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#100900 01/21/07 01:06 PM
Joined: Mar 2005
Posts: 240
howdy folks have questions about generators I hope you can help with.
with the recent storms we've had out here in the pacific northwest and extend outages everbody wants an auto generator now.
home depot sells a variety from 7kw up propane or natural gas fed.
the 7kw to 16kw come with factory assembled load center and auto transfer switch which
seem pretty straight forward to install.
art 445 seems to be somewhat light though in its chapter, as far as the do's and don'ts. is this a world of electrical which is best left alone (mean learning curve) kinda like factory assembled structure remodels?
some questions that i specifically have are
1. can one put a generator under a two story deck?
2. working clearences the same as panelboards
3. how close to a house may one set gen close to main service(meter)
5. how close to gas meter(3' typical seperation)
I am sure there are lots I am missisng to ask so if all of your years of wisdom you can think of things that can save the learning curve and insure better installation I would GREATLY appreciate.


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#100901 01/21/07 01:46 PM
Joined: Oct 2006
Posts: 65
you should be able to find the answers 3 places- Manufacters instructions, AHJ, and POCO and gas supplier. Between the 3, you should find out what you need. Some requirements will change on your locality.

#100902 01/21/07 02:50 PM
Joined: Oct 2005
Posts: 100
JJM Offline
I'm going to offer some general answers, but keep in mind local codes and your AHJ will be the final governing authority.

You can install a genset under deck. But keep in mind general clearances, 3 feet all around for servicing and access, and the exhuast 5 feet away from any building openings, such as doors, windows, dryer vents, makeup air for HVAC, etc. CO poisioning is a real issue, so keep that in mind.

I've seen plenty of installs that violate these clearance rules though... not saying it's right though.

It's advisable to keep the genset within a reasonable proximity to services (minimum 3 feet) but the genset can be installed as far away as you want too. Just keep in mind the further away you install the genset, the greater the material costs will be, not just length-wise, but size-wise as well.

You'll need to run much larger wires, which means larger conduit, and you'll need much larger gas lines. For example, you could run a typical 16KW genset with 20 feet of 3/4" pipe. Move the genset 150 feet away, and now you need 1 1/4" piping. Ditto for the cabling. Then these items need to be reduced back to the sizes the genset allows.

The biggest question is whether you'll be using NG, or LP liquid or gas. You may run into issues with NG if the meter doesn't deliver the required volume for the genset and other gas appliances, as well as the regulated pressure, as many gensets require elevated gas pressure, though newer ones are more forgiving. Volume is still an issue though.


#100903 01/22/07 01:13 AM
Joined: Mar 2005
Posts: 49
What part of the Pacific NW? I'm in Bothell, WA.

Your questions are insightfull. I'll have to be thinking about these issues myself.

One thing different from your questions you might consider is the choice of fuel. You mentioned NG or propane. In addition to NG uncertain capacity and pressure as mentioned, I've been told NG tends to not have consistent BTU content. This seems to be quite variable, regardless of what the gas company says.

Also, King County for example will not use NG for a couple of reasons. One being the BTU content, but another is its availability following an earthquake. The NG supply or service line could be broken during an earthquake leaving a generator fueless, right when it's needed. King County uses diesel.

#100904 01/22/07 01:10 PM
Joined: Mar 2005
Posts: 1,213
I wouldn't say generators are any worse than any other eletrical installation. You just have to be careful with the neutral now that you have two separately derived sources.

The smallest generators I've work with are 200kW, so I can't offer much practical advice about residential installation. We use diesel pretty much exclusively, but it's more for damage control and ease-of-support reasons. Above all, I'd say let common sense prevail! Coordinate with your mechanical subcontractor to make sure your conduit and his/her gas lines aren't interfering. As well as the exhaust, you'll also have to be careful about fuel tank vents- they need to be vented 12' above grade and 5' from any openings.

UFC/NFPA1 offers some more codes that NEC is silent on:

[This message has been edited by SteveFehr (edited 01-22-2007).]

#100905 01/22/07 08:40 PM
Joined: Mar 2005
Posts: 240
thanks guys for the help.
rich I am in olympia didnt see anything in the WAC even covering art 445.
one more question on the 5 foot rule, what if on first loor(grade elevation) ther are no windows but on second floor there is one.
other then the fact its probably not a good idea to put it here can one do it and suffice code?
this paticular house is on a hill and there is only one practical spot.

#100906 01/24/07 10:53 AM
Joined: Jun 2004
Posts: 220
I would add that at least on particular brand sold at the Home Depot, doesnt want the genset placed under a deck. This brand comes with a 25' whip from the transfer switch, which is then mated to the liquidtight whip from the genset. I try to place it within that 25' radius, because adding to the whip alters the system, and invalidates the warranty.
I don't have NG here, so we run all the gas gensets on LPG. The gas company requires a min. distance of 10' from the tank, not sure about NG though.
The static pressure required for the newer generation is 10"-12" of water column pressure. The older ones run on 11"-14"W.C. As mentioned earlier, volume is a huge deal, as gas companies seen to unersize the pipe even with the manufacturs instructions right in there face. Min. size is 3/4" pipe up to about 15', depending on 90's which add 2.5' to the length of the run. Also temperature affects the volume tremendously, at least on the LP side. These generators are tested at the factory on NG.
If you dont have the manual, go to and you will find all the answers to installation.

#100907 01/24/07 10:58 PM
Joined: Jul 2004
Posts: 9,828
Likes: 22
I just went through this with my LPG company with a gas pool heater (and a rough in for a genset).
I heard the same setback as Trekkie. 10' from everything on the tank (property line, electrical equipment, structures etc). The regulator and gas lines can pretty much be anywhere.
One thing that might be important to know is a LPG system around here is "high pressure" (around 10 PSI) up to the final regulator where it gets dropped to 10 inches of water (WC). That is in the side of my pool heater and not installed yet on the genny rough. That means you can use a smaller line than the "10 WC" designated size. With Natural Gas you still measure to the final regulator but as I remember this is the one where the meter is.
I discussed this with the gas inspector, "book" (what we call 110.3(B)) in hand. He explained the high pressure thing.

Greg Fretwell
#100908 02/13/07 11:21 AM
Joined: Apr 2002
Posts: 178
What are the concerning issues with the neutrals Steve?You bond the neutrals together at the service panel correct? The xfer switch switches only the L1 and L2? I`m asking for some insight for future reference.

#100909 02/13/07 12:54 PM
Joined: Mar 2005
Posts: 1,213
Reel-Break: the key is that you must never have a situation where the neutral is derived to ground at multiple points, as this would allow current to travel through the ground wire. There are two ways to do this:

1) ground the service and the generator neutral one one point (usually the service entrance or the transfer switch)

2) Switch the neutral at the transfer switch

There's some very good information in this thread:

Edit: you need to check the internal wiring of the generator, too, to make sure the neutral isn't grounded inside the chassis.

[This message has been edited by SteveFehr (edited 02-13-2007).]

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