can a central AC run off a portable generator?

logastellusAugust 29, 2013

Hi. We're doing a new construction in SC, 3700 sq ft home.

Would like to do a rough in for a future portable generator, I doubt we'll get a full standby model.

My question is, can a portable generator (say 10kw, about the largest I found) run an AC system?

At least for a single, 2nd floor?

Thank you!!

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Depends on the specifications of the unit you want to run.

In most cases, a 10 KW would run roughly a 2 ton maybe 2 1/2 ton a/c.

Looking at a 10 KW portable, price is pretty close to an 11 KW home stand by with a 200 amp transfer switch.

Then you'll have load management capabilities already built in and no worries about gas and all the other stuff associated with portables.

    Bookmark   August 29, 2013 at 11:17AM
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Based on the info you provided in the HVAC forum, I'd say a 10 KW would work fine.

The minimum circuit ampacity on the condenser is 9 amps.

    Bookmark   August 29, 2013 at 11:30AM
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ok thanks.

The builder is saying even a 12kw is not enough and getting a 10kw portable would "damage the AC or the generator itself."

That just doesn't sound right, does it?

    Bookmark   August 29, 2013 at 11:45AM
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What he's probably talking about is the startup current that the AC may require.

    Bookmark   August 29, 2013 at 1:00PM
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yea but even the startup wattage is about 5-6kW for a

    Bookmark   August 29, 2013 at 1:08PM
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Your contractor is pretty much correct with his reccomendations. A 10-12kw unit is marginal for central AC. It doesnt leave much room for anything else. With that being said, I can run my four ton AC with my 12kw Winco generator without problem. I installed a "hard start kit" on the AC which basically reduces the starting demand of the compressor.

If you want to rough in something for future use, I would reccomend a factory interlock on the main panel connected to a 50 Amp inlet. They are fairly cheap and with that installed, you can get a 15-26+ kw "portable" generator later to use with it.

Winco make an 18kw surge unit and Genejunk makes a few 24+kwunits that are on wheels.

    Bookmark   August 29, 2013 at 1:27PM
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OK, I'll relay this info.

I got a few more details from the AC product numbers:

Model No.1 4TTB3018E1
Electrical Data V/Ph/Hz 2 208/230/1/60
Min Branch Cir Ampacity 9
Br. Cir. Prot. Rtg. - Max (Amps) 15
Compressor CLIMATUFFî
RL Amps - LR Amps 6.4 - 40
Outdoor Fan FL Amps 0.74

So, if I'm reading this correctly.. It uses 208V, with a locked rotor Amps of 40.

Therefore, 208*40=8320W to start this unit.


    Bookmark   August 29, 2013 at 2:41PM
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That is a good ball-park number. The only way to know for certain is to check your particular installed unit with an amp meter.

For reference, my unit has an LRA of 109, but the start current is only about 42 amps.

This post was edited by Auger01 on Thu, Aug 29, 13 at 15:47

    Bookmark   August 29, 2013 at 3:44PM
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Yikes. That's a huge variance!

Well, for now we're doing what you suggested - 50amp switch/wire and we'll figure out the generators later.

Question - portables that run on natural gas - I see a few that are tri-fuel, so do those hook up to a gas line that's similar to where an outdoor grill would get hooked up to??

    Bookmark   August 29, 2013 at 4:05PM
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Yes, but a 10-12kw generator uses much more natural gas than the typical grill so it is very important to size the line from the gas meter and the meter itself for the size generator that you are considering.

A 10kw generator will usually have a 20hp engine. Figure 10,000 BTU per Hp for a grand total of 200,000 BTU's just for the generator. Once you add all the other gas appliances in the house, you will probably need a bigger gas meter than the typical house.

It is much easier and cheaper to put that in now during new construction rather than do it later.

The typical 1/2" line for a gas grill is only able to supply at most 80,000 which would limit you to a 4kw generator.

    Bookmark   August 29, 2013 at 6:51PM
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Inverter-driven compressors use much less start-up power and run much more efficiently at partial speed than other types of motors.

You might not think of it this way, but the more efficient the cooling system is, the smaller the generator you will need.

    Bookmark   August 29, 2013 at 8:19PM
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Auger - that's helpful thank you. One thing I stumbled on here is that it says to derate the power by 20% if using NG. That's a pretty raw deal.. :( That means to get true 10kw on NG, I need to plan for a 12kw generator.. Not cool.

    Bookmark   August 29, 2013 at 10:39PM
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If an engine is set up optimally for gasoline, there is always a penalty for natural gas fuel. The gaseous fuel, however, has much greater knock resistance that enables higher compression. That means that NG is the better fuel if designed for. Volumetric efficiency can be greatly increased with a supercharger or turbocharger. Thermodynamic efficiency can be greatly increased by increasing compression ratio.

    Bookmark   August 30, 2013 at 2:25PM
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Don't forget that sometimes the air conditioner will try to kick on while several other things are also turned on.

So the total wattage of the generator has to cover both the starting requirements of the AC unit and a variety of other things.

In many cases, a large generator will use more fuel than a small generator for the same load when that total load is quite low.

For these reasons many folks are discouraged from using a generator for central AC (or running the AC when using a generator).

    Bookmark   September 1, 2013 at 9:26AM
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Just got through making my 240V 8000watt generator power my A/C and home. Major issue was surge or inrush.
My LRA is 73 amps. Tripped the generator breaker every time. Have a config that allows the condenser to run from only generator so no complications of other currents tripping the breaker. Found a device that does a soft start on the compressor. reduced my compressor surge at least 50%. Still searching for most accurate method to measure inrush. This device may have reduced the inrush by more than 60%. My A/C runs with house basics of refrigerator, freezer, two rooms of lights, fans, TVs. Have to manage dryer, and electric stove. You have to get electrician help on connection, isolation from city power, 4 wire hookup etc.

Can be done. Soft Start technology is the key to inrush issues for portable generators that max out at 8KW with 10KW surge which I think is the portable's limit today.
This also should be strongly considered for standby generator sizing. Those companies use the LRA to insure they have enough power for all conditions. But for every extra 10 amps of reduced surge is a 2400 watt reduction.

Every 3KW increase will cost you approx. $400 for a standby generator. The first step from, 8K to 10K, portable to standby, is around $1500.

    Bookmark   September 2, 2013 at 10:08AM
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There is now a third choice for genset installations. It used to be that you have a large generator that will run all loads, or a smaller one and manually manage loads if they are managed at all.

Now automatic load management is available for residential gensets. What that means is that individual, large users of power are equipped with relays to control power availability. For example, if your central AC wants to start up, the refrig and freezer will turn off while the genset deals with the surge. After it is up and running smoothly, they may turn back on. They won't if the genset won't provide all the power necessary. There may well be limits to how long a refrigerator or freezer can be off before forcing the AC to turn off to keep the food cold. Sump pumps might be set to the highest priority.

I have no idea how much that might cost and how reliable it might be. It is possible that it costs more than a big, dumb genet, but if you use the genset a lot, it might be worth it. You'd be able to run at closer to maximum efficiency for extended periods.

    Bookmark   September 2, 2013 at 11:04AM
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With my 4000 watt portable, I do the load management manually. Mine is just one gallon and the refueling requires my attention anyway. Since 1989, we have had only two outages extending for more than 24 hours.
We have a large commercial freezer in the basement. That and the refrigerator are the highest priority.

    Bookmark   September 2, 2013 at 12:12PM
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I have a 8kw portable (really its up to 7500) and i ran my 3 ton central AC with my last power outage. I did not leave it on long as i was afraid i couldnt run my water well pump but it did run without any issues.

i have it hooked up to my 200amp panel with an interlock kit and 30amp breaker. w/o the central AC i can run about 75% of the lights, appliances (gas stove), fridge and tvs/computer/cable, well pump, boiler without any stress to the generator. the stanbdy was just too much at over $7k installed for a 14kw model. hopefully one day.

    Bookmark   September 16, 2013 at 4:45PM
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response to mnk716 16 Sept post.

Please relate the manufacturer and model of your A/C. A 3 ton unit drags nearly 80 amps at startup. Do you have an inrush device already built-in? Glad it works for you.
There are no A/Cs in my area that can run off a 8KW portable without an inrush reduction device.

    Bookmark   September 16, 2013 at 5:24PM
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I have a Generac XG8000e which says has startup watts up to 10k. I DO NOT have an inrush device. i wanted to see if it would run the central a/c. i had the fridge on and some lights but no running water as i am on a well. the generator has a powerbar (i dont know how accurate it is). i shut off all the lights and i turned on the thermostat and it ran.

i did hear the generator seem to stall but it went back to normal rpm after a few seconds. i turned on the tv and some lights and the power bar was at 3 out of 4. i let it ran for 10 minutes and no unusual sounds from the central air and the compressor was running with cold air coming out. After 10 minutes i shut down the thermostat and the a/c stopped normally.

i ran the ac again when power was restored and no issues. no breaker trips with the generator. on a really hot day it would be a last resort but then i believe i wouldnt be able to run my well pump (i didnt try to do both).

it was more to see if it could handle it then to run it full time and it did. w/o the a/c i could run most of the circuits that i usually do with normal power. i even ran the washing machine but not the dryer. i was out of power this summer for 4 hours and even without ac i still watched tv, washed the clothes, lights, video games with no problems.

    Bookmark   September 16, 2013 at 5:48PM
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The AC and the water pump both pose heavy starting loads. For a 10K peak generator, the practical choice is to run them only one at a time.

    Bookmark   September 17, 2013 at 9:26AM
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