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How would you expand an existing electrical panel?

user-2310254 | Posted in Mechanicals on

I have a townhouse with a mix of electric and gas appliances. I don’t like combustion inside a home, so I plan to make some changes to eliminate the gas burners. (These units are all original to the home and nearing end-of-life.)

The townhouse has an 11-year-old 150 amp service with a couple of unused breaker slots. I know it won’t be enough for one or two new HVAC units (heat pumps, mini splits, or possibly ducted minis), an electric water heater (probably a 75 gallon Rheem Marathon but maybe also a heat pump unit), an induction cooktop, and a Level 2 charging station.

Would it make more sense to keep the existing panel and add a subpanel or swap the existing panel for a new 200 amp panel? Or is there are better way to expand the service?

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  1. walta100 | | #1

    150 amps is quite a bit of power.

    Are approved tandem breaker sold to fit your brand of panel?
    What are the current 240 volt loads?
    How many square feet?
    What are the current insulation levels in the walls and ceiling?
    How many common walls to conditioned spaces?
    How many windows and how good are they?
    Do you have the budget to do a “Deep Energy Retrofit” of this space?

    You will likely need the owner’s associons approval to upgrade the service. It is unlikely the utilities very expensive electrical equipment was oversized by 25% on the off chance someone would up size there service.


  2. Andrew_C | | #2

    RE Walter's comments -
    I have never lived in a house or condo where I thought that the electrical panel was too large with too many empty breaker slots. Conversely, I've often been constrained when adding new circuits for (whatever). If you're changing things, 150A service is small, IMO. You won't regret 200A service and more slots. [I can hear Martin's eyes rolling at the decadence of some of us grid-tied people, but I don't turn everything on at once...]

    A charging station likely changes a lot of things. If you're adding that plus an electric water heater and an electric range, you're probably going to have to get an electrician involved anyway...see what they suggest for your situation. Walter is right, there are a lot of variables to consider, including the layout of your townhouse and garage.

  3. user-2310254 | | #3


    I agree. The project will also require running some new 240 volt circuits to different parts of the townhouse.


    The existing 150 amp panel only has one open slot. (That may be for two 120 volt, 15 amp breakers, but I don't know much about how these things are typically configured.) The service to the unit is rated for 200 amps, however. So maybe I can replace the panel or add a sub panel without affecting anything outside the unit.

    Let me see if I can answer your other questions.

    Q: Are approved tandem breaker sold to fit your brand of panel?

    A: I don't know. The panel is about 11 years old, so maybe yes.

    Q: What are the current 240 volt loads:

    A: From the inspection photo, it appears there are four 240 volt breakers.

    Q: How many square feet.

    A: The townhouse is 2,800 square feet with an elevator.

    Q: What are the current insulation levels in the walls and ceiling?

    A: I don't have a clue on the walls. Probably whatever was code minimum in 2007. I'm sure there is no insulation under the slab. The insulation in the partially ventilated attic is inadequate. I plan to move the thermal boundary to the sheathing and air seal and insulate the attic to at least R-38.

    Q: How many common walls to conditioned spaces?

    A: If you mean to adjoining units, there are two common walls. The townhouse is an interior unit.

    Q: How many windows and how good are they?

    A: There are about 20 double hung, dual-pane windows and three doors with half-height fixed glass. I'm not an expert but think the windows are not very good. They don't appear to be super leaky or anything, but they don't inspire a lot of confidence when opened or closed. The HOA is responsible for the windows, and I wouldn't be able to replace them without board approval. That said, the weather is rarely nice enough in our climate to open windows. If the door blasters indicates they are a big problem, I could install some type of interior storm window.

    Q: Do you have the budget to do a Deep Energy Retrofit on this space?

    A: I don't think this unit is a good candidate for that type of effort. (And my wife would kill me if I started cutting holes to blow in insulation.) Overall, it is in great shape. But I do plan to do as much air sealing and insulating as is practical. FYI, the unit is real stone and brick on the front and back.

  4. GBA Editor
    Martin Holladay | | #4

    I agree with Andrew: You're going to have to get an electrician involved. So call one up.

  5. this_page_left_blank | | #5

    You say there's 200A service there already. Replacing the current panel with a 200A one is by far the simplest and best solution.

  6. seabornman | | #6

    Before you call electrician, find one of the free on-line electrical load calculators (I used one from to get an idea what total load could be. Some electricians are not so diligent in assessing your loads. When I had solar system installed I had a new 200 amp panel installed that backfed the existing panel, so I had the advantage of having plenty of space for breakers, did not have to disturb all of the existing circuits, and could bring new circuits to the new panel.

  7. jackofalltrades777 | | #7

    Like others have said. Call a licensed electrician.

    If you have to pull the panel box. That is a lot of work and requires cutting the main power box from the electric company. Most homes have 200amp service with some larger homes running on 400amp service.

    I had 400amp power pulled underground to the home since I plan on having multiple homes (two) on this multi-acre site. I have a 400amp panel that splits the power to 200amps per house.

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