I am trying to figure costs on a sub station for our fire dept. What I need to find out is how big of a standby generator do we need.
We will be operating about 12 plugs, 7 lights,(cfl’s), 6 floodlights, 1 refrigerator, and 3 ton hvac. everything else will be gas.
Any ideas?
Where there’s a will, there are 500 relatives
Replies
Basically you need a generator that will start and run the 3-ton compressor. Everything else is (relatively) negligible beside that. You can really shave it close if you somehow arrange to shut down other loads while the compressor is starting.
And a lot has to do with the choice of compressor -- some are much easier starting (lower peak current) than others.
Thats what I thought too, but when I called the electric supply they said I needed my elec. to tell me what I needed. We dont have one nailed down yet and I dont want to bother one of them with something that is 6 months down the road.99% of the time when the power goes out their will be no one there. It wont be used as a shelter or anything. I was thinking about 13-15 KW but I just want to be sure as I am not an elec. by any means.We will be going with a propane model BTW.Where there's a will, there are 500 relatives
Edited 10/23/2007 2:51 pm by bambam
Well, the problem is that I'm not really up on what the starting current is for a 3-ton compressor, etc. But that's the big issue.If you know the HVAC manufacturer they'd be able to give you starting current and maybe even suggestions as to generator margins.
If your view never changes you're following the wrong leader
We do know the Hvac sub. Thats good advice, thanks.Where there's a will, there are 500 relatives
Is the 3 ton hvac vital in a power outage?
Yeah, that's a valid question. Without the HVAC you can probably get by with a unit about 1/4 the size. You could still have fan and gas heat, just no AC or heat pump.
If your view never changes you're following the wrong leader
At times it would be. If the power goes off in the area then some of the FFs would head to the station. Plus in the event of a disaster like Rita we woild be passing out needed stuff like ice, water and food. When that happens some of the older crowd may come and get cool there.But most of the time no it would not be vital.Where there's a will, there are 500 relatives
Another thing to consider is whether you want this unit to be auto-start/auto-cutover. If so that sets a practical minimum size for the unit and implies some extra costs for the automatic equipment. Plus with a manual unit it could be portable and used for emergency power elsewhere when needed.
What voltage is the electrical service, and is it single or three phase? For a fire house I'll guess they have 120/208 three phase.
I have a Caterpillar generator sizing program here on my computer; if I enter just the 3 ton HVAC, with all the standard default values (standard efficiency, starting across the line, etc.) it says a 25kw generator will be needed. Cat's sizing program does tend to be a little conservative but that's probably in the ball park for what you'll need. I didn't add in any other loads, but as Dan said generally they don't have much influence on the sizing - if it's big enough to run the HVAC, the lights and outlets probably won't be a problem.
I don't usually deal with generators that small, but I'll guess a diesel unit will be in the $15,000-$20,000 range and a natural gas unit will be a few thousand dollars cheaper.
Edited to add: I'd think the generator supplier (whether it's Cat, Detroit Diesel/Onan, or any of the others) should be able to size up the generator for you as long as you give them a list of the loads. They'll do the same thing I just did, and enter the numbers in a computer program and it will spit out the recommended size. It only takes a few minutes to do.
Edited 10/23/2007 5:06 pm by Stuart
If your talking about the power supply I think it will be 3 phase. The property is right on the border of another utility that I am not familar with. Theirs is probably also 3 phase though.Where there's a will, there are 500 relatives
you will probably need something larger than your 13-15Kw
the minumum size should be a unit that can deliver the Locked Rotor Amps ( LRA) of the AC
maybe upwards of 90 Amps for 3 ton but I'm just guessing
that info is on the dataplate on the unit and Stuarts' 25Kw is probably closer to what is realistic
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, wer ist jetzt der Idiot ?
I would rather have more than I need than not enough. But alas, money is tight.
I dont think there would be that much price diffrence in the 25 Kw.Where there's a will, there are 500 relatives
Just thinking outloud. Since your considering long term, disaster backup power, how do you refill your air bottles. Is that equipment gas or electric, or even on site? Could be at another station..like I said, just thinking outloud.TFB (Bill)
The cascade is at another station and if we get our own it will be portable with its own power supply. That or it will be just an air station that we refill elswhere.
Thanks though, this is the kind of feed back I need.Where there's a will, there are 500 relatives
I just thought of something you may want to look in to: the federal government has a program where military and government surplus equipment can be sold to local fire departments for pennies on the dollar. Last year I went to look at a generator for one of the local communities here in Minnesota. There's a government surplus depot in the area and they had a number of generators, and we could have got one for just a few thousand bucks. As it turned out, the only ones they had at the time that were the right size in kw were a weird military voltage that wouldn't have worked, but they did have some others that did have 'regular' voltages. It was kind of like a rummage sale, some of the stuff was great and some of it wasn't so great, you have to poke around for a while to find the gems in the rough.
I'm pretty sure it's a program that's available nationwide, and not just here. I think this is a link to the appropriate GSA website; if not have your fire chief check into it. There may be some information available from the state fire marshall.
http://www.gsa.gov/Portal/gsa/ep/contentView.do?contentType=GSA_BASIC&contentId=10790&noc=T
This depot we went to was pretty neat - they had all kinds of cool stuff there, pickup trucks, Hummvees, Boston Whaler boats, big fire trucks like you see at airports, fire pumps, all kinds of things. The city I was consulting for did pick up a boat and a pickup truck for the fire department's use, and they were ridiculously cheap. Some of the equipment was pretty tired (for instance, the Hummvees had come back from the middle east) but other equipment was in nearly new condition.
Cool link. ThanksWhere there's a will, there are 500 relatives
to add to what reno wrote.picking out a standby generator is a bit more involved then just buying one..transfer switches exercisers, load banks...maintenance.
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the minumum size should be a unit that can deliver the Locked Rotor Amps ( LRA)
Are you saying that the continuous-duty rating of the generator needs to match the compressor's start-up current?
Ed
No, the surge capacity must. Generally the HP of the genset motor must be near that level (momentum can carry it a little further), but the steady-state current capacity of the actual generator can be lower. Just as wiring can carry, for a brief time, a surge that's 1.5x the circuit's rating, so can the generator.
If your view never changes you're following the wrong leader
no not at all, it has to be able to start that AC which could be a very large draw, which is the LRA another reason for getting some qualified people to determine that for him
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I was just wondering about your comment because 25 KVA sounded kinda oversized to me, even with an LRA of 90.
Ed
Edited 10/25/2007 10:14 am ET by edlee
I ran the numbers with a different genset sizing program, and this time came up with a 20kW/25kVA generator requirement (the last size I posted was 25kw, which came from an admittedly conservative sizing program.)
The total running load is only about 3kVA but the starting load is more like 21kVA and that's where you need the size. This is a case where the running load is so much less than the generator's capacity that there could be a problem with wet stacking on the generator engine - it would need to be run at full load with a load bank occasionally (basically, to blow the carbon out once in a while.)
there's the rub.bambam might have to throw in some 1000w quartz lights they could switch on when the genny is running or an elec stove or two.
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I didn't run the numbers. but the original 13-15 Kw seemed small while Stuarts calcs were closer to what was going to start that thingin the past I would mention here to use 2500 w/hp to size portable or standby since that was a rule of thumb for the engineers I had spoken with who designed a few of the systems I worked on..granted they are huge units and not something a HO would ever buy.unless they are Bill Gates.
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I'll mention again that it's worthwhile looking into an AC unit with an "soft start" feature. Through various techniques the starting load can be reduced 30% or so for a given tonnage.
If your view never changes you're following the wrong leader
I didn't now you could soft start a compressor, but it is a great idea..
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., wer ist jetzt der Idiot ?
A 3 ton AC would draw about 4,000 watts.
I really don't know tht much about generators, but I would expect that times 2 would start it.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
I looked up Generac web site and they said that their 10 Kw would run a 3 ton unit. I dont really know how to size it myself though. They did have an 18 Kw that looked promising. Supposed to power a small house.Where there's a will, there are 500 relatives
You're right ... you need a sparky.
You'll need to do a lot of talking with the PoCo. They may not talk to you.
The generator suppliers are also a great resource. make them earn the sale.
There are any number of threads here about generators ... there are a lot of other issues. This is not like buying a new washing machine. Generators are one thing that is NOT for the DIY.
We will be using an elec., just not sure wich one yet. I still have to find which utility it is. We just got the land yesterday.Where there's a will, there are 500 relatives
You'll need to do a lot of talking with the PoCo. They may not talk to you.
Huh?
Most PoCo would want to know who has standby emergency generators. Hospitals, police and fire departments are on our first responce list durring any outage. Knowing that they are, or can be, self sufficient for some critical period of time means we can respond faster to those on the list that don't have that capibility.
Fire departments and police are first reponders durring power outages. Public safety is a primary concern durring outages. We depend on both, as well as our own resources, to accomplish that goal.
We have trained techs and engineers that can help.
Why would any PoCo not want to help?
Dave
A PoCo has obligations to the community, as well as a duty to protect it's equipment - and linemen. One of their more common policies is to require that any service changes have load calculations ... which they often require to be signed off on by either a licensed engineer or electrical contractor. Generator installations are a particular concern, because of the risk posed by 'back fed' lines. It wasn't all that long ago that PoCo's required absolute proof of the destruction of all generating equipment, before connecting outlying farms to the 'grid.' That's why the PoCo may not allow DIY generator installs.