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11-01-2011, 11:18 PM   #1
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## How much generator do I need?

I bought my house last year and it is wired already for a backup generator to feed the service panel. I'm sure this work is all legit -- it passed the home inspection and all. I can't engage the generator without opening the main breaker first.

My question is this -- there are 2 ganged 30-amp breakers for the incoming power for the generator, total of 60 amps. If the house is running on 120 volt system (and volts * amps = watts), that would mean that the most juice I could push through that service panel would be 7200 watts (60 amp * 120 volts), and I should look for a generator of that size. Right?

11-02-2011, 12:01 AM   #2
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 Originally Posted by mattdu I bought my house last year and it is wired already for a backup generator to feed the service panel. I'm sure this work is all legit -- it passed the home inspection and all. I can't engage the generator without opening the main breaker first. My question is this -- there are 2 ganged 30-amp breakers for the incoming power for the generator, total of 60 amps. If the house is running on 120 volt system (and volts * amps = watts), that would mean that the most juice I could push through that service panel would be 7200 watts (60 amp * 120 volts), and I should look for a generator of that size. Right?
A 7200 watt generator is fine.

Note that generators have 3 different ratings

Surge, full run, half run.

 11-02-2011, 05:54 AM #3 Electrical Contractor     Join Date: Sep 2008 Location: Delmarva Posts: 3,368 Rewards Points: 2,000 Your math is a bit off, but you ended up with the correct solution. Your breaker is a double-pole 30 Amp, which means you have 30 Amps, not 60 Amps. Your panel is operating at 240 Volts. You still end up with 7200 watts, however. __________________ -KB Life is uncertain -- eat dessert first!!

 11-02-2011, 07:34 AM #4 Member     Join Date: Jan 2008 Location: Long Island Posts: 3,630 Rewards Points: 2,000 keep in mind you want to be somewhat conservative during a power outage and the chances of pulling 30 amps thru both breakers is going to be rare..heating,house lights,home electronics,refrig's typical winter night tops 10 amps per breaker...always safe to have more.if it is gasoline the less drawn the longer the tank lasts __________________ http://www.youtube.com/watch?v=U1qEZHhJubY
 11-02-2011, 08:07 AM #5 Newbie   Join Date: Nov 2011 Posts: 23 Rewards Points: 10 I have a generator as well wired via a transfer switch with a 30 amp into the main. So would it be reasonable to assume from this thread that: anything more than 7200 will probably flip the breaker, total effective power I have available would be 7200 - surge requirements, and if the largest surge I needed to cover was 1200 watts ( like a fridge) I could then cover 6000 watts worth of household stuff? And so when buying a generator the minimums I would want are a 7200 watt generator with a running 6000 wattage? Any more could not be used anyway? On a side note I want to upgrade to cover the house AC. It has a rather large breaker so I know it takes juice. Rather than using that there is info on the panel. What commonly provided AC panel info can I use to calculate my generator needs? Sent from my iPad using DIY Forum
11-02-2011, 08:47 AM   #6
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 Originally Posted by mattdu I bought my house last year and it is wired already for a backup generator to feed the service panel. I'm sure this work is all legit -- it passed the home inspection and all. I can't engage the generator without opening the main breaker first.
I would not trust ANY "home inspection" for something like this. Those inspectors are quick to tell you they are not code experts. Pay a real electrician to check it out. You can, quite literally, kill somebody if the hookup has been incorrectly done.

 The Following User Says Thank You to md2lgyk For This Useful Post: gregzoll (11-04-2011)
11-02-2011, 09:23 AM   #7
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Quote:
 Originally Posted by mattdu I bought my house last year and it is wired already for a backup generator to feed the service panel. I'm sure this work is all legit -- it passed the home inspection and all. I can't engage the generator without opening the main breaker first.
I'll second md2lgyk here but with an option...

If a permit was pulled, which it should have been, your wiring inspector or AHJ should have a copy on file.

 11-02-2011, 03:04 PM #8 Member     Join Date: Jan 2008 Location: Long Island Posts: 3,630 Rewards Points: 2,000 add up the amps on the compressor and condenser fan out there.the furnace fan is on a seperate breaker check that total on the furnace tag make that a seperate total. to get a ballpark value its voltage X amps=watts __________________ http://www.youtube.com/watch?v=U1qEZHhJubY
11-02-2011, 04:55 PM   #9
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 Originally Posted by MikeInDetroit I have a generator as well wired via a transfer switch with a 30 amp into the main. So would it be reasonable to assume from this thread that: anything more than 7200 will probably flip the breaker, total effective power I have available would be 7200 - surge requirements, and if the largest surge I needed to cover was 1200 watts ( like a fridge) I could then cover 6000 watts worth of household stuff? And so when buying a generator the minimums I would want are a 7200 watt generator with a running 6000 wattage? Any more could not be used anyway?
A 30 amp breaker does not automatically trip once you reach 30.00 amps. Most will allow a signifigant overload for a very short period of time to accomidate starting or "surge" loads. So the answer to your question is a qualified "no".

Quote:
 Originally Posted by MikeInDetroit On a side note I want to upgrade to cover the house AC. It has a rather large breaker so I know it takes juice. Rather than using that there is info on the panel. What commonly provided AC panel info can I use to calculate my generator needs? Sent from my iPad using DIY Forum
There is the running load amps which is usually abbreviated as "RLA" or "FLA." However the statring load is usually about two to four times the full load amps. Your generator must be capable of handling the starting load. You will also see a LRA number which is the locked rotor amps of the compressor motor. Usually all this info is on a plate somewhere on the outside unit. If your genny can hanlde the LRA them you will have no problems starting the AC.

For example, my four ton AC needs 43 amps or about 10KW to start but only requires 17 amps or about 4500 watts to run. The LRA on mine is about 108 amps.

11-04-2011, 07:33 AM   #10
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Terrific feedback. Thank you. Imbedded answer to why GFCI switches are so needed. There is imbedded delay to accommodate surges that might create issues if an immediate cut off was required.

Speaking of which, i had a problem with two GFCI switches triggering each other when they shared a ground. I share a ground with six outlets in my outside landscaping. So i have one GFCI over the six I have not started using them yet Before i do i wonder if i should just replace all with GFCI to avoid triggering problems?

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