OT: AC's and Generators

[buddybikes]

Lived through Isaiah yesterday, nasty winds. We have a 11kw generator and 1.5 or 2 ton AC (Trane). The AC kicked on and over loaded the generator on start. Thus we were sleeping in bit warm humid conditions. I turn off AC and generator handled rest of small house fine. 920sq house... AC people came today and put new capacitor in it. They were going to put some kind of "boost" on it, but decided not to. Tried again, no luck. Now with power on the AC working fine.

So now wondering if time to retire this 13 year old AC, put this boost thing in or will AC always trip the Generac.

As you can see from my writing, I am far from electrician or HVAC.

thx

[54ny77]

Something's amiss.

How much power does the AC draw?

Best is to ask a licensed electrician fluent with generators on the load capacity of your generator (which I presume is whole house, or is it just feeding dedicated circuits?).

You might have other things sucking up power continuously that when the AC turned on, is the proverbial straw that broke it's back.

p.s. it's helpful to go thru the house and identify the continuous power usage requirement of all electronics and lighting , that way you will know if your generator can potentially be overloaded.

You'd be surprised how much juice a running electric stove, microwave, AC, fridge etc etc can use. Alot, basically.

[Louis]

Originally Posted by 54ny77

How much power does the AC draw?

This ^^^ is the key. I'm guessing that like a water pump, the initial peak load on start-up is higher than the "running" load, but it may not be that much higher. You should be able to find that out for your AC.

As was pointed out above, there's also all the other stuff in the house, and what they're doing when the AC tries to start. 11 kW is not a wimpy generator, but if everything else just happens to phase exactly in the wrong way...

[Davist]

Most likely inrush current on the compressor. Can be 7x full load draw for inertial stuff. The newer ACs have a VFD on them (to reduce this among other things) which could solve the issue, but they're expensive as well. Cripe I have a 6.5kW generator and turning on the 1200W microwave bogs it down with just the fridge, chest freezer, a couple LED lights and the sump pump running.

[donevwil]

Look up the startup wattage for the AC. Good to know startup and running wattages for all appliances you want to run simultaneously.

I shopped for a portable generator to address the mandatory blackouts during last year's fire season and for two fridges, a wine cabinet and a small window AC was recommended a ~8k generator to reliably accommodate startup for all. Certainly possible you're under generatored for what you're running, especially considering the much higher draw for an older AC. Maybe choose things to turn off/unplug when you want your AC next time? A fridge can go unplugged for hours if it's kept closed.

[Peter P.]

Originally Posted by Davist

Most likely inrush current on the compressor.

THIS.

The surge current when the compressor starts, combined with whatever existing load is on the generator at the time, trips the breaker.

Sounds like your generator is undersized based on your maximum current load.

[MerckxMad]

On Long Island. We were crushed by yesterday’s storm. Power out for a good portion of the island. Road crews still clearing downed trees. Anyways, after Sandy my brother in law electrician got me a portable Generac (8000kw?) and a sub-panel. He explained peak and running loads to me and then wrote in bold letters On the panel that it would not power central AC. It will handle fridge, fan, couple of lights, and modem/router. It can power the fan in the furnace if we watch the load, but definitely not an AC. That’s all I retained about the generator.

[Dave]

You need a generator in the 16-22kw range and probably a new ac unit. It makes sense for a modern ac unit to have a 3 phase motor and a VFD that will convert the two phases in 230v household power to 3 phase. A VFD can soft start a motor and vary the motor speed.

My neighborhood has an irrigation pump powered by a VFD that will handle 25hp, but it can pull 140 amps and requires a 175 amp breaker. We barely get by running it and one 5hp city water booster pump, at the same time. The 5hp VFD can pull up to 28 amps. It all runs off a standard household electrical service with a 200 amp breaker. The amperage rating for continuous duty is only 160 amps.

The largest 230v single phase motor is only 10hp and would have a starting current around 250 amps.

[buddybikes]

Kind of surprised about power requirements people seem to be mentioning. Or do people (99%) just not use any AC. How many 24Kw generators out there?

Yes I know we need (due to age) replace our unit, but scheduling is now issue (scheduling in Oct).

Wondering if I need to go out and buy small window unit.

My asthma doesn't like high humidity/temp at times...

[Blown Reek]

Originally Posted by steveandbarb1

Kind of surprised about power requirements people seem to be mentioning. Or do people (99%) just not use any AC. How many 24Kw generators out there?

My crappy little gas powered 9500/7500 Champion generator hooked up to a manual transfer switch powered everything (but the AC and washer/dryer) in my house last hurricane with zero issue. That's refrigerator, ceiling fans, lights, outlets, etc. I didn't try to power the AC because it was a 30 amp generator, but based on the wattage I was running, I was waaaaay under the generator's limit. Three 5-gallon cans ran the generator for 24 hours, and the 5 days we were without power, that thing was running constantly, except for when I changed the oil due to hours run.

When we moved into our new house, I was "going to do it right" and priced out a Honda EB10000 (10,000 watt) generator. After doing my due diligence, I ended up with a 38kW Generac with a 1,000 gallon underground tank.

[Tandem Rider]

Inrush current on a small compressor such as a refrigerator will often be in the 25-40 amp range. 25 amps x 120 volts = 3000 watts. A slightly bigger compressor, such as on a small AC will normally be in the 45-90 range, with older generations being higher. 45 amps x 240 volts = 10,800 watts.

Have you had the generator checked? Did the generator previously handle everything at once including the AC? A good generator guy should be able to check your system and let you know if it's big enough or not.

ASHRAE life expectancy on a residential AC is 15 years so you are still good there. Glad to hear you have power back on.

[buddybikes]

This is our first big summer storm. Handled winter without issue (forced hw/gas).

I am having my first tune up for unit next week (pre scheduled). we put 8k$ into this, we will live with it, and hopefully deal with cooling issue.

[merlinmurph]

Originally Posted by Peter P.

THIS.

The surge current when the compressor starts, combined with whatever existing load is on the generator at the time, trips the breaker.

Sounds like your generator is undersized based on your maximum current load.

Yup, I'll pile on. This is a common topic on RV forums and the answer is the same. I have no personal experience, just what I've read.

[mdeth1313]

I have a 15k generator that runs my electric hot water heater, well pump and most of my upstairs. I was told NOT to attempt to hook up central air as it would consume a ton of power and the generator doesn't provide clean enough power to run it.

We have a small window unit that can be plugged in for emergencies. Usually the basement is cool enough to survive.

There are some generators that can provide the clean power necessary.

Now if NYSEG would get their act together I could turn off the generator!

[buddybikes]

Challenge is with Generac - lose power -> Generac wait x seconds then starts up -> whatever devices were on kick in. So: 1 would need to be home, 2 run into basement to trip breaker to AC. This appears unreasonable in real life.

Older individuals live with Generac's.

AC guy coming today to tune up and check all voltages/amps. Will advise what life left this this guy. Next Fri Generac tune up, so all codes can be downloaded and discuss startup options