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logastellus

can a central AC run off a portable generator?

logastellus
10 years ago

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!!

Comments (45)

  • wirenut1110
    10 years ago

    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.

  • wirenut1110
    10 years ago

    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.

  • logastellus
    Original Author
    10 years ago

    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?

  • weedmeister
    10 years ago

    What he's probably talking about is the startup current that the AC may require.

  • logastellus
    Original Author
    10 years ago

    yea but even the startup wattage is about 5-6kW for a

  • Auger01
    10 years ago

    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.

  • logastellus
    Original Author
    10 years ago

    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.

    Right?

  • Auger01
    10 years ago

    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

  • logastellus
    Original Author
    10 years ago

    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??

  • Auger01
    10 years ago

    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.

  • ionized_gw
    10 years ago

    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.

  • logastellus
    Original Author
    10 years ago

    Auger - that's helpful thank you. One thing I stumbled on here http://www.electricgeneratorsdirect.com/manuals/winco_hps6000he-e_hps9000ve-c_hps12000he-a_manual_2008.pdf 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.

  • ionized_gw
    10 years ago

    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.

  • ajaynejr
    10 years ago

    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).

  • Jcris3973
    10 years ago

    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.

  • ionized_gw
    10 years ago

    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.

  • bus_driver
    10 years ago

    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.

  • Jcris3973
    10 years ago

    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.

  • mnk716
    10 years ago

    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.

  • bus_driver
    10 years ago

    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.

  • dannyandrhondao
    8 years ago

    For those who want facts not have to's or need to's. During a recent power outage I personally witnessed my neighbor run his 4 ton central air, several ceiling fans, refrigerator freezer and a TV on a 5500/6500 Onan portable gen set.He and I both use an InterLock kit to distribute power. All he shut off was the hot water! Yes the generator would strain when the AC kicked on but the breakers held and nothing was damaged on the 6 hour run. I have a 5000/5500 cheapie China gen set which ram my same 4 ton AC and some lights during the same outage. While I am not recommending any of this, it does work. We are at sea level and use no inrush reduction devices. I am a retired Cummins/Onan mechanic and kept an eye and ear on both gen sets. So much for having to have a 12 to 15 KW generator just to start a 4 ton AC alone! B.S.

  • d2305
    8 years ago

    Better get a big one. I need to run mine as it's hurricane season.

  • chas045
    8 years ago

    I suppose my experience is way too late for the original poster, but I do recall that on another thread, someone with a single whole house AC was not able to start his unit. I have two units; a heat pump for second floor and a standard unit for the first floor. The smaller units should be easier to start. I certainly can run one unit with my 7000 watt generator.

    However, one should probably consider the possibility of an extended outage. If gas or propane appeared to be available, then running the AC may be reasonable, but if availability was questionable, then it would be better to focus on the basics.

  • ionized_gw
    8 years ago

    dannyandrhondao, how do you know there was not cryptic damage to the AC compressors?


    Some modern, variable speed HVAC compressors have little to no start up surge. The rest have a lot.

  • dannyandrhondao
    8 years ago

    Like I said, I am not recommending this but there was no appreciable drop in RPM's or voltage when the compressor cycled. The difference in cost between a 8 KW and a 15 KW generator will also buy a lot of compressors. I never said you should do this daily or at all. Your choice.

  • ionized_gw
    8 years ago

    that is good information to have, thanks dannyandrhondao.

  • Jason Bloom
    7 years ago

    Can anyone tell me the minimum size generator I should have to start this unit?

    Total Amps 26.4

    Maximum Circuit Amps 32.5


  • Ron Natalie
    7 years ago

    Well, you also have to add a little bit for your interior air handler. That data plate only gives you the compressor and its fan. Assuming something like a 1/2 HP motor for that brings it up another 5A or so.
    38*240 = is just a hair under 10,000 VA.

    However, something is amiss here. If the unit is really 5 tons, it should be consuming at least 17KW. Someone is not telling the truth.

  • PRO
    3rd.Generation Hot Tamales
    6 years ago

    Ok my name is Roy Stewart of 3rd.Generation Hot Tamales and I'm wondering if I'll be able to run a Central HVAC unit 30V in a portable office off a 3550 watt to 4550 watt generator that is 120V 30amp.

  • PRO
    3rd.Generation Hot Tamales
    6 years ago

    Also the portable office has a electronic panele on it....@#blessitbe

  • ionized_gw
    6 years ago

    Ray, there is a lot in your post that does not make sense. First, you can't be doing anything with 30 volts. Second, central AC is generally 208/240V not 120V.

  • Michael Miller
    6 years ago

    Its that time of the year again in Florida. Hurricane season. LOL I'd really like a portable Generator, but comparing options.

    I have a brand new 3.5 Ton AC. I've also installed the hard start/start assist capacitor. Before the start assist it pulled 31A startup / 9A running. Now after start assist it pulled 23A startup / 9A running. Looks like for just compressor I would need 2160 running watts and 5520 starting watts. Pencil in another 5A for air handler (1200 watts) looks like i'm at 6720 starting 3360 running.

    Debating on finding a portable generator with 60A or 50A. anyone have recommendations?

  • ionized_gw
    6 years ago

    Some people are down on Predator, but Consumer Reports rates 685291 well (albeit noisy) and the price is excellent. I have a more powerful model that has seen very little use --a couple of hours under light load.

    I'd ask for a better fuel cutoff valve and a fuel filter.

  • User
    5 years ago

    I bought and installed (easy installation) a medium sized window A/C unit and plugged it directly (via heavy duty extension cord) into my 3000/4000 Watt generator along with refrigerator, TV, and a couple of lights. No problems. Never stalled the generator. Closed all bedroom doors. Cooled the living room, dining room, kitchen, and family room to 75 degrees on a 90 degree Florida day (after hurricane).

  • ionized_gw
    5 years ago

    Yup, the portables are getting better all the time. Those that rely on central units and don't experience them, might not have an appreciation of how much they have changed. Before I got my current system in I ran on three borrow portable window units for a time. Certainly not as nice as a good central system, but not bad at all. I predict that soon inverter-driven compressors and variable speed (ECM) fans will make an entry even in that inexpensive market. What a difference that will make!

  • Tinas Husband
    5 years ago

    last year during a 3 day long hurricane outage i was able to run JUST my 3ton Goodman package a/c on a 5500 watt 6500 watt surge generator, the key is a hard start.A hard start basically sucks in allot of power ,stores it, and when the a/c kicks on it dumps all of it to the compressor at once, this almost completely eliminates the high amp draw on start up. The power was also running through allot of wire to get to the a/c .Over 100ft of 10ga wire being back fed from my side yard shed then into the main Panel, then it travels into the houses panel then out to the a/c i bet it would have done even better hooked directly into the a/c circuit.Hard start is the key you can get one for 20 or less online.

  • Paul Sloan
    5 years ago

    Have a whole house 14kw Kohler with Load Shed – when I upgraded my central AC from 3 to 4 ton I had a problem with AC startup dropping out all on Load shed and almost stalling compressor. I bought a good meter with “in rush” detection which allowed me to check start up draw on compressor – start up current was 101 amps yes 101 amps. Started researching how to deal with this AC and Generator issue and after many online searches found Hyper Engineering’s SureStart soft start module. Had my AC tech install and my “inrush” current dropped to 25 amps….from 101 amps to 25 amps was almost unbelievable, I live in FL and AC runs almost constantly and module has been in for a year. Generator does a full load 20 minute run test every week and I AC / Generator / Load Shed works like a charm. Purchase at https://www.gen-pro.biz/

  • HU-466753606
    3 years ago
    last modified: 3 years ago

    So, for the folks who are running their central AC on a small generator, there are some things to consider so you don’t damage your ac. Just because it’s working doesn’t mean it’s not harming your central ac. 1st you need to have a quick reading voltage meter your typical cheapie can’t read ms long voltage changes. What your looking for is when your ac starts what is the instantaneous voltage drop. If your seeing under 208V when starting your ac, your accelerating wear on your motor windings on the compressor. Your compressor takes a certain amount of wattage which varies depending on temperature. If your ac says 90lra at 208v you need 18,720 starting watts, to start the ac without a voltage drop. If your generator can’t supply the needed power, you’ll slow the engine down and bog the generator causing its output voltage to drop, and say it’s outputting only 185v starting the ac. That means your ac is now drawing 18720/185=101 amps, and for a longer period of time. at low voltage the ac will take longer to start, heating the motor winding on your ac significantly more, once the insulation degrade or the wiring melts in the ac your compressor is done, So it’s not that it can’t be done, but it will wear your compressor a lot more quickly. Also if your ac doesnt sound the same while running it’s because the motor is vibrating more due to a highly distorted ac wave form or high THD from the generator. This extra vibration will wear the compressor more quickly and if bad enough destroy the motor. So some things to keep in mind when running a central ac on a generator. something highly recommended for small generators is leave the ac running all the time to reduce starts which damage the motors and or install a soft start kit to reduce LRA. If the ac is noisy on generator power get a different generator with a low thd, make sure it’s grounded properly if needed and consider upgrading your ac run capacitor to a 440v. Real bottom line, a cheap generator can run a very expensive ac. But if it kills your ac you didn’t really save money by buying a cheap generator.

  • Brian O'Rourke
    3 years ago

    I have a 6250 Briggs and Stratton "Storm Responder"

    My friend, who hard worked the breaker box with a 30 amp breaker, he said the following:


    If you run everything in the house normally, television, refrigerator, lights you are only at 31% power consumption on the generator. If you want to run your central air conditioning we would need to install a 50 amp breaker not a 30 amp breaker. Yes this generator might power your central air conditioning unit but you are running into issues here. You can damage the AC unit. If you really want to run central air, you need at least 8500 W may be 10000.


    Save yourself the money and buy a window unit for air conditioning. Put it in your bedroom since there is cable TV up there.... You can have air conditioning and TV and not need to buy a 10000 W generator.


    The money you will save on buying the smaller generator, you can easily buy 2 or 3 window air conditioning units. How long is your power going to be out anyway? Usually not that long. Why waste the money? My 6500 W generator easily powers to refrigerators lights, television in 3 window units with power left over to spare... Your 6250 will more than do the job.

  • John Youngs
    3 years ago

    On the coast of NC storms and outages are pesky. Struggled for years with a 10kw Generac that would barely run a 10k BTU window unit and some small stuff. Repaired the unit 5 times for burned diodes, wires and connectors. It would not run a 3 ton water to air heat pump. The inrush current for the heat pump was 60 amps 240v. The locked rotor for the heat pump was 82A plus fan. The run current on the plate was 16A and measured 9A running.


    Finally bought a surplus single phase 16kw diesel gen. after much research and it works everything without drama. Burns .5 gal/hr at half capacity. Kabota 2 liter diesel. The same capacity propane gen can burn 50 gal. a day at half capacity (Generac).


    Lessons learned: Hard start kits can be helpful to reduce the HVAC start surge by more than 20%. Most generators will not deliver the advertised current without risk to gen or motor driven stuff. HVAC can be tricky.

    Three phase generators often will only give about 60% or less of the three phase rating and most will be at 208v instead of 240v. Don't believe the advertised single phase numbers given by most. A 12 wire three phase gen. wired for single phase puts several windings (120 degrees out of phase ) some in series, some in parallel, which cripples the current capacity (higher impedance) . Some have a voltage knob to adjust the 208v higher with more excitation and internal heat. Avoid three phase units because the engine will be needlessly larger and consume more fuel and possibly clog the under used injectors (stacking). Most of the over 20kw gens are three phase because they are slightly cheaper to build and offer more flexible applications for rentals.




  • mtvhike
    3 years ago

    Why would anyone even consider a 3-phase generator for residential use?

  • Isadore Jr Carret
    3 years ago

    Rule of thumb, 3kw per ton of ac

  • Stax
    3 years ago

    Ignore much of John Youngs' diatribe about three phase generators and motors!

  • Kirk Rollins
    2 years ago

    If you look at your AC, you will see your fan and compressor FLA and RLA, add these up and multiply by 240 ro get the wattage required for continuous operation, this will need to be about 50 to 75% of your generator’s running wattage. Be sure to note the starting amps required, which can be 4 -5 times the running amps. A 22kW generator is needed for a standard 13 seer unit because the starting amps are so high.