what size generator?

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What size genset would it take to run a 3.5 ton residential central air con unit with air handler included?
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What is the power requirements for the units? There should be some ratings for both on nameplates. Add 20% or more and you have the size you need. You may need both 220V and 110V for them.
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No! do not add 20%! Triple it! Motor starting on a generator needs 2-1/2 to 3 times the motor load for starting. The air handler will not be much of a problem, but the compressor in the condensing unit will be a hog. 3.5 tons probably consumes 20 amps @ 240 volts. Add 5-10 amps for the air handler. 20 amps @ 240 volts gets you 4800 watts, to run it all you may need up to a 15,000 watt unit. If you plan on anyother loads the size increases even more. A 10,000 watt genny may run it all, but I would not bet on it! Greg
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This is Turtle.
You need to get the amp draw of the condenser and the amp draw of the air handler or gas furnace and add up and see what size generator it will take.
Now i did run my 3.5 Rheem 8 seer condenser and my Arcoaire gas furnace 100k with 14 seer evaperator coin with TXV on my 6KW Generator and conpared the amp draw reading with a meter and the rating of the 6KW generator and it fell well below the generator amp draw it was rated for. now check the amp draw off your unit for different unit require more or less to run the cooling systems even when the btu rating is the same. The hotter the weather is / the more amps maybe required to run the condenser.
Do your home work before buying a generator.
TURTLE
TURTLE
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And then double or triple the amps drawn by the motor-driven stuff to account for startup surges.
This is the same reason you're supposed to use time-delay fuses on such equipment if you're using fuses. [Breakers inherently act as time-delay fuses to permit momentary current spikes]

The amperage draw during operation will remain about the same. What changes is the duty cycle, and to a lesser extent, the _number_ of startup surges the generator gets cycled through.
If you just size the generator for continuous operation, startup surges will be very hard on it. A lamp will show severe brownouts during motor startup. It may be bad enough to cause the overload to trip. It will certainly be reducing the lifetime of the motors.
--
Chris Lewis, Una confibula non set est
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My 7500 -13500 watt gen can run my 2 ton fine but for you maybe a 12000w - 15000 watt unit. Surge load will be 3-4x run load. Many 3600 rpm gens are only good for 3500 or so hrs . Look into 1800 rpm
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This is Turtle.
If you knew about generator, You would know that what ever the generator is rated for the surge rating is usely double that and will well account for starting surges of any load that would run normal at. The starting surge created is something to concide but they are not the beast you make them out to be.
Now with what you say here if I need a 6KW working load you want me to buy a 12KW to a 18KW generator to run the 6KW load. If you would read on the paper work of the generator that the 6KW will usely take about 8KW to a 10KW surge load to get it started without brown outing at all. If you would take the time and read the paper work you will read about the surge load and the running load. There is such of a thing as surge load rating on generator which will take care of starting amps for a short time to get it started. Yea, I know on the Space ships they don't need surge ratings for they have all the power they need.
TURTLE
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TURTLE writes:

I think you may misunderstand these specifications. The "surge" rating on the generator is what it can take as an overload for short periods without overheating. This is different than the "surge" needs of starting a motor.
The problem with a portable generator starting an electric motor is that the generator has NO instantaneous "surge" capacity. The gas engine is running at some power (torque X RPM), and that is ALL the power it can deliver when the electric motor load is connected, until the fuel governor can compensate, which takes several seconds. The engine slows down, the voltage dips, the frequency falls, and the phase lags. Some electric motors can tolerate that and start, some can't.
What a bigger portable generator buys you is less of this dip.
You could start a bigger motor by switching off a resistive dummy load and on to the motor with a transfer switch. This gets the generator running on maximum throttle. I kind of play at that with the garage lights vs the garage door opener motors, with a kind of game to see how little I can perturb the generator's surging, by switching the lights off right when pressing the door button. Maybe have the wife turn off the oven while you turn on the A/C?
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For example, a compressor motor could require as much as three times the amps for a half a second. Let's give a concrete example: you have a 1Kw compressor and a 1.1Kw generator. The generator motor is happily pushing 100W worth of power (100W of gasoline) to a lightbulb, then the compressor tries to start. It wants 3Kw of power to start turning. Where's that going to come from? The generator's governer can't react fast enough (1/60th of a second or faster) to maintain speed. All it has is rotational inertia to try to supply an instantaneous 3.1Kw load. The generator slows down, voltage drops, and the compressor then takes longer to start, suffering higher temperatures trying to cope with the brownout, prolonging the generator overload. Since the voltage and frequency is lower, the generator is overheating too. Meanwhile the governor is trying to react to the genset slowing down.
What ultimately happens depends on whether the governor manages to catch up before the breaker trips or something dies first. For example, does the compressor start turning sufficiently fast that the start up surge die down to close to the generator continuous rating before the breaker trips?
Yes, generators have surge ratings. But with portable units, they're typically only 20-25% of the continuous rating (eg: a 5Kw generator has a 6 or 6.5Kw surge) - not a factor of 2 or 3. And their surge capacity is too slow to pick up motor startup spikes.
Generator manufacturers, electrical engineers, DIY books, government "how to provide your own power during an outage" brochures _all_ take into account 2-3 times overfactors for motor loads. I think they have a reason for it.
--
Chris Lewis, Una confibula non set est
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Turtle , on a Honda brochure the models are rated at or near surge and they specificaly state to figure in surge load in their units as a figure of total watts needed. Yes they show 3x surge needed for certaing things. Generac and most other gens give a running load as rating, like my 7500watt generac has a 13500 watt surge. You are safest to figure in surge or you may possibly not start your apliance or worse mess up your gen.
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This is Turtle.
I have a 6KW Generac Generator and has surge rating 9.8KW. I can hook up a 3.5 ton central unit and a gas furnace on it with a few lights and will pull fully loaded of 5.1KW at most. It will run it just fine and the only thing is when the condenser fires up. You hear a 1/2 second change of the engine speed and the lights don't dim or blink. Now the gas furnace and the condenser don't come on together for the condenser comes on then 30 seconds later the furnace comes on. then in reverse the thing shuts down the same way. I don't see all the carnage that is spoke about here.
I figured up the Surge that is taking place on the condenser unit and it is 21.1KW needed to start it by using lock rotor amps from the compressor amperage. By What is said here I should get me a 21KW Generator to run a 5.1KW load. Yes I know you have to have a deviding line as to the size using the surge rating but if i went with a surge rating of 21.1KW . I would still have to have a 15KW running load rating to get the surge rating of 21.1KW. The cost of the 15KW generator would out weight any cost of repairs to the hvac system or buying a new 6KW generator ever so often.
So what I'm tring to say is if I want the surge rating to be good I just can't afford a generator to run the hvac system at all , one or 2 times every 5 years. Then just running the generator during power outages -- doing it right would cost me about $10K to $15K or just tear up something just a little and run a $.5K generator. The $9.5K to $14.5K savings will buy a lot of repairs to anything that breaks. Now me being in the HVAC repair business will make the hvac repair cost go way down. Being cost effective is all that counts in a operation of anything and leaves out '' the way it should be '' .
TURTLE
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True enough. But...
There's lots of factors involved. Equipment is built with a sizeable safety factor - you'd see this with larger generators. Larger electrical equipment is often designed with an idea of trying to minimize startup surge. What's okay during an ice storm may not be okay during a heat wave. It's not entirely necessary to hit the "locked rotor" maximum, because that's of extremely short duration, a compromise between locked rotor and continuous is the prudent level.
Whether you've made the reasonable compromise or not, is hard to tell in detail without in depth engineering, or at least a consult with the manufacturer of the equipment.
I'd be more confident with a decent industrial grade generator and a 3.5 ton A/C compressor than I would be with 30 year old refrigerator that needs 600W on a 600W generator. Even a Honda.
I personally wouldn't do what you did without consulting with the A/C manufacturer.
You pays your money and you takes your chances.
And it's not always that you just have the generator fry.
As the great Ice Storm up here demonstrated, sometimes a cheapo generator failure takes the house with it.
--
Chris Lewis, Una confibula non set est
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What is running amps of your condensing unit? I suspect it is 15-20 amps, probably closer to the 15 amp figure. 15 amps X 240 volts 600 watts. Double that and you are at 7200 watts. Well bellow your surge rating, but slightly over the run continuous rating. Three times just puts you over the top in your case. I believe your genny is right on for size. Any smaller and it would not pull it! It may depend on the type of compressor in the condensing unit also. I suspect a scroll would start easier than a recip. Everything I have seen gives a 2-1/2 to 3 times motor run amps for starting when sizing generators. That sizing may be over kill for some applications, but in this case it is better to have a bigger generator, than too small of one! Greg
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If it starts your stuff, it works. Many times ive seen small units just not start the load or even stall. I think it is a saftey-overload factor. If it works, fine. But you can stagger the load also, turn fan on first. It is odd, Honda will rate only a 10% surge on most models, their commercial- industrial EW171k1 rates 4000w max, 4000w continous. Most companies give a 40% surge.
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m Ransley writes:

Of course they want to sell you a bigger genset and CYA by overestimating motor starting loads. As Turtle notes, the actual needs are far easier than the worst-case figures on the labels.
Funny how the same companies overstate some power specs (air compressor horsepower) and understate others (generator motor-starting ability).
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RB writes:

Running 3.5 tons = 42000 BTU/hr = about 4.2 kW assuming a 10 EER.
Starting currents are much higher than running, but an A/C compressor is on the lower end of this phenomenon, since it is (supposed to be) unloaded when starting. The rule of thumb for motors is 2X to 3X of running load is the starting load, so you're in the range of needing at least 8 to 12 kW. This is on the upper end of "portable" sized generators.
Get a smart thermostat, like a White-Rogers Series 90. This will delay starting the air handler until a programmable number of seconds after staring the compressor, which will save you a kW or so of surge capacity.
There is no sure way to know without actually trying it out. I suspect many generators are optimistically specified.
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They're (supposedly) unloaded at instantaneous start. However, pumps pick up load faster than virtually any other load. It's not like a table saw screaming away at 3600RPM that draws virtually no power until you load it down with cutting wood. Pumps draw essentially full load all the time. Many systems try to deliberately unload the pump when it's off (air compressors and the like), because trying to start a piston pump under full compression often means the motor won't start turning at all. Electric motors have poor torque trying to start.
--
Chris Lewis, Una confibula non set est
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A similar question. How about generator size to run a well pump? I can live without air conditioning but I need water.
Reading responses to the OP's question is a little different from mine. If there is a spec plate for the submersable pump, it's 200' underground. I know it's wired through a 30a, 240v breaker, but that doesn't tell me what the real run or starting current is. Any comments?
Bob S.
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Bob S. wrote:

my house (2 refrigerators, a chest freezer, computers, TVs. The well pump does dim the lights for a second when it kicks in.
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Means little to the poster's question. Your pump maybe alot smaller, and uses less power. His pump is possibly a 4-5 HP model, yours? Who knows for sure about either one at this point. Yours could be 1 to 1-1/2 HP which may run fine on your genny. No way it is 4-5 HP. Greg
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