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I have some basic electrical knowledge but have never wired anything.

I plan on wiring a 288SF cabin; it be just one room and will have a separate small bathroom. I will need a mini split system to provide heating and cooling and I was thinking about using a tank less water heater to save space.

Here is my idea of how I would wire it :

  • 20 AMP Bathroom Fan and lights
  • 20 AMP Bathroom Outlets
  • 40 AMP Mini Split Unit
  • 20 AMP Outlets Outlets Cabin (8 outlets)
  • 20 AMP Outlets Kitchen
  • 75 AMP Tankless Water Heater
  • 20 AMP Lights Main Cabin and outdoor light

That would come up to 215 AMP which seems like a lot ! I am guessing that I will need to get a sub panel at my house where I was planning on getting power from.

Do those numbers look about right ?

Thank you for your input, I do appreciate.

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  • You might want to consider using point of use water heaters instead of one large unit for the whole place. You could also just get a 30 gallon lowboy heater. So a 150 or 200 amp sub panel in the cabin fed from your house. The feed to it would be a lot smaller than you calculated because everything won't be running at the same time.
    – JACK
    Commented Oct 2, 2020 at 22:40
  • How many square feet is the cabin, what do you plan to use for a range/stove there, and have you looked at other options besides central tankless water heating? Also, who is your electric utility, what is the nameplate minimum circuit amps on the mini-split, and how many amps is your existing electric service? Commented Oct 2, 2020 at 23:17
  • One 15A circuit is likely plenty for all the lights and the bathroom fan, if looking to economize, unless you are an incandescent / halogen using hater of LEDs.
    – Ecnerwal
    Commented Oct 3, 2020 at 4:40

1 Answer 1

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The added up size of the breakers is not equal to (generally it's much greater than) the actual load.

There are code guidelines, or you can look at what's expected to be powered and what its actual amp draw is.

Tankless electric water heaters are foolishness, and the huge amperage requirement is one reason why (the lack of power savings is the other main one.)

Also you are apparently in 120V/240V land and don't realize that 2 20A loads on opposite phases at 120V are the same as one 20A load at 240V. That alone gets you down to 175A on your current list.

You also need a minimum of 2 20A circuits for kitchen outlets, and it's generally advisable to have at least one other circuit (15A is normally plenty) for the refrigerator and nothing else.

For a start, look at the actual running amps of your mini-split, .vs. its 40A breaker size. If you insist on the tankless foolishness, it's actual amp draw .vs. breaker size will matter as well, but it won't spare you much in that case. FWIW, that's one HECK of a breaker for a minisplit on a 288 sf cabin - I'm doing 1000 well insulated square feet with one that asks for 15 and uses maybe half of that.

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  • Thank you for your answer, I do appreciate. I am okay with going with a small water heater rather than a tankless one if the energy savings are significant. It looks like a 20 gal water heater will demand only about 16 AMP. As for the mini split, I just did some basic research and I believe that the model I did find was a 35 amp demand or so. You are right, I did not realize the difference between the 120v vs the 240v. I am very new at all this, do I get to choose if the new sub panel will be 120v or 240v ? How does this work ?
    – Msar
    Commented Oct 3, 2020 at 12:01
  • Keep learning or give hiring help a consideration. Sub-panels are 240V by default in the US/Canada electric system, and you always have 120V available (240 is "Line to Line" and 120 is "Line to Neutral"). A tank type heater reduces your amperage load since it can take some time to heat water. Modern (well insulated) tank type electric resistance heaters are basically the same energy cost as tankless electric resistance heaters if you actually use any hot water (they are well insulated, so the "standby cost" of keeping water hot is very low, and that's all you "save" with tankless.)
    – Ecnerwal
    Commented Oct 3, 2020 at 12:17

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