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Old 10-14-2010, 10:22 AM   #1
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Generator connection questions


I have some questions regarding how to connect a portable generator to a house.

The generator is rated 8000W with 12000W peak. The unit has a 4-prong outlet that states that it is 30A 120/240V. My plan is to backfeed into the main house panel, with an "interlock switch" to prevent backfeed to the supply power grid.

I have a few questions:

1) The stated "30A" on the generator does not seem compatible with 8000W (more like 7200W by my math), let alone 12kW (surge). What am I missing here? Is that outlet able to provide the full measure of the generator's capability?

2) I had planned to run #10 to make the connection back to the panel (based upon "30A" as stated on the generator). Is this adequate?

3) What size breaker should I backfeed through into the main panel? 30A? Seems like that would trip during a surge of 12kW.

Thanks

Eric

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Old 10-14-2010, 08:21 PM   #2
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Generator connection questions


Quote:
Originally Posted by mn1247 View Post
I have some questions regarding how to connect a portable generator to a house.

The generator is rated 8000W with 12000W peak. The unit has a 4-prong outlet that states that it is 30A 120/240V. My plan is to backfeed into the main house panel, with an "interlock switch" to prevent backfeed to the supply power grid.

I have a few questions:
Ok read on .,,,

Quote:
1) The stated "30A" on the generator does not seem compatible with 8000W (more like 7200W by my math), let alone 12kW (surge). What am I missing here? Is that outlet able to provide the full measure of the generator's capability?
The 8000 watt is the factory normal wattage rating and the surge rating is used for when you have any electrical motor load to tempory support during start up. and some case it may have nonstandard breaker size in there like 32 amp breaker in there { sometime it will be 30 as well }

Quote:
2) I had planned to run #10 to make the connection back to the panel (based upon "30A" as stated on the generator). Is this adequate?
If super short run yeah it is not a issue but you will need a inlet box for the generator cord to plug in.

Quote:
3) What size breaker should I backfeed through into the main panel? 30A? Seems like that would trip during a surge of 12kW.
There is few ways to do this in legal way one is use the subpanel with interlock on it or use manual transfer switch to switch over the circuits as you need.

However with any generators you have to watch the wattage and keep the load balnced.

And make sure you get proper perimits and do follow the codes no short cuts in here. { many place are cracking down with bad set ups.}

Merci.
Marc

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Old 10-15-2010, 09:26 AM   #3
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Generator connection questions


Does your generator have some 120 volt receptacles in addition to the 30 amp 4 prong receptacle?

The 30 amp receptacle won't be hurt by occasional 33 amp current draw. The next size receptacle is rated 40 amps.

If the generator is able to and is going to supply more than 24 amps for long periods of time (continuous load) then you need 8 gauge wire including for short runs. You would use 10 gauge for intermittent loads of 30 amps.

There are four current draws to be aware of:

1. Continuous, the load the generator and the wires can take for long periods of time, including for heating and air conditioning.
2. Intermittent, the load the generator and the wires can take for perhaps half an hour at a time,
3. Starting, the load the generator can deliver for perhaps a minute, don't worry about the wire gauge.
4. Maximum, above this load the generator will go off frequency, or even stall.

I would expect the 8000 watts to be continuous but the instruction manual may say otherwise. Still it is unlikely to be drawing 7800 to 8000 watts regularly and continuously.

You can get a slow trip 30 amp breaker if you have one tool or appliance that has an unusually high starting load approaching the generator peak of 12000 watts while its own running load is around 8000 watts or less. Smaller appliances are not likely to push the power draw to 12000 watts.

The commonly mentioned wire rating (20 amps for 12 gauge) is for intermittent, use 80% of tha amps for continuous.
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Last edited by AllanJ; 10-15-2010 at 09:51 AM.
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Old 10-15-2010, 11:11 AM   #4
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Generator connection questions


Allen.,

Hate to crcizte ya but for the next size up for receptale there is no 40 amp recetpale on the listing the real next step is 50 amp rated recptale.

The 50 amp receptale is common used on both 40 and 50 amp circuits.

To OP just make sure you get plan B ready in case the generator do have 50 amp twistlock but ya never know with this wattage class it will go either way but of course it should have 30 amp twistlock on the generator.

You may have to change the inlet box and conductor size to handle larger loads like 10mm˛ or 16mm˛ { #8 or #6 AWG ( 40 / 50 A ) }so just be aware with it.

Merci.
Marc
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Old 10-16-2010, 08:16 AM   #5
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Generator connection questions


Thanks for the replies.

The unit is a Generac XP8000E. It has one 4-prong locking receptacle rated 30A 120/140V and two 20-amp 120V GFCI receptacles.

Did I buy the wrong unit? I don't really have much use for the two 20-amp outlets - my only use of the generator is to power my house. So, the 30A outlet is the only one I had planned to use.

So, I'm left wondering whether much of the capacity of the generator is inaccessible. The 30A outlet has a "surge" capacity of 33.6A (per Generac tech support). So, it appears the maximum ratings of my unit are quite a bit lower than the "8000W / 12000W" stated on the box.

I'm amazed that the manufacturer didn't include at least one outlet that is sized to handle the entire 8000/12000W specs, for house connections.

Am I understanding this correctly? Should I be looking at buying a different generator?

Thanks

Eric
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Old 10-16-2010, 08:41 PM   #6
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Generator connection questions


The manufacturere "got away" with rating the generator at 8000/12000 watts by providing the two 120 volt receptacles.

Does the instructions include a section about taking off a cover plate and connecting house wires directly to some terminals, or taking the wires off the 30 amp receptacle and wiring them into house wiring?

Otherwise that model generator is not "meant to be" wired into your house system and the fact it has built in receptacles as opposed to wire-it-yourself leads tells you that.

Nothing prevents you from wiring it into your house system anyway using a cord and plug extending from your house system and living with the current draw limitations of the 30 amp receptacle.

That said, I doubt you would have any problem just plugging into the 30 amp receptacle, unless some of the things you will use are big with startup current surges such as a whole house air conditioner or a table saw or an air compressor.
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Last edited by AllanJ; 10-16-2010 at 08:56 PM.
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Old 10-17-2010, 08:28 AM   #7
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Generator connection questions


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Originally Posted by AllanJ View Post
The manufacturere "got away" with rating the generator at 8000/12000 watts by providing the two 120 volt receptacles.

Does the instructions include a section about taking off a cover plate and connecting house wires directly to some terminals, or taking the wires off the 30 amp receptacle and wiring them into house wiring?

Otherwise that model generator is not "meant to be" wired into your house system and the fact it has built in receptacles as opposed to wire-it-yourself leads tells you that.

Nothing prevents you from wiring it into your house system anyway using a cord and plug extending from your house system and living with the current draw limitations of the 30 amp receptacle.

That said, I doubt you would have any problem just plugging into the 30 amp receptacle, unless some of the things you will use are big with startup current surges such as a whole house air conditioner or a table saw or an air compressor.
No such section! I guess I bought the wrong one. I may try to exchange it.

The biggest surges are 1) Well-pump 2) Heater fan 3) 'Fridge x 2. I doubt we'll need the AC.

Eric

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