Yet another generator question

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  #1  
Old 08-07-11, 03:58 PM
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Yet another generator question

Over the time I've been here, I've read a number of generator/transfer switch questions and answers, and thought I knew it all... that is until I wanted to actually do it for myself.

I am looking at a 10,000w generator (8,000w running). It has a 30A twist-lock receptacle (L14-30) on it which would be connected to an inlet at the house and to the transfer switch. To me, 30A @ 240v = 7200 watts. So when a few devices turn on at the same time and pull 9Kw, I'm concerned about the generator breaker tripping. Even pulling the running 8Kw could end up tripping the breaker, right? I feel like I'm missing something.

Question 2 is about the inlet at the house. Again, it would seem correct to install a 50A twist-lock inlet, connected to a similarly sized transfer panel. But it seems like the whole setup is limited by that 30A receptacle and 30A breaker on the generator.

Question 3 involves me actually reading the manual. One of the generators that I'm considering has the following statement:
This generator incorporates GFCI (Ground Fault Circuit
Interrupter) outlet protection and has its neutral bonded to
ground to comply to OSHA inspections on job sites. This
generator will not function when connected to a 2 pole
transfer switch since the home or building main breaker box
also has a neutral bonded to ground. When both the
generator and the home or building breaker box contains a
neutral bonded to ground, the generators GFCI will open and
no outlets will function.
Umm.... basically meaning it won't work with a transfer switch? Later in the manual it talks about requiring a transfer switch if connecting to a building electrical system (which makes sense).

Will I need to remove the neutral/ground jumper even though the manual says it could be bad?

Thanks!

-Mike
 
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  #2  
Old 08-07-11, 04:59 PM
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OSHA requires that generators used on jobsites have GFCI protected receptacles. This may, or may not, include the four-wire twistlock receptacle outputting 240/120 volts.

Assuming that the four-wire receptacle DOES have GFCI protection you can use a three-wire transfer SWITCH with a separate circuit breaker panel for your standby loads. The center pole of the switch would switch the neutral from the service panel (where it is bonded to the equipment ground) to an isolated neutral from the generator. This would make the generator a "separately derived source" in accordance with NEC provisions. The neutral-equipment ground bond would then exist at the "source" of power in either switch position.

Otherwise it would be necessary to remove the neutral-equipment ground bond at the generator. Doing this would make the generator in violation of OSHA if it were ever used for jobsite power. A switch could be installed on the generator to allow either bonded neutral or non-bonded neutral operation depending on the load but the mere presence of the switch may be an OSHA violation.
 
  #3  
Old 08-07-11, 05:13 PM
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Thanks Furd - I appreciate the OSHA clarification. I don't plan on using this generator on a job site, so I'm not too concerned about that. Just about the safety at the house!

I'm still perplexed though by the 30A breaker/receptacle...
 
  #4  
Old 08-07-11, 05:59 PM
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Generally speaking most electrical items such as circuit breakers and receptacles should be limited to no more than 80% of their rating for continuous loads. The NEC describes a "continuous load" as being more than three hours. Otherwise it is acceptable to operate at the rated load for anything less than three hours.

8000 watts at 240 volts is 33.33 amperes, or approximately ten percent overload for the receptacle and circuit breaker. Such an overload, while not recommended, is acceptable for a short duration. The 10,000 watt surge rating of the generator only applies for a few seconds to at most maybe a minute and is just to get larger electric motors running. Quite honestly, I would not think of any continuous loading of that particular machine to any more than about 6500 watts.

For what it's worth, I can live reasonably comfortably with my 2500 watt (3000 watt surge) 120 volt (only) generator supplying power to my refrigerator, furnace, television and DVD or computer and still do some minor cooking with either (not both at the same time) a countertop convection oven or microwave oven. This also includes a few lights. Of course I also live alone and I do not have a well or sewer pump to worry about.
 
  #5  
Old 08-07-11, 07:00 PM
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I would install a switch on the generator to bond/unbond the neutral. I would also put on one of those red things over it, so in order for the switch to be on the red cover must be up.

I'm suprised the generator doesn't have a 14-50 receptacle on a 40A breaker.

My family can live off a 4Kva generator which powers everything except the range and dryer. We run our fridge, freezer, boiler, lights, tv's, computers, ceiling fans, microwave, deep fryer, tablesaw, and a string of lights. Our grill has a gas burner on it which we use to cook and have city water/sewer.
 
  #6  
Old 08-07-11, 08:07 PM
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Thanks for the input. I'm actually helping out my parents get a backup system for a horse barn w/ house.

The 10Kw (8Kw) generator is actually based on what a generator installer recommended for a fixed propane installation including well pump and a bit of electric heating (which I know is horrendous in terms of power-suck). The whole installation minus propane tank was $9K, which was waaay more than I thought was worth. I sold my Mom on a portable gas generator and associated equipment. Total price will be around $2K. A pretty good savings even if you do have to maintain the gas generator more often than the propane.

Regardless, I'll redo the calculations and see if we can get away with a smaller gen.

Thanks again!

-Mike
 
  #7  
Old 08-07-11, 09:41 PM
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The conversion from gasoline to propane is pretty simple and can be done for less than $500, often less than $300 depending on how you want to do it. I probably spent around $250 on my Yamaha generator because I wanted it to look the same as when it was gasoline.

http://uscarburetion.com/
 
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Old 08-10-11, 08:20 AM
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Thanks Furd.

One (hopefully) last question.

Should the wiring/transfer switch be sized based on the 30A breaker on the generator (10ga) or the continuous power of the gen (8000w = 33A = 8ga)?

Thanks,
Mike
 
  #9  
Old 08-10-11, 09:58 AM
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I would use 8 awg.
_______________
 
  #10  
Old 08-10-11, 01:07 PM
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You can use #10 if you want. The bottom line is that the OCPD on the circuit is 30A so it is a 30A circuit. There would be nothing wrong with upsizing if you want, but it would not be required.

As with most things, manufacturers tend to use the most optimistic estimate and then exaggerate from there. The generator probably will realistically max out in the mid 20s of amps continuous under normal use without bogging and/or stalling. The nameplate ratings are done in a lab with a tuned engine, brand new fuel, pristine clean carb, etc. When an ice storm hits in the middle of the night you won't find conditions like that out beside your garage.
 
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