Stand-by genset

To John T/Billy Shafer- 20 kw all electric house standby, new house. Is it big enough, 100 amp indoor or 200 amp outdoor w/load control. When it transfers is 10 seconds enough to keep an a/c or fridg running? Dan

I am not John T or Billy but I do have a 20 KW (YES GENRAC) Billy hates them. It is now 7 years old and some where around 370 hrs. Trouble free. I built the house with two 100 amp panels calling one essential and one non. The generator picks up the essential side within 20 seconds. I have well, hot water heater, and freezer plugs as well as lots of lighting. Our heat is heat pump with propane backup. The true compressor is not on the generator but blower for the gas side is. In hind site I really think the 20 would carry the entire house. really never hear it struggle. Best money I ever spent. Had one 72 hour ice storm outage three years ago but most or just two or three hours now and then. Locate it outside near your service entrance.

I am not John T or Billy either and don't pretend to be. We have a 14K Champion located near service entrance and it is more than adequate. Propane furnace with heat pump and all circuits are in service when the generator runs. In a power outage I could utilize the electricity in the shed too if so inclined. It is automatic switch over when the power goes out with a weekly 20 minute exercise. A nine hour power outage has been about our longest but it sure is nice not to have to get the tractor and PTO generator out and hook up the cord and then having to monitor the utility service for when our power comes back on.

No a 10 seconds switch over to standby power will not keep a refrigerator or A/C compressor online uninterrupted. There is probably some specific time value set for this but I have no idea what it would be. When those compressors are running against pressure they stop pretty quickly. I would guess that you would need to be able to switch over in something less than a fraction of a second to keep one running continuously. No big deal, when the power drops they just have a timer that keeps them shut down long enough for the refrigerant pressure to equalize and then turns them back on.

(quoted from post at 10:06:05 08/22/21) No a 10 seconds switch over to standby power will not keep a refrigerator or A/C compressor online uninterrupted. There is probably some specific time value set for this but I have no idea what it would be. When those compressors are running against pressure they stop pretty quickly. I would guess that you would need to be able to switch over in something less than a fraction of a second to keep one running continuously. No big deal, when the power drops they just have a timer that keeps them shut down long enough for the refrigerant pressure to equalize and then turns them back on.

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"when the power drops they just have a timer that keeps them shut down long enough for the refrigerant pressure to equalize and then turns them back on"

While that is TRUE with most "central air" units, window AC's, household refrigerators and freezers will typically try to start against high side/condenser pressure and trip the overload a time or three 'til the pressure equalizes and the compressor can start.

20 KW gives you 83 amps on 240. The 100A transfer switch will be adequate, as long as you have it feeding a sub-panel with all your critical loads.
If your house has a 200A service feed, and you want to switch the whole house, you need a 200A service-rated panel, UNLESS you have your main disconnect (nothing between this and the meter) prior to the ATS.
I have no experience with the load management modules, but I have heard, from friends that have such a system, that they are sometimes problematic, when various equipment tries to come on-line at the same time. The management modules don't know what to do.
In my opinion, use the 100A ATS feeding a 100A sub-panel, and put all your critical loads (A/C, freezers, refrigerator, water heater (if electric), and some lighting on it. I have my 15KW unit wired this way, and it runs all those loads without issues. My stove is gas.
If you do put the stove on the emergency circuit, don't use it unless you turn off the water heater or A/C first. I don't think the 20KW will do all those loads at once.
Remember that the A/C has large starting current requirements - 2-3 times running requirements. The nameplate will give you that data. The water heater, using 4500 watt elements, is 4500 watts continuously, while it is heating the water. That is 1/4 your generator output.
Good luck with your installation. zuhnc

jm,
Good idea, two 100 amp load centers. I never would have thunk of that??

I just run an extension cord through kitchen window for Refrig, coffee, micrwave, hot plate. Another extension cord to basement for well.

Or I go live on second house 25 miles away, which I did in 2011
after a 90 mph wind took power lines down for 15 days. The neighbor hood looked like a war zone.

Danny, 20 KW is adequate to run many typical homes subject of course to actual loads (fridges, freezers, heat plants etc) of which I have no idea and its short term surge capacity to allow for compressor/motor starting which can be 3 to 6 times running current. NO a 10 second transfer delay wont keep an AC or fridge running and typically they could have a built in delay circuit to prevent re start under load or short cycling. I trust it will have safe and proper transfer methods but if it was professionally installed Id say it does have. A unit that big is likely configured as a Separate Derived Source and your Transfer Switch switches both Hots PLUS the Neutral and it has a Neutral Ground Bond and connection to a Grounding Electrode. The thing is on smaller gensets where you plug the cord into a receptacle to backfeed the homes panel those gennys should have a Floating Neutral.

Congratulations see what Billy has to say he's the expert in this field

John T

Thanks John, Im trying to get some info before I go help my brother with his house. Hes building a 2500 sq ft forever house in the hills in Tenn. He plans on all elec. ,the genset hes looking at is a Koehler 20kw with a ATS dealer installed using propane. Thanks again for the reply, gives me a little more info go on. Thanks , Dan

Well if it's dealer installed, presumably it was sized for the anticipated load based on talking to your brother. The only person your brother would have to blame is himself if it turns out to be inadequate. He's the one that didn't provide the dealer with all the information he needed to size the generator.

But really, if 20KW isn't enough he's doing something wrong. Maybe he won't be able to turn on EVERYTHING at once, but when do you have EVERYTHING on all at once under normal circumstances?

The 20KW Kohler is a good unit, and will serve you well. It should handle the whole house, unless you have electric heat and no heat pump. Those elements suck a lot of power.
Ensure that there is a MAIN fuse/breaker between the meter and the transfer switch, if you are switching the whole house. NEC also requires the transfer switch be of the same capacity as the commercial feed into the panel; i.e, if you have a 200A main panel that is fed from the meter, then the ATS MUST be 200A as well.
If the ATS is service-entrance rated, it should have the main disconnect fuse/breaker built in.
If you have a sub-panel, fed through the ATS from the main panel, then most of these requirements don't apply, but it sounds as if you are switching the whole house.
If you go to the Kohler generator section on SmokStak (TM), you will get more information than you want about Kohler standby generators; lots of experience there.
Typically, permanently home standby sets are NOT a separately derived source and do not switch the neutral; cost of the ATS is about double that of a standard unit. But, your local codes and AHJ will have the final say on that. zuhnc

This post was edited by zuhnc on 08/23/2021 at 03:58 pm.

A 20 kw gen set will run a average 12-1500 sq ft all electric home, but for his 2500 sq ft house I'd look at a min 25 kw unit

Our backup generators transfer switch waits 15 seconds before starting the generator and transferring power, this prevents unnecessary starting of the generator in cases where the power blinked off for a few seconds.
On ours even if the power comes back on 5 seconds after the generator fires up it will switch back to line power but continue to run for another 5 minutes which is it's normal cool down cycle

As long as nobody runs the electric clothes dryer , the electric stove, the hot tub and are L2 charging their electric car at the same time . The 20Kw unit on a ATS and 200 amp panel is plenty
. I would install a hard start kit on any central air unit .
Reduced the inrush current on my parents 6 ton unit from 166amps to 96 Amps.