I’m installing a eco smart 27 tankless water heater, which requires (3) 8 gauge wire with (3) 40 amp double pull breaks. I am having to install a sub panel for this, and I am wondering what gauge wire do I use to supply power to the sub breaker box with the 8 gauge wires? Thanks in advance 😎

  • What size is your electrical service, how many square feet is your house, and why are you going tankless here? – ThreePhaseEel Jan 6 '19 at 3:53

You size the sub panel feeders to the size of the over current protection you are using to protect your subpanel. For example if you are installing a 60A supanel then you would install a 60A breaker in the main panel. Then using NEC Table 310.15(B)(16) you would find that a #6 cu in the second column or a #4 AL in the fifth column is capable of carrying 65A so you would use those conductors.

Note that your Eco smart 27 can actually carry up to about 113A. Since it is SMART it probably only allows a certain amount of current depending on your flow, but it is not SMART enough to know how much current is being used by the remaining loads in your dwelling. It could overload the service.

Rather than feed through your panel causing unnecessary strain on your electrical system. I would recommend you install a separate enclosed breaker by you main disconnect and feed you water heater from it. You are allowed to have multiple main disconnects per the NEC 230.71.

Finally I would check with someone and make sure your service is large enough to provide the adequate power for your new load. It could be your AHJ, utility company or a trained professional.

Hope this helps and good luck.

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You can’t install that water heater on a 60 amp sub-panel. The manufacturer recommends a 200 amp service with that heater so it needs to be fed from the main panel. The main panel needs to be a 200 amp service.

Do you have a 200 amp service?

I have the same water heater.

I do not recommend it to others since it puts a nasty notch in the sine wave of the system that causes the lights to flicker and puts a nasty buzz on my guitar amp. 😞

Good luck!

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