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I am having an interlock and power inlet installed for a portable generator to plug into for my home.

I asked my electrician about adding an AC soft start kit to reduce the amount of start up watts and thus reduce the size of generator I need to run the AC in an emergency.

He advised me that even with the soft start kit, the 4 ton AC would draw at least half the 12,000 running watts from the generator, thus requiring the large generator versus getting a 9500 running watts generator.

On other forums and also on YouTube I see ppl running their 4 ton AC with a soft start kit and a 7,000 running watt generator.

What is correct here? I would like to save myself a few thousand dollars and be able to get a smaller generator if possible.

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    Are the youtube people using anything else with the generator? You might want your fridge at least, maybe a microwave/coffee maker/tv/lights. AC should have a label with volts amps needed(watts).
    – crip659
    Jul 8, 2023 at 0:03
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    Well just don't run those things at the same time as the A/C. You can't go by neighbors' experience with 4 ton A/Cs because A/C efficiency has dramatically improved in recent years, and their 4-ton may take a lot less electricity than yours, or vice versa. Modern split systems also use inverter drive and are soft-start by design. Jul 8, 2023 at 0:59

3 Answers 3

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If you can afford it, then go for a larger generator. I would suggest a diesel one as it requires less maintenance (unless you are able to service it yourself).

There are generator rental companies around. If not too expensive, you could rent one and try it out.

We are rural, I had a 5KVA (loosely 5000 watts) generator first. It required a pull start which my wife could not operate and it definitely did not cope with my 5KVA AC units.

In the new house, I have an 11KVA generator with auto-start. We have three 5KVA heat pumps. When the power fails, I flick two of them off manually, just to be safe. And I can run the whole house on it (except the oven and induction tops). It powers my electric water heater, all the lights, water pump, septic system, microwave, washing machine, all lights, 3 fridges, 2 freezers, computers for 4 people, and the media room.

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I say go for it.

At a previous house I had an old-school 1-speed central A/C for the whole house that was rated at 5 tons (60,000 BTU/hr) and which used a 40 amp 2-pole breaker. I successfully ran this A/C with a 5000 watt diesel generator that had a 6250 watt surge rating.

How I did it:

I installed a hard-start kit on the A/C compressor, a 3-second time delay relay on the condenser's fan motor, and I set the inside air handler's 2-speed blower motor to low speed / constant fan. Lacking any of those three adaptations, it wouldn't run or would blow the generator's breaker, but with all three it ran just fine.

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Soft starts have a broad diversity, and motor quality varies, but I would expect AC nameplate would spec "minimum circuit ampacity" of around 15A, soft start needing 1.5x the nameplate, you are landing 5400w plus fan motor for air handling unit needing another 500w.

So you have 3800w left over if you get the 9600w. How frugal do you want to be? If you have lots of kids, an uncooperative spouse, it might not work for you. I personally prefer to go a little small for generators, they tend to be easier to start, to be quieter, use less fuel, and are easier to move and lift.

But I'm a little confused 9.5kw versus 12kw transfer switch? Most people are installing main panel breaker interlock style transfer switches, if you are I don't see a real downside to sizing for 12kw. To get the full capacity of a 9500w you would need a 40A breaker, likely same cost as 50A. Wire size for a 50A breaker should be one size larger than the 40A if he uses using NM cable, no change in wire size if using any 75°C rated wire. Receptacle for 40A is the same as 50A. I might get some disagreement, but even if you decide on the 9.5kw the only down side is you may have to make your cord if the 40A generator only has a 30A receptacle. Tell your electrician to go ahead and size the xfer breaker for 12kw.

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