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Need help sizing generator to run central air

southfork

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#1
I have a 4 ton 16 seer unit, although the lra is listed as 109 the actual draw on each leg to compressor was 29/30 , after installing a -5/2/1 hard start kit the start amps are 19/20 on each leg so around 40 amps, blower is on time delay so does not enter equation, Im thinking a 12k generator should handle this, obviously no stove or hot water heater running, has anyone sized their ac for a generator and run it sucessfully.
 

DodgebyDave

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#2
Get the biggest and best one you can afford.
 

BackwardsEngineeer

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#3
South,
Is this for occasional usage? Reason I ask is that there is limited, if any, service factor any more. So you may purchase a unit and honestly stay below rated output and still have premature failure by operating close to unit max rating. I have started to oversize items like these by as much 1.5 to 2 times what is needed if it is a critical application. My best example is that back in the 70's I changed out a mess of 100 HP 480v 3 phase motors, those beasts created in the 40's weighed almost 600# the new ones 300#. Less steel, less copper, less bearing pretty much less everything. Now those 300#ers are being replaced with 215# units. Somehow same application has gone from a 600# to 215# answer. Pretty clear to see you better not overload that 215# for very long or bad things are going to happen.....
my2c....
 

nickndfl

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#4
I used a $99 5000 BTU window unit to cool 1 bedroom and plug it into a 10,000 kw gasoline generator. When you try to do the whole house a' la carte you can run into plenty of hurdles. You will need to calculate the pull of everything that's running.

The absolute best method is to install a Generac whole house propane generator. Then you can party on much easier, but it will cost you.
 

southfork

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South,
Is this for occasional usage? Reason I ask is that there is limited, if any, service factor any more. So you may purchase a unit and honestly stay below rated output and still have premature failure by operating close to unit max rating. I have started to oversize items like these by as much 1.5 to 2 times what is needed if it is a critical application. My best example is that back in the 70's I changed out a mess of 100 HP 480v 3 phase motors, those beasts created in the 40's weighed almost 600# the new ones 300#. Less steel, less copper, less bearing pretty much less everything. Now those 300#ers are being replaced with 215# units. Somehow same application has gone from a 600# to 215# answer. Pretty clear to see you better not overload that 215# for very long or bad things are going to h
appen.....
my2c....
Just for hurricane protection, im thinking a 12k might do it but dont want to take a chance, biggest draw the central ac, running amp one leg is 9/10 amps, 20 total , ac heats the hot water tank so no power there, really just the fridge and a tv , light or two but all are led/,s
 

ttazzman

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#6
I run whole house on a 17kw unit.......i have a 5 ton (2-stage)(starts on lower stage) geothermal unit (house) and 1.5ton geothermal unit (shop) ....everything else is also electric .....of course i wouldnt run items like washer dryer range etc in conjuction with hvac....i do turn off the shop hvac if power goes out.......but the 17kw unit has ZERO issues running it....fwiw breaker sizes are hvac compressor unit 40a 240......air handler 15a 240....also have 30amp 240 for well pump---our hot water is combo groundsoure/gas <<only thing gas in the house other than the generator .......we are on propane (higher rated than natgas)......you can usually look at your hvac unit specs and get the start up load and the running loads and make decisions based on that but obviously you cant start/run everything in the house at the same time. usually the highest single start load rules

so for me 17000watts / 240volts = 70amps of useable .......for you 12000watts /240volts = 50amps of useable........my loads are similar to yours and i allowed for 20% extra and i had a guy that owed me a genset and he had a new 17kw in stock

you should also consider the actual run time of each genset mine is about 3-5 days on a 500gal propane tank depending on load

fwiw being that geothermal is our only practical heat source that is the primary reason for the gen backup is more for cold weather than hot weather

useful article http://www.generators.smps.us/start.html
 
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GOLDBRIX

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#7
I used a $99 5000 BTU window unit to cool 1 bedroom and plug it into a 10,000 kw gasoline generator. When you try to do the whole house a' la carte you can run into plenty of hurdles. You will need to calculate the pull of everything that's running.

The absolute best method is to install a Generac whole house propane generator. Then you can party on much easier, but it will cost you.
I like the GENERAC idea my brother loves his. When his Fla. neighborhood went dark during a hurricane a few years ago his house was the only one with power. He had the neighbors come over to rest, sleep, and wash up . In the morning all the men started cutting up the trees that blocked the roadways out way before the power company could get there.

He said two more neighbors have bought themselves Generac systems.
 

Unca Walt

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#9
I run whole house on a 17kw unit.......i have a 5 ton (2-stage)(starts on lower stage) geothermal unit (house) and 1.5ton geothermal unit (shop) ....everything else is also electric .....of course i wouldnt run items like washer dryer range etc in conjuction with hvac....i do turn off the shop hvac if power goes out.......but the 17kw unit has ZERO issues running it....fwiw breaker sizes are hvac compressor unit 40a 240......air handler 15a 240....also have 30amp 240 for well pump---our hot water is combo groundsoure/gas <<only thing gas in the house other than the generator .......we are on propane (higher rated than natgas)......you can usually look at your hvac unit specs and get the start up load and the running loads and make decisions based on that but obviously you cant start/run everything in the house at the same time. usually the highest single start load rules

so for me 17000watts / 240volts = 70amps of useable .......for you 12000watts /240volts = 50amps of useable........my loads are similar to yours and i allowed for 20% extra and i had a guy that owed me a genset and he had a new 17kw in stock

you should also consider the actual run time of each genset mine is about 3-5 days on a 500gal propane tank depending on load

fwiw being that geothermal is our only practical heat source that is the primary reason for the gen backup is more for cold weather than hot weather

useful article http://www.generators.smps.us/start.html

ttazz has it spot on. In my observation, folks do careful analysis of power needs for A/C... and TOTALLY MISS the critical part:

Where the fug is all that energy gonna come from?

Answer: Either propane or gasoline. That is really it. And as ttazz points out, you get a whopping 3-5 days out of a 500 fargin gallon propane tank. Gasoline is higher in energy per unit than propane, but the problems with gasoline are much greater.

As an old engineer, I put my slide rule (no shit, I still have one!!) to it and came to a conclusion, to wit:

DON'T try to air-condition your whole fargin house. You will have to have a gasoline tanker truck parked in your driveway, or a 10,000 gallon propane tank buried in your back yard.

The solution is embarrassingly easy. Go get two items:

1. From WalMart




Product TitleGE 5000 BTU Mechanical Air Conditioner, 115V, Gray, ...
Average rating:4.5598out of5stars, based on343reviews343ratings

Current Price
$116.00
List Price$129.00
Free shipping

2. Honda eu2000 generator from... OMG: WalMart again. $999. Sumbitch even puts out DC if you want to charge batteries. TINS



This sooper-quiet darling is so easy to start, my dainty wife can start it easily. It is so fargin quiet that you have to walk over to it to listen to see if it is running.

Oh... and remember that gasoline tanker truck? This beauty runs EIGHT (8) hours on a tad LESS than a gallon of gas.

That ain't a typo. All night on a gallon of gas. Running a window A/C.

I have two of them. And I have two window A/C's... one for the bedroom, one for the living room. So my entire house is cool (way the hell down here in West Palm Fargin Beach). For two gallons a day.

I have two other generators (one for backup, one for lights, fridge/freezer, TV.) During power outages (and they can last for more than a week!!!) I cook outdoors on my propane grill. <-- It has a little propane tank that generally lasts a few months.

So lookit your/my outlay for peace of mind: $2K for two top-of-the-line quality generators, $232 for two A/C's that easily cool your whole house.

SUMMARY:

If your power is out for 3 days... where, oh where are you gonna get 300 gallons of propane delivered from for another 3 days?

Why not install Unca's Sooper System, and get away with A/C and general power for $2300 and ten 5-gallon gas cans full of gas (fitty gallons @ $2.50/gallon = $125.

Total outlay: UNDER $2500, and no more fargin worries at all.


LATE FARGIN EDIT GEEZ!!! I fergot the fargin main selling point: Outlay in bucks for Unca's setup is very probably less than 1/5 the cost of the OP concept... and the FUEL lasts ten fargin times as long
 
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GOLDBRIX

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#10
Can't argue with a man that can use a slide rule.

I like the set-up you call for Unca.
 

Alton

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#11
Generac is the de facto standard gennie for outdoor shows. Always reliable and provides rated power without problem. Typical outdoor shows needed 200 - 400 amps. That quantity has been dropping due to the advent of high speed switching power supplies in class D amplifiers. The change over to digitally based lighting and LED products has not yet significantly reduced power demand for lighting due to the use of many motorized lighting fixtures and assorted theater lighting instruments.

That said, standard rule of thumb and NEC guideline for hardwire service panels is 80% of service panel rating. So if you're using a 200 amp service panel you only want to run 160 amps through it at any given time. I've found it's good to leave yourself this level of "headroom" in case an appliance wants to take a dump on you or other weirdness happens. This headroom in current draw is also a useful "standard" for generators. So if your AC wants to see 30A/leg to be happy and healthy you would be smart to let it see that from a gennie that's rated to at least 40A/leg. Bigger here, would be better to allow for future circuit additions. Now you just have to sort it all out with Mr. Wallet to see what's workable for you.
 

ttazzman

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#12
Walt hit some important considerations as to fuel and ussage, in my situation the decision to put in a automatic unit was primarily for heating purposes especially in winter if we are traveling to protect the home from freezing.......i am in the somewhat unique position to have several generator options ....tractor pto powered and hundreds of gallons of diesel, meduim sized Gas powered gen/welder combos.....and small inverter units like Walt posted......I 100% agree with walts way of dealing with the issue from a cost, flexibility, resale etc options....as it was the option i used when i was as city dweller ....couple of pics of our primary unit setup in place now almost 10 years with no problems....
 

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hoarder

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#13
Some words of caution. Honda generators use very little fuel, which is why they often won't start and sometimes won't run at all when it's less than 20 degrees outside....too damn lean. I tried drilling out the jet on one of mine and it wouldn't run hardly at all until I bought a new jet and put it back in. Usually they are OK if you can carry it inside to warm up before you start it. Strong back helps. Generac larger generators and their older generators are pretty good, but their smaller generators made in the last decade or so have been getting some mediocre reports. For those who are shopping for new generators under 10 KW, you aren't going to get much bang for your buck.
 

nickndfl

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I like the GENERAC idea my brother loves his. When his Fla. neighborhood went dark during a hurricane a few years ago his house was the only one with power. He had the neighbors come over to rest, sleep, and wash up . In the morning all the men started cutting up the trees that blocked the roadways out way before the power company could get there.

He said two more neighbors have bought themselves Generac systems.
I been in 5 hurricanes and say there is no substitute for a Generac. I would also get a window a/c unit just in case the central system breaks down without a power failure. You can also go with a smaller gas generator which uses less fuel to complement the Generac. If you plan on keeping the house there is no better way to go.
 

tigerwillow1

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#15
Just as a data point, I have a 5 ton heat pump and a 10 kW PTO generator. The tractor engine is 23 HP, so that's the limiting factor. It's good for about 7 kW. The heat pump draws a bit under 5 kW running, but when I attempt to start it with the PTO generator, the tractor engine dies. I have propane available for backup heating so my main concern is running the well pump. It draws about 3.5 kW and the PTO generator handles it ok. For lighter loads I use an eu2000i drawing from a 500 gal propane tank.
 

Unca Walt

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Some words of caution. Honda generators use very little fuel, which is why they often won't start and sometimes won't run at all when it's less than 20 degrees outside....too damn lean. I tried drilling out the jet on one of mine and it wouldn't run hardly at all until I bought a new jet and put it back in. Usually they are OK if you can carry it inside to warm up before you start it. Strong back helps. Generac larger generators and their older generators are pretty good, but their smaller generators made in the last decade or so have been getting some mediocre reports. For those who are shopping for new generators under 10 KW, you aren't going to get much bang for your buck.

Your post is spot on. But if the temperature where I am sitting right now lookin' across at my bamboo, mango, cocoplum, and coconut trees gets to 20 degrees... We are in nuclear winter.

They ain't too heavy for the redhaired wanton I live with; they weigh 47 pounds.

So I just leaves it inna garage. Without fuel in it***

***There was a time I did not know this. No gas in the tank or line. You MUST drain your carbonator before storing.
 

DodgebyDave

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#17
snow jan 19, 1977.jpg
 

southfork

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#18
I have a coleman ive used in last two storms,, but now at 67 i need to try to cool the whole house, im going to buy a 10/12 btu portable unit vent them thru plywood on windows for back up, but still want to use the central. The duromax might be an option at 1099, puts out around 39 plus amps, to get one that does 50 amps more than doubles the price not to mention the gas consumption