'm well aware that the generator probably wouldn't handle the AC, but I wanted to try it out to see if it actually would or not.

biggles 07-08-2012 08:28 AM

might consider a fused disconnect seperate from the house breaker for the condenser.the generator voltage might be "Dirty" causing the trip.

AllanJ 07-08-2012 08:32 AM

If voltage drops such as due to momentary overloading of the generator, many motors will draw more amperes than the rating on the nameplate states. In turn this can cause a breaker to trip unexpectedly.

The typical warning in the instruction manual for dehumidifiers and small air conditioners is to wait two minutes after a turn off before restarting it again, to allow internal pressures to equalize and prevent overloading of the compressor.

stickboy1375 07-08-2012 08:43 AM

Quote:

Originally Posted by wiz561 (Post 960542)
Thanks all for the responses....

I have a 7000/9000 watt generator and a 5 ton ac unit.

I have a 7000/9000 watt generator and can't fire my 3 ton AC unit... just as a heads up for you.

kontoose 07-08-2012 03:07 PM

The N.E.C (National Electrical Code) gives the rating of the breaker at 250% (max) of the "name plate amperage of the compressor, but you can use the running amperage measured with an amp probe...it'll be close enough.
So, 18.5amps X 250% = 46.25amps (Use the next size up. The breaker needs to be a 50amp 2-pole).
The wire size needs to be 125% of the "name plate" amperage --- 18.5amps X 125%= 23.125amps. (You could have used #10AWG THWN copper conductors, so #8AWG is more than enough.
A 7000/9000 watt generator will develop between 30.43 and 39.1amps @ 230volts. This is enough to run the A/C, but remember, the compressor will pull between X4 and X6 the running amperage at the instant the compressor is turned on, (4X18.5amps = 74amps, and 6X18.5 = 111amps), dropping to the running amperage of 18.5amps during the "start time cycle" (which ought to be less than 1/4 of a second (depending on load pressure)).
The issue is, that when an excessive load is thrown across the (spinning) rotor winding's of the generator, then the load is converted to drag on the rotor, and if the generator is too small to overcome the drag, then the rotor slows down, and a voltage drop occurs. To compensate, the compressor tries to draw more current...etc., etc. and a vicious cycle is born, and the breaker trips...trying to prevent the A/C winding's burning up. Get a bigger generator.

beenthere 07-09-2012 05:03 AM

Install a hard start kit on the A/C.

stickboy1375 07-09-2012 06:34 AM

Quote:

Originally Posted by beenthere (Post 961306)
Install a hard start kit on the A/C.

I doubt that will help start a 5 ton unit with a 7kw generator.

beenthere 07-09-2012 06:36 PM

Quote:

Originally Posted by stickboy1375 (Post 961326)
I doubt that will help start a 5 ton unit with a 7kw generator.

Actually, it does. Ok, its on a 7.5 KW gennie. But a good hard start kit(5-2-1) will get the comp off locked rotor before the gennie has a problem.

I have 3 ton units on 5KW gennies. They all have hard starts.

stickboy1375 07-09-2012 06:45 PM

Quote:

Originally Posted by beenthere (Post 961733)
Actually, it does. Ok, its on a 7.5 KW gennie. But a good hard start kit(5-2-1) will get the comp off locked rotor before the gennie has a problem.

I have 3 ton units on 5KW gennies. They all have hard starts.




a 5 ton unit is WAY over a 7 kw generator, I have a 3 ton unit and a 7kw generator and it won't start it... No hard start kit, but with the rest of my house going, there is very left to give, so if all you want is AC, then yes, I suppose it will work. :) ( The 3 ton unit, not the 5)


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