Natural Gas vs. Diesel Generators

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tmillard

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Hi, I have a quick question...

I am providing a 50kw generator for a school that will be used for egress lighting only. Since this falls under NEC Arcticle 700 as an Emergency System, I pretty much am limited to a diesel unit since I need 2 hours of on site fuel.

I remember reading something a while back that I couldn't use a natural gas gen with a utility hook up since that fuel source was considered interruptible.

Does this make sense?

Thanks...

Tom M.
 
As I read 700.12(B)(2), you have to have two hours worth of fuel on site. I think that you don't get to take credit for natural gas that is in pipes located off-site. If you had an on-site natural gas tank, with enough pressure to keep the generator going for two hours, it might be OK (as far as the NEC is concerned). However, the local Fire Marshall might have something to say about a gas tank that size being anywhere near a school. So it will come down to an AHJ decision.

By the way, why is your generator so big? 50KW can power over 600 light fixtures, assuming 2 bulbs per fixture at 40 watts each are on the Emergency circuit.
 
Liquid Propane is another fuel option. However, there are problems with propane as well. Liquid propane can be fed to the engine as a liquid or as a vapor. Liquid fed systems require a fuel evaporator unit on the generator. These are often difficult to start during cold weather. Vapor fed systems start fine, but they require the fuel tank to supply the vaporization energy. As a result, the fuel storage tank must be large enough to be able to vaporize fuel as fast as the generator consumes it.
Diesel fuel is generally stored directly below the genset in a sub-base tank. Makes for a clean installation. With the various heaters (fuel, block, alternator, etc) that these units have now, they work very well even in cold weather. If I were not allowed to install a diesel unit, I would opt for a vapor fed LP unit with a properly sized LP tank.
 
Let me add one more plug for diesel engines: They have a better load response time than natural gas. Emergency systems are supposed to be able to supply the load in 10 seconds upon loss of power. Gas engines have a harder time doing that--not that it's impossible--especially if it's way oversized.

I'm with Charlie: How'd you come up with that size?

(Just curious)

-Dale
 
It almost seems that battery-supplied backup would be more economical and reliable, not to mention instantaneous.
 
Charlie:

I read 700.12 (B)(2) differently. It says "a on site fuel supply shall have a on-premise fuel supply sufficient for not less than 2 hours". IMO, if you don't have a "on-site" fuel supply, you are not required to have the 2 hour "on-premise" fuel supply.

700.12(B)(3) clearly says a natural gas installation can be used where acceptable to the AHJ, and where there is a low probability of a simultaneous failure. We have done just that here in Illinois. However, it wouldn't be a good idea in California, since there is a good chance an earthquake could take out both the natural gas and electricity supply.

Stve
 
A word of caution about Diesel. It can go bad. In your application, the only time the gen set is going to run is when the power goes out and for 30 minutes during the weekly excercise. I guess if you are carefull not to oversize the tank and you should be able to avoid any trouble.

As for battery systems, school maintenance folks hate them. If batteries are not maintained the won't work. I would ask the owner if they have a preference (they usually do).

As for LP / Natural Gas you may have it on site already. Most large kitchens have gas of some sort. (if your school is big enough to need a 50kw genset for emerg lights it probably has a kitchen). Also most science labs (especially upper school) use gas of some sort.
 
steve66 said:
IMO, if you don't have a "on-site" fuel supply, you are not required to have the 2 hour "on-premise" fuel supply.
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This subject does have me a bit confused. Do you have an opinion regarding the difference between "on-site" and "on-premise"? And for that matter, what does "supply" mean, in the phrase "fuel supply"? Specifically, is a pipe that originates off the property a "fuel supply," or does that phrase only apply to tanks?

 
I'm with you Charlie. I'm scratching my head about this one as well. Hmmmmm! I guess we see diesel units here because the engines are smaller and less expensive to purchase. I thought it had to do with the fuel supply, but it may just be as simple as purchase price alone!
 
Charlie:

This is strictly my opinion, but I think it relates to a site with several buildings and more than one generator. It might apply to a university campus, for example. We have a similar arrangement at a local hospital. A large "on-site" fuel tank is common to all the generators. It feeds an oil supply loop that feeds a "day tank" at each generator. So the day tank at each generator would be the "on-premisis" tank.

At a class one time, we were given an example of a similar arrangement, but due to oversight, the pump that ran the loop wasn't on emergency power. Sometime after a test run, one generator only had about a 1/4 tank of gas left. A power failure occured, and that generator only ran for about 15 minutes before running out of gas.

So I think the "2 hour" minimum "on premisis" supply is supposed to be enough to get a low fuel level warning, and give the maintence staff time to investigate before before the generator runs out of gas.

Steve
 
steve66 said:
I read 700.12 (B)(2) differently. It says "a on site fuel supply shall have a on-premise fuel supply sufficient for not less than 2 hours". IMO, if you don't have a "on-site" fuel supply, you are not required to have the 2 hour "on-premise" fuel supply.
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I just went back an re-read 700.12(B). IMO, that is not what it says. It says that if you have an internal combustion engine,
. . . an on-site fuel supply shall be provided with an on-premise fuel supply sufficient for not less than two hours . . . .
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I read that as saying,

(1) . . . an on-site fuel supply shall be provided . . .

AND

(2) (it shall be equipped) with an on-premise fuel supply . . .

I think this is explicitly forbidding reliance on an off-site supply that is fed to the site via pipes. They don't want us to rely on a pump that is located off-site as the method of getting fuel to the engine. They want a tank on site, so that the pumps that bring fuel from that tank to the engine are also on-site, are under the control of the same management, and are powered by the emergency generator.

But this brings up a problem for generators that use natural gas as their fuel. Two of my co-workers with expertise in machines and all things mechanical tell me that a machine that runs on natural gas is, by definition, an "internal combustion engine." Thus, 700.12(B) applies, and you need an on-site fuel supply.

For those of you who encounter emergency generators that run on natural gas, how do you read 700.12(B), and how do you get the generators to comply?
 
charlie b said:
For those of you who encounter emergency generators that run on natural gas, how do you read 700.12(B), and how do you get the generators to comply?
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We install them, the gas fitter brings a gas line to them and we get an inspection. ;)

It is actually a decision made well above my position, I only install them, I don't specify them.

I believe this is 'how'

Exception: Where acceptable to the authority having jurisdiction, the use of other than on-site fuels shall be permitted where there is a low probability of a simultaneous failure of both the off-site fuel delivery system and power from the outside electrical utility company.
Click to expand...

That said I have read other posts from inspectors that never accept off site fuel.

I believe Larry (Sandsnow) is in an area like that.
 
tmillard said:
Hi, I have a quick question...

I am providing a 50kw generator for a school that will be used for egress lighting only. Since this falls under NEC Arcticle 700 as an Emergency System, I pretty much am limited to a diesel unit since I need 2 hours of on site fuel.

I remember reading something a while back that I couldn't use a natural gas gen with a utility hook up since that fuel source was considered interruptible.

Does this make sense?

Thanks...
Click to expand...

Two things.

1) An inverter would work better for this application.

2) An on-site diesel source is more reliable than natural gas. Especially in earthquake prone areas. Relying on an outside source to provide you fuel in an emergency situation gives me the heebie-jeebies. :)
 
Gen Update

Gen Update

Thanks for all the responces.

The director of facilities for the school district requested we install the generator. Originally they wanted to feed the kitchens coolers and freezers from this gen but it was value engineered out for budget purposes. They will probably add a second transfer switch and pick up these loads next year. We are just above the 30% load so there shouldn't be any wet stacking issues.

As for the fuel issue, I prefer a diesel unit with a skid mounted fuel tank. As long as you run the gen once a week you shouldn't have any fuel issues. Fuel conditioner can take care of any cold weather gelling issues, etc.

I believe it would be almost impossible to find an AHJ that will sign off allowing you to use natural gas supplied from a utility.

Thanks...

Tom

Thomas Millard, P.E. LEED AP
 
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