Load sharing device – GreenBuildingAdvisor

I’m not so sure you need a load miser here. Heat pump water heaters don’t draw all that much electricity, certainly no where near what an electric resistance water heater would. You might very well be fine without it, even with 100A service. If you did have an issue, I would probably set the stove up to be the load that is dropped.

The “heat pump” I assume is a multisplit? Maybe a central unit? I’m not quite clear on that. Regardless of what type it is, the indoor and outdoor units are part of the same system, and need to be treated as such. That means you CANNOT drop out one side and not the other. The indoor units CANNOT work without the outdoor unit. If you have to set any of that to drop out, it is an all or nothing deal. I would not set this as the interruptible load at all, not any part of the system.

If you have stages, I would set the water heater to drop out first, since it will be least noticeable by you. The tank will hold a reserve of hot water, so you can continue to get hot water even if the unit isn’t powered, although it will run out over time. I’d set the stove as the next thing to drop out, since it’s less important than your heating system, but also more disruptive to your home if it’s disconnected by the load miser.

Do you have a list of your large electrical loads? If all you have that are “large” are the stove, heat pump system, and heat pump water heater, that’s not really all that much. The stove may have a 40A or 50A breaker, but it probably averages less than half of that while you’re using it, probably even less usually. A typical range is 4 burners and an oven, and that breaker can run *all* of that at the same time. Running only one burner is likely running around 6A, maybe 10A for a “big” burner. A heat pump water heater is probably a 120V circuit, maybe 10A, but only on one leg of your service. That is a bit more complex to explain, but it isn’t a “big” load. The heat pump is probably something under 30A, and likely uses less than that most of the time.

There is a little thing in the electrical world we call “the 80% rule”. It means you’re not supposed to load up a circuit to more than 80% of rated capacity for “continuous” loads, which are defined as loads that are on for more than 3 hours at a time. That means all your normal stuff probably won’t use more than 80% of the breaker rating ever, especially the heat pump, so it’s really a 24A max load. You can actually use ALL of your 100A service, as long as it’s not for more than 3 hours, and those loads are likely to cycle within that time, so you won’t be running all out the entire 3 hour window. Think about it: you probably finished boiling that pot of water before 3 hours, the pie in the oven is done. The water heater maybe ran once in that time. The heat pump might have been running the entire time on a very hot day, but nothing else was.

So be careful about going overly conservative with a load management system that disconnects loads — you might not need it to be so aggressive.

One other thing: electricians don’t generally do repairs on equipment. Electricians install equipment, and replace things that go bad. As an example, if a light switch fails, your electrician is going to replace it, not try to take it apart and fix the internal contacts. The same goes for those load misers — it’s unlikely an electrician is going to be able to replace one that has failed. The electrician may need to replace the entire unit.

I recommend you also ask your electrician to check the electrical load of your entire home with things running. This is very easy to do using what is known as a “clamp meter”, which is an amp meter that can be temporarily clamped around a wire to measure current. This needs to be done in AT LEAST TWO PLACES (both service entrance “hot” conductors, which are typically either black or red wires in the main panel). See what the *actual* load is to determine if you really need to disconnect anything.

Bill