Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Electrical

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Search this Thread Display Modes
Old 07-07-2012, 09:48 AM   #1
Member
 
Join Date: Nov 2008
Posts: 242
Share |
Default

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!

wiz561 is offline   Reply With Quote
Old 07-07-2012, 10:57 AM   #2
Newbie
 
Join Date: Dec 2011
Posts: 21
Default

AC breaker trips on generator


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.

JACK HOTTEL is offline   Reply With Quote
Old 07-07-2012, 10:59 AM   #3
Member
 
Join Date: Nov 2008
Posts: 242
Default

AC breaker trips on generator


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!
wiz561 is offline   Reply With Quote
Old 07-07-2012, 05:12 PM   #4
Member
 
Join Date: Jul 2012
Posts: 119
Default

AC breaker trips on generator


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.
kontoose is offline   Reply With Quote
Old 07-07-2012, 05:38 PM   #5
Licensed Electrical Cont.
 
Speedy Petey's Avatar
 
Join Date: Feb 2004
Location: NY State
Posts: 6,798
Default

AC breaker trips on generator


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

How are you backfeeding the panel?
__________________
Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
Answers based on the 2008 & 2011 NEC.
Speedy Petey is online now   Reply With Quote
Old 07-08-2012, 06:21 AM   #6
Member
 
biggles's Avatar
 
Join Date: Jan 2008
Location: Long Island
Posts: 3,621
Default

AC breaker trips on generator


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.
biggles is offline   Reply With Quote
Old 07-08-2012, 07:08 AM   #7
Member
 
Join Date: Nov 2008
Posts: 242
Default

AC breaker trips on generator


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.
wiz561 is offline   Reply With Quote
Old 07-08-2012, 07:28 AM   #8
Member
 
biggles's Avatar
 
Join Date: Jan 2008
Location: Long Island
Posts: 3,621
Default

AC breaker trips on generator


might consider a fused disconnect seperate from the house breaker for the condenser.the generator voltage might be "Dirty" causing the trip.
biggles is offline   Reply With Quote
Old 07-08-2012, 07:32 AM   #9
Member
 
Join Date: Nov 2007
Location: Nashua, NH, USA
Posts: 6,930
Default

AC breaker trips on generator


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.
__________________
Stop wasting time re-adjusting the pattern. Have several lawn sprinklers, one for each pattern.
AllanJ is online now   Reply With Quote
The Following User Says Thank You to AllanJ For This Useful Post:
Speedy Petey (07-08-2012)
Old 07-08-2012, 07:43 AM   #10
E2 Electrician
 
stickboy1375's Avatar
 
Join Date: Apr 2012
Location: Litchfield, CT
Posts: 4,897
Default

AC breaker trips on generator


Quote:
Originally Posted by wiz561 View Post
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.
stickboy1375 is online now   Reply With Quote
Old 07-08-2012, 02:07 PM   #11
Member
 
Join Date: Jul 2012
Posts: 119
Default

AC breaker trips on generator


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.
kontoose is offline   Reply With Quote
The Following User Says Thank You to kontoose For This Useful Post:
zappa (07-09-2012)
Old 07-09-2012, 04:03 AM   #12
An old Tradesmen
 
Join Date: Oct 2008
Location: PA
Posts: 24,741
Default

AC breaker trips on generator


Install a hard start kit on the A/C.
beenthere is offline   Reply With Quote
Old 07-09-2012, 05:34 AM   #13
E2 Electrician
 
stickboy1375's Avatar
 
Join Date: Apr 2012
Location: Litchfield, CT
Posts: 4,897
Default

AC breaker trips on generator


Quote:
Originally Posted by beenthere View Post
Install a hard start kit on the A/C.
I doubt that will help start a 5 ton unit with a 7kw generator.
stickboy1375 is online now   Reply With Quote
The Following User Says Thank You to stickboy1375 For This Useful Post:
Speedy Petey (07-09-2012)
Old 07-09-2012, 05:36 PM   #14
An old Tradesmen
 
Join Date: Oct 2008
Location: PA
Posts: 24,741
Default

AC breaker trips on generator


Quote:
Originally Posted by stickboy1375 View Post
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.
beenthere is offline   Reply With Quote
Old 07-09-2012, 05:45 PM   #15
E2 Electrician
 
stickboy1375's Avatar
 
Join Date: Apr 2012
Location: Litchfield, CT
Posts: 4,897
Default

AC breaker trips on generator


Quote:
Originally Posted by beenthere View Post
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)


Last edited by stickboy1375; 07-09-2012 at 05:49 PM.
stickboy1375 is online now   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Adding 220 20A breaker but it trips when turned on mi_fiveo Electrical 23 05-19-2012 11:46 AM
Well pump trips breaker at start, but then starts when breaker reset J. David Electrical 34 02-25-2012 11:23 AM
New pool pump trips breaker?! New home owner needs advice. KenR7A Electrical 22 01-05-2011 09:42 AM
Wiring Size Calculations for New Service CorinthWest Electrical 7 12-07-2010 11:36 AM
Central Air Trips Breaker RobSmith HVAC 6 08-05-2008 09:06 AM




Top of Page | View New Posts

Copyright © 2003-2014 Escalate Media. All Rights Reserved.