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I recently purchased a house that has a flush mounted main panel on the exterior garage wall. I would like to install a surface mounted subpanel on the inside of the garage either right behind the main panel or to the side.

I need circuits for a 240V 30A table saw, 120V 20A for shop tools, and two 120V 20A for a future kitchen remodel project.

What would be the best way to go about installing and connecting the sub to the main?

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    First do a load calculation to see how much amps you use for the panel. Overloading the main panel not fun. After that can get a 200 amp big panel for future use, even if you can only send 30 or 40 amps to it.
    – crip659
    Commented Jan 31 at 0:47
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    Is this a 'rule of six' panel? The quad main in the middle is weird.
    – KMJ
    Commented Jan 31 at 0:50
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    The quad is a the main breaker (as the label confirms). Nothing weird about it. The reason is probably a combination of: it was easier to build high-amp breakers in the early days by paralleling two smaller ones, and it also enabled this narrow meter-main package by having both the main breaker and branch breakers all stabbed onto the bus in the same way.
    – nobody
    Commented Jan 31 at 1:28

2 Answers 2

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Load Calculation

This should always be the starting point for any major electrical upgrades. You will need to do two of these - one for the current Meter Main and another for the new subpanel.

This is a particular complex calculation based on NEC 220.82 which includes many things such as:

  • Total living area (square feet)
  • Kitchen dedicated receptacle circuits
  • Laundry dedicated circuit
  • Cooking appliances (gets a little complicated there)
  • Other large appliances such as dishwasher, water heater, etcv.
  • HVAC - largest of Heating or Air Conditioning
  • Water Heater
  • EV Charging
  • Other fixed loads such as pumps

You first do this for your entire service. If your service is 100A and your Load Calculation is 100A or more, you are already in trouble. If your service is 200A and your Load Calculation is 120A then you have 80A to spare. Etc.

Then you do this for the new subpanel. The total you come up with (a) has to fit (except for any existing loads that are just being moved around) within your total service (e.g., 80A in the second example above) because it will increase your total Load Calculation, and (b) that determines the maximum breaker size, which in turn determines the minimum wire size.

Using the above example, if your service is 200A and your existing Load Calculation is 120A then you have 80A to work with. If your subpanel calculation is 60A then you could install anywhere from 60A to 80A feed with no problem. I think you can even install a larger feed but your Load Calculation still limits how much you can use. In this example, if you installed a 90A feed because you used aluminum 2 AWG wire and wanted to feed as much as possible for future use, you would still only be able to load it up to 80A of Load Calculation usage - e.g., the 60A your Load Calculation shows + 20A for EV charging but not anything beyond that even though the subpanel could technically handle it.

Assuming all this is good, you get to the next step:

Breakers and Wires

Your Meter Main looks like it is full. So a key step here will be to move a few circuits (2 full spaces or 4 half spaces) to the subpanel. Ideally pick circuits where the wires are running in the correct direction so that you don't end up with wires that are a foot too short.

The breaker in the Meter Main is a standard double breaker. The wire size is any size at least as large as an ampacity table shows. For a short distance such as this, you can go with copper or aluminum, but aluminum is the usual wire type for subpanel feeds. You can use an appropriate cable, but individual wires (4 wires - hot, hot, neutral, ground) in conduit makes a lot of sense here. Why? Because big cable is a pain to bend and you have to deal with bend radius. With conduit you build the path for the cable and then insert the individual wires.

Based on the description, I think conduit would work very well here. If you align things reasonably, you should be able to:

  • Use a knockout in the top, bottom or side of the Meter Main
  • Use a knockout in the top, bottom or side of the subpanel
  • Drill one hole (just larger than the conduit) through the wall
  • Build a conduit section between the two panels

If that conduit section is less than 24" in total length then you have the advantage of being able to route multiple circuits (so not just the subpanel feed but also extend other circuits to move them to the new panel, if needed) through the conduit without a multiple circuit derate, though you do still have to be concerned with conduit fill.

The subpanel does not have to have a main breaker, but it can. In fact, a "main panel" used as a subpanel is often very cost-effective because they are priced very low in package deals at Home Depot, Lowes, etc. including:

  • Large panel - 20 or 30 spaces or more
  • Main breaker - doesn't matter if it is larger than your feed - 200A main breaker with a 60A feed is perfectly fine. The only thing you need to check is wire size as the large main breakers have minimum wire sizes that can be a problem.
  • "Bonus" breakers - typically some 15A and/or 20A single breakers and 20A and/or 30A double-breakers, but varies.

Any subpanel needs the same amount of space in front of it, and they nearly all fit the same "between studs" width so 8-space subpanel or 42-space main panel is in certain ways the same.

Your existing panel appears to be a Square D Homeline panel. If you get a Square D Homeline panel for your subpanel then you can move breakers between the panels. For example, you could get a Homeline 24 space 100A panel for $109 (January 2024 price at Home Depot online) that includes 3 x 20A single breakers and 2 x 30A double breakers.

Most circuits will need AFCI and/or GFCI. AFCI is normally with a special (expensive) breaker. GFCI can be with the breaker or at the receptacle for 120A circuits, but for 240V circuits (e.g., 30A table saw) it has to be at the breaker. You probably will be OK with existing circuits moved from one panel to the other.

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I can't read all the instructions and can't see any sticker behind the deadfront on the inside of the panel, but generally on outdoor panels there is labelling that restricts penetration to a particular distance below the bussing.

In light of this I would route the conduit out the bottom of the panel, squeezing the new penetration between the two existing conduits, then a foot or so below the panel use an LB condolet to change direction to go through the wall. Another LB could the be used inside to redirect the conduit up to the desired mounting height.

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