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I am upgrading my service to 200 AMP. Since the location of the old service is behind a cabinet, we moved the main panel to a different area. This put it more than 15 feet away so we have a 200 amp service disconnect outside and my "main panel" is now considered a sub-panel.

I'm trying to install a new 36kW instant hot water tank. Tempra 36 Plus http://www.stiebel-eltron-usa.com/sites/default/files/pdf/install-tempra.pdf

I'm concerned because this installation guides calls out this model needs a min 300 Amp service. It requires 3 X 50 Amp breakers. My inspector gave my load calc the ok and this is the main reason we are upgrading our service. This hot water tank is pretty over sized for my home so I'm not really worried about drawing the full 150 amps.

Am I able to use 50 amp breakers in a 200 amp panel, or is there some size restriction?

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    Wait... The manufacturer's installation instructions for the heater says it requires a minimum 300 ampere service, but you only upgraded to a 200 ampere service? If the heater draws 150 amperes of the 200 ampere service, that only leaves 50 amperes for the rest of the home. Do you have A/C, Electric range, electric heat?
    – Tester101
    Commented Apr 14, 2016 at 2:33

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Your 200 amp service can supply your 150 amp water heater. But will anything else in your house require power at the same time? Such as a microwave, room or house heater, room or house air conditioner, stove or oven? Have you considered using the 24 instead of the 36?

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  • We have heaters, oven, micro, and dryer. Will the heater always require 150 amps even if it's not going to use its max power?
    – Frank Sposaro
    Commented Apr 14, 2016 at 6:52
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    @FrankSposaroMSFT It is going to be likely that it will need the 150 AMPs while other appliances together also need more than 50. Heaters typically work with in a full blast then turn off cycle with the length of each section decided by a thermostat. Which means it will be drawing all those 150 AMPs during the actual heating.
    – ratchet freak
    Commented Apr 14, 2016 at 8:26
  • She means, are those appliances electric. Gas furnace, range or dryer do not count.
    – Harper - Reinstate Monica
    Commented Apr 15, 2016 at 4:37
  • Your Tempra 36 Plus does feature "full power-modulation based on both the incoming water temperature and the water flow rate. In layman's terms, the Tempra 36 will automatically sense the water flow rate and temperature, calculate how much power is needed to do the job, and will apply exactly that much power so it can precisely maintain the desired output temperature regardless of whether the flow rate changes or not." But in the worst case, you can still overload your 200 amp service.
    – Philip Ngai
    Commented Apr 15, 2016 at 16:08
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    Thanks for all of the input. We ultimately ditched the electric instant and went a rheem 50 gallon. Got the one with two heaters in it, jacked the temp up to 140 or 150f, then put a relief tank along with booster/mixer value.
    – Frank Sposaro
    Commented Jun 29, 2017 at 6:04

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