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Sizing a whole house generator for a 3 Ton Air Conditioner

Delmar

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Joined
Dec 9, 2019
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817
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Lake Conroe Texas
Grandma got spooked from the Texas cold and blackouts and wants a generator to run her furnace. Must operate autonomous which pretty much must be a whole house unit with auto transfer switch. Might as well do it right and size to run the AC unit during summer hurricanes.

Please confirm my sizing calculations for the 14A running, 77A locked compressor. It appears the absolute minimum is 7.5KW however the Generac charts suggest a 16KW which seems excessive.

Will a 10KW generator be acceptable? I can eliminate the microwave, clothes & dishwasher and other nonessential appliances from the transfer switch.

Major benefit of the generator for me: her house is nearby so will be a reliable source of backup power to run the wall charger on my Powerwheels golf cart.
 

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I switched my portable 5500kw and 2200kw generators over to tri-fuel (gasoline, propane, and natural gas) to operate under numerous zombie or liberal apocalypses scenarios. I have the manual back feed set up that would not be conducive to grandma. I have a much larger A/C, so I do not think I will get to running it with my gens. But for as small as grandmas unit is, look for a soft start device. That will drastically reduce your start up amps and let you run the A/C reliably with a much smaller gen.
 
14A, 240V is 3360W (5 HP)
Ideally, you'd have an inverter and battery capable of delivering the 18 kW starting surge.

Specs for my SI-6048 are 120V, 5750W continuous at 25 degrees C, 11 kW for 3 seconds, 180A (21.6 kW) for 60 milliseconds.
There is an off-chance one of these with 120/240V transformer could start the compressor.
A pair of them for 120/240V, no problem.

Some other inverters I've seen mentioned on the forum had 18 kW surge. It was a 6kW high-frequency inverter, with 20 seconds 18 kW surge. Only $1200



I think such an inverter with a small auto-start generator (and optional PV) would be a good way to go.
 
your best solution is the soft start like others have said there about 300-400$ https://www.microair.net/ 14 amps is 3360watts, so I would check with a meter tho, 2 1/2 ton only pulls2200 watts which is lower than the listed current . I would presume yours is about 2800 watts running. I've done a lot of research with mirco air and it should be able to get the surge down to about 7000 watts . I what i would recomend is to buy and centech or uni-t inrush current meter for 60$ and the micro air soft start and install it and see what your real inrush needs are and that point you can then buy a perfectly suited generator as opposed to just going way over sized. but yeah the inrush amp are real. . my ac pulls 18,000 watts on start up , this was also verified with a inrush meter.
 
I helped another contractor out when he replace a 3 ton heat pump with a new heat pump. The new heat pump would trip out the 8 KW water turbine on site. We installed a Sure-Start soft starter and was able to get it to start and run (just). The water turbine has a lot of mass for starting so I think 12KW-16KW generator would be needed. Your A/C unit has the same 3 ton Scroll compressor as my 3 ton unit. Soft-Starter don't work on all compressor. Scroll compressor start unload, rotary compressor(common in rv) start unloaded doe to system design. Piston compressor start fully load and need full torque and will not work or last on a soft-start.
 

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Grandma got spooked from the Texas cold and blackouts and wants a generator to run her furnace. Must operate autonomous which pretty much must be a whole house unit with auto transfer switch. Might as well do it right and size to run the AC unit during summer hurricanes.

Please confirm my sizing calculations for the 14A running, 77A locked compressor. It appears the absolute minimum is 7.5KW however the Generac charts suggest a 16KW which seems excessive.

Will a 10KW generator be acceptable? I can eliminate the microwave, clothes & dishwasher and other nonessential appliances from the transfer switch.

Major benefit of the generator for me: her house is nearby so will be a reliable source of backup power to run the wall charger on my Powerwheels golf cart.
Let me save you a ton of money, get a motor soft start for the AC, a hyper engineering soft start. My surge went from 71 amps or 16-17k watts to 6000-7000. Running wattage is around 4500. I would have to look at the name plate. My 13 KW could not start it. Now it can. Keep in mind The larger you go the more fuel it will burn, it will cost more and it self tests monthly which also burns fuel. You can save yourself a ton of headache and just use window units.

Also skip generac. They suck. The engines and alternators are great. Everything else on it is made in china and plastic. They break often and cost a fortune for small parts.

Champion makes a really nice 8.5k unit that uses a common honda clone single piston engine. In the event of a failed engine outside of warranty, it shouldn't be more than 400-500 dollars to get a replacement engine. Champion parts are very reasonably prices. One of the reasons I use them, its cheaper to repair than buy a new one. Something not very common anymore.

The champion standbys go for 2500-3000ish I think. They have a 10 year warranty on grid, not off grid meaning its a standby unit. 24 volt starter so its going to work no matter how cold it gets. Lots of good reviews and off grid people (who have no warranty) getting 3000+ hours out of it with 0 issues. I highly recommend that and skipping the central air. Get a big ass window unit if you must and cool only certain parts of the house.
 
Is cost a real factor?

I am also in Texas and I am getting a 27kw Generac, the RG027. But, I don't need 27kw (Actually 25kw on NG). It was expensive, but I think its worth it.

The only reason I got the 27kw RG027 is that its liquid cooled and just a little more money than the RG022 which is just 20KW on NG. The Liquid cooled generators run at 1800RPM and have real car engines, in this case a 2.4l Mitsubishi engine, and they are more efficient. the RG027 running at half load which is 12.5kw uses less natural gas than the air cooled unit at half load producing just 11kw.

The cheaper 999cc 3600rpm air cooled units just don't seem like they are built for the long haul, so if you are getting a permanent install, I would go up to the liquid cooled units if you can stomach the cost. Its my opinion they will last much longer, and be cheaper in the long run, and they are quieter. Being liquid cooled they can also take the brutal Texas heat much better (Which will only get worse...)

My backup plan for power until the generator arrives, and if the Natural Gas ever goes out is a 8750w Champion Inverter Generator from Amazon for just $917 (I think Deveak above might be talking about the same one) paired with a Hyper Engineering SureStart. I tested it last week, and it powers my 4 Ton AC plus a bunch of stuff in the house no problem. I'm using a 30a inlet with a generator interlock kit. I also ordered the NG Conversion kit for it, so if my big generator dies, or it doesn't arrive before a hurricane, I can still power the house "infidelity" on NG with the portable generator

Personally, I think the choice should be an expensive liquid cooled unit, and/or a small air cooled portable unit. Skip the permanent air cooled unit, I don't think it should be an option
 
I bought the Generac 14KW at Lowes for $4097. This was one of the few ways to purchase the generator without installation. Already completed the gas connection so the generator now runs. It's louder than I expected.

Was originally planning the transfer switch install myself, however am getting lazy and may have a local electrician install for me. Probably could install in a day what would take me a month of running to the hardware store for one last terminal-connector-breaker-wire.

Per above the sizing was not for cost, it was for Damage running generator with no load? The large surge is only needed to get the AC running.
 
i would look at used cummins, detroit diesel or caterpillar generators.
I picked up a 125KW Cat for $8000, rebuilt it for $2K,

10 times the output and half what you will pay for a generac
 
How about replacing this power (surge) hog with one or more mini-splits or whole-house heat-pumps that don't have "surge". Home Depot has 36,000 BTU mini-spits. For example, my Lennox 4 ton heat-pump compressor has a max of 18a@240v = 4,320w - e.g. a more common 7000w generator would work.

The size of generator(s) discussed above might cost as much as a mini-split + a common generator :)
 
The Generac 14KW at Lowes was $4200 with tax delivered to my door, plus around $200 for the battery and various gas line fittings. Next Monday a local electrical contractor will install the transfer switch for $1000 with new breakers (I could have done it but I'm lazy and its hot outside). Total installed price is around $5400. Would have been double+ if I went the easy way and contracted a Generac installer.
 
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i agree with offgridinthecity if its just for AC mini splits is a great way to avoid needing a large generator.

those standard 7K gas driven units are loud, i would definately listen to whatever unit you are considering and how loud it is when running if its located close to living space
 
those standard 7K gas driven units are loud, i would definately listen to whatever unit you are considering and how loud it is when running if its located close to living space
Regardless of generator noise, it will still be quieter than Grandmas complaining when she does not have TV or HVAC.
 
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i would look at used cummins, detroit diesel or caterpillar generators.
I picked up a 125KW Cat for $8000, rebuilt it for $2K,

10 times the output and half what you will pay for a generac

Did you install that in your 2009 Bounder?
 
lol bounder can barely pull it, not legally. i think its about 6000lbs plus the steel trailer is probably another 1200-1500
Generator is for the farm mostly for irrigation well pumps. in a dry summer we can easily go thru 3000 gallons of diesel a month.
Farming gets expensive lol.
 
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