I'm planning a generator installation (Generac PowerPact 7.5kw) with an ATS with 8 circuits (i.e. Not whole house).

I want 240v hot water to always be on when on utility power. When the ATS switches to generator power, I want the hot water heater to be off and require manual intervention to turn it on. When power is switched back to utility, I want hot water always on.

I do not want it setup so that when power switches to generator, I have to go and turn off power to hot water.

You ask: Why is this hard and not just have separate breakers for utility vs generator? As I understand, the breaker in the ATS replaces the original breaker in the main box. So utility and generator power go through the same breaker and can't be controlled independently. But perhaps there's another way to wire it so I can have the situation described above in my second paragraph?

Update in regards to comments:

I'm planning on a Generac Powerpact 7.5.kw. I am in the northeast USA. The ATS is a subpanel off the main panel. I don't have a planned diagram but it's this kind of setup (minus optional sub panel on bottom left: https://i.imgur.com/VMhOpwe.png.

In regards to xy problem: Hot water uses 4500 watts and I have a furnace (~1100 watts running/~1500 watts startup) and well pump (~1500 watts running/~3300 watts startup). Hot water is not a necessity and puts me over my generator power limit. But I'd like the option to turn off the furnace or well pump and turn on hot water temporarily.

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    How many kW of generator are we dealing with here? Also, normally, with a non-whole-house ATS, the ATS is essentially a subpanel off the main panel Mar 22 '21 at 1:20
  • if you switch the hot water on in 1st power outage and neglect to reset it back when mains is back on, do you care if it is still on in 2nd power outage? if you want an automatic reset, what if the mains comes back on for just a few seconds?
    – Skaperen
    Mar 22 '21 at 2:26
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    We need make and models of the generator, ATS and your main panel as well as your planned wiring diagram. Also, where on this planet are you? That can make a big difference on the advice ppl here give you. I agree with @ThreePhaseEel in that normally an 8 circuit ATS would only have important loads on it and connected as a sub-panel. The water heater would remain on the main panel as well as other high-amperage loads. Mar 22 '21 at 4:16
  • I don’t see a problem don’t put the water heater on one of the 8 circuits for the ATS. A water heater is a large load normally in the 5-6KW size a system with only 8 circuits is normally a smaller generator and the water heater would not be connected to the ATS circuits.
    – Ed Beal
    Mar 22 '21 at 14:17
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    I sense an XY problem here. This platform works better when you tell us your evil plan :) Mar 22 '21 at 17:51

What you need is a critical-loads subpanel

This is an ordinary subpanel, in which the critical-load circuits are permanently moved. (this isn't as annoying as it sounds). The ATS automatically switches its source between utility and generator.

As Ed Beal notes, it's unusual to put a large load like hot water on a backup generator for a couple of reasons: First, tanked electric water heaters are well-insulated, and will hold heat for 12-24 hours. So it's simply not a problem except for long outages. Second, they are one of four enormous loads in your house, the others being electric dryer, electric range and A/C. Because of their large size, they are generally excluded from critical loads, because they require such a large generator, and wipes out any ability to use a portable generator.

Generally an ATS (automatic transfer switch) implies permanently installed generator, since it defeats the purpose if the ATS can't auto-start the generator, and that's not generally supported with portables.

Pad generators can easily be sized to pick up any planned load, and when an owner is committed to supporting some or all of these large loads, that's what they do. NEC requires a load calculation be done to assure the generator is large enough.

You may be looking for a load shed device

However, NEC has an exception - when doing your load calculation, you can exclude loads which are on automatic "load shed" devices.

A load-shed device is an interruptor, which monitors the input power, looking for sagging voltage, slowing AC frequency or other signs of the generator bogging down. They then drop out their connected load.

What they do not make, unfortunately, is a smart "load balancer" that draws off only the excess capacity of the generator; it would have to "dim" the water heater, but that would work stupendously well with water heaters. Be a disaster with a motor load. Too bad this is vaporware... the problem is how to sense when the generator is at peak performance. Generators don't exactly have an output signal for that.

For what it's worth, water heaters are well insulated. Once they reach target temperature, the water will stay hot for as long as a day, even with the power off.

To answer your question, you need a contactor.

Which is a big relay. And I would use either a DPDT contactor with an interlock, or a 3PDT contactor. 2 poles are for water heater power; the third is to allow a "hold-on" feature.

Here are the bones of it. The common goes to the water heater. The NC (connected when de-energized) contacts go to the utility (i.e. its normal 30A breaker in the main panel). At its quiescent state, it's on utility. Its NO (connected when energized) contacts, you have several choices there. The coil contacts likewise; you have choices there as well.

The reason to make NC (coil off) the utility side is so that coil doesn't have to get held "on" 24x7x364. Wasteful and unnecessary vampire load, that!

How to wire that contactor's coil to manually switch onto generator load is complicated, and I have an easier way.

Or, you could run the water heater on 120V

Which lowers its power usage to a very practical amount for a small generator, only ~1200 watts.

It takes the water heater 4 times as long to recover, but it works. You can crunch the numbers easily enough, thanks to a convenient unit called BTU. (note that HVAC equipment uses this unit incorrectly.)

  • 1 BTU heats 1 pound of water 1 degree F.
  • 1 KWH (1000 watts for 1 hour) equals 3410 BTU.
  • 250 pounds of water in a 30-gallon water tank.

Go look at the nameplate (take the ONE largest wattage, don't add them, they don't run simultaneously) and punch it into the above, you'll see full recovery at nameplate wattage takes around an hour. Okay, that's on utility.

On half voltage, the water heater's power will be 1/4 of that. Recovery will take 4 times as long. Expect ~1200 watts give or take. "Well gosh, that's well within the range of a common 120V circuit!"

So instead of a 240V/30A breaker, it goes to a 120V/15A breaker. With the contactor we just discussed, the NO side is connected: 1 wire goes to hot of the critical-loads sub, and the other wire goes to neutral of that same sub.

We still need to get the contactor coil to pick up. We don't want it picking up on utility (and the critical-loads sub will be hot on utility mode)... so we can either take coil power hotshot off the generator side of the critical-loads panel interlock... or, here's what I'd do.

I'd get a plain old Intermatic mechanical timer... the kind you twist. I would feed the contactor coil through that. It will work and run down the clock whether power is on or off. Basically whenever you set the timer, the water heater will be taking 120V off the critical-loads subpanel instead of 240V off the utility-only subpanel.

Since a typical water heater can fully recover in <<4 hours (run the math above), I wouldn't bother with a timer longer than that. You set the timer AFTER you use hot water, so it can recover, and the next person can "just use it".

  • Thank you so much for this extremely thorough and helpful reply!
    – Matt
    Mar 22 '21 at 19:23
  • Why does water heater at 120V run at 1/4 power instead of 1/2 power?
    – Matt
    Mar 22 '21 at 22:07
  • @Matt -- P = V^2/R, in short, and immersion heaters are pretty much archetypical resistive loads Mar 23 '21 at 0:09
  • Interestingly enough, theoretically, one could build a load-modulating device that used a constant-input-frequency loop to phase-control a heating load, since generator output frequency is a fairly reliable indicator of loading (in fact, the Generac loadshed boxes work by frequency sensing -- it's how they are able to work without a control connection to the genset) Mar 23 '21 at 0:13
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    @Matt As ThreePhaseEel says, immersion heaters are resistors. Resistors have a value in ohms. Ohm's Law: Voltage drop = current x resistance. 240V = e.g. 22A x Resistance. So resistance is 240/22 ohms. Now, cut voltage in half. Current must also fall in half, since resistance isn't changing. So you fall from 240Vx22A to 120Vx11A. Mar 23 '21 at 0:24

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