Remaining capacity of service panel

Discussion in 'Electrical Forum discussion & Blog' started by SmithHSC, Sep 23, 2020.

  1. SmithHSC

    SmithHSC New Member

    Joined:
    Oct 16, 2018
    Location:
    South Carolina
    We have just had solar panels installed for our home (yesterday). I am looking into replacing two water heaters with tankless heaters. I currently have one whole house water heater (double pole 30 amp breaker in main service panel) and a smaller water heater in an addition with one shower and one sink (double pole 30 amp breaker in sub panel). My question is about the capacity of my panels and what I can add to them. My explanation may be lengthy and I apologize.

    The main service panel has had a 150 amp main breaker. In this panel are four 20 amp single breakers, one 15 amp single breaker, one 20 amp double pole breaker, two 30 amp double pole breakers, one 40 amp double pole breaker, two 50 amp double pole breakers, and one 60 amp double pole breaker. I believe that one of the 50 amp double pole breakers leads to the sub panel.

    There are three 20 amp breakers, not included in the description above, which were removed from the main service panel and placed outside with the solar panel equipment. I don't have a grasp on how that impacts the capacity of the main service panel now. There are now at least five single slots available for use in the main panel.

    My first question is about the main service panel. If I get a tankless water heater, obviously I would be replacing on 30 amp double pole breaker with a larger amperage. How much can I add to this panel as it stands now?

    My second question is similar. If I get a smaller tankless water heater for the sub panel, replacing a 30 amp double breaker in the sub panel, how much can I add to the sub panel. For logistics reasons we cannot use just one water heater for the entire house.

    Thank you for enduring my lengthy post and for your advice.
     
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    Electric tankless WH are almost a joke except for a single end use, and that is best limited to hand washing!

    A typical gas-fired tankless is in the 200K-BTU range, which equates to almost 59Kw, or if fed with 240vac, about 246 amps. No way you'll get two of those powered from your solar array or current panel. It would take one very huge battery storage system, too.

    Now, electric tanks, yes, but if efficiency is your goal, a heat pump version would be much more efficient where it can multiply the watts used to run the pump with ambient heat maybe as much as a 3:1 ratio.
     
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  4. SmithHSC

    SmithHSC New Member

    Joined:
    Oct 16, 2018
    Location:
    South Carolina
    Jadnashua, I'll accept your assessment of electric tankless water heaters with the caveat that I notice that you're in New Hampshire while I'm in South Carolina where this summer the water has come out of the ground ready to boil lobsters. I exaggerate. Unfortunately, gas or propane is not a practical option for me. I will look into the heat pump water heater.

    And that brings me back to my original questions. What can I add to my breaker boxes?
     
  5. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    There is a procedure in the National Electric Code called a load calculation that you would need to follow to answer that question. You can't just go by the breaker sizes. For all the breakers that feed things other than lights and receptacles, you need to find out what equipment the breaker is supplying and get its nameplate info. That would include all the breakers bigger than 20 amps, and might include some of the 15 and 20 amp breakers.

    So you can read up on how to do a load calculation and perhaps find an online calculator to help you. The resulting load will be a number like 30,000 VA, which at 240V is 125A. Since your main breaker is a 150A breaker, you can go up to 36,000 VA. So if you came up with 30,000 VA you'd be able to add another 6000 VA, or 25A at 240V.

    Make sure you use the optional calculation method, for residences the standard method is grossly oversized and the optional method is just somewhat oversized. As a practical matter, 98% you can just add a breaker for a heat pump water heater and you won't overload your panel to the point of tripping your main breaker. But you might exceed what the electrical code allows, because it provides a generous margin.

    Cheers, Wayne
     
  6. drick

    drick In the Trades

    Joined:
    May 16, 2008
    What Wayne said, but I'll add do not try to add an on demand electric water heater without performing a load calculation.
     
    Last edited: Sep 23, 2020
  7. Stuff

    Stuff Well-Known Member

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    Mar 7, 2013
    Location:
    Pennsylvania
  8. WorthFlorida

    WorthFlorida The wife is still training me.

    Joined:
    Oct 28, 2009
    Occupation:
    Retired
    Location:
    Orlando, Florida
    Assuming your two WH's are electric. The electric WH heat pump will draw only a little more power to run the compressor if both heat pump operation and electric elements are activated. Heat pump water heaters is a standard electric tank with an AC unit on top plus with computer control board. The condenser coils are wrapped around the outside of the water tank that transfer heat to the water. When the demand decreases the water temp, the electric elements will operate just like a standard tank. Only during this time would you be using more power than your current water heaters.

    Still do a load calculations for tankless system to get an idea what you are up against.
     
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    When I lived in Kuwait for awhile, the water tank was on the roof. During the day, the water coming out of that tank could get hot enough to scald you. So, to reasonably take a shower, we shut the WH off, and over time, it cooled to the room temperature. Then, we used the cold for hot, and the hot for cold, but boy, was it nasty if you ran out of 'cold' from the WH! So, I know about having warm water. But, you do have winter, certainly not as cold as we may get in NH, but the ground water will be much cooler than the end of summer. Tankless systems are generally rated for a 50-degree inlet water temp. Your hot output will go up if it's warmer, and down if it's below that, but electric powered ones still need a very significant input power to provide any decent volume and temperature rise...one BTU will raise one pound of water, one degree, but there's a limit on how fast you can apply that amount of power. One watt-hour = 3.41 BTU. You can run the math...

    A gallon of water weighs about 8.33#, and a tub filler can usually flow about 6gpm, or about 50# of water. Gas-fired burners are rated at BTU/hour, so in one minute, a 199K BTU unit provides 3317 BTU. Divide that by the 50# of water, and that would allow you to raise the incoming water about 66-degrees, barely enough at a 50-degree inlet, and pretty low, if you're filling a dishwasher, but that fill is slower.

    But, basically, forget about an electric tankless system unless you might live in the tropics.
     
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