Rewired the service panel today for 240V 50A. The bump up in size is an annoying accommodation that we have to make for a very specific circumstance in our build. Normally I only put 30A or 40A 120V in small spaces like this because it’s well sized for a smaller off-grid photovoltaic system, but in this case it’s actually the other off-grid features of the house that are creating a problem.
We are installing a hybrid woodfired solar hot water system that requires a large tank as a reservoir and the easiest way to do this is just to use a commercial 30 gallon water heater, but at that point it only makes sense to use the elements that are in that water heater as our back up electric water heat. You can of course wire a 240 water heater as 120, it just heats a lot slower, but fast hot 240 is going to have better thermal stratification meaning that you can put it on a switch and just heat up a small portion of the tank to take a quick shower and then turn it off and let the off grid portions of the set up do the work. So that’s how we ended up at 240. The nice thing about this is as long as there are no other 240 loads you can literally rewire the panel and plug for 120 in twenty minutes and run the whole system as 120, which is how I would set this up when we switch to fully off grid.
Normally I wouldn’t even have an electric back up but it’s not clear that we are going to be able to use the woodstove in the city which eliminates our winter source of hot water. I lived off grid with exactly that set up for 10 years and surprisingly took less than a dozen disappointing showers. Going that route or just going with a 7 gallon switchable mini tank keeps you solidly inside a 30A 120V system.
Because a tiny house is used more like an RV, NEC load calculations don’t work very well for service sizing, I don’t have the space to explain that here but if anyone wants clarification just ask and I will explain it in a comment below. Also I am aware that the last photo is missing a couple nail plates!
– Brian
