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As a followup to this question of mine, I'm now uncertain if I can add another dual-pole breaker to this subpanel at all.

subpanel inside

The bus bar on the left side has only one slot left, and the right side is full. The lower slots have no metal conductor in the breaker slots - only plastic.

Does this mean I cannot use those lower slots? Or do I have an option to replace the bus bars with longer ones? (I'm guessing no)

It's a Square-D Homeline panel.

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Whoever put this in was really trying to pinch pennies. It is an 8-space panel that, except for the shorter bus bars, could just as easily be a 12-space panel.

In fact, it looks like Square D used the exact same box and front panel, and most of the other parts (except bus bars 8 vs 12) for:

  • 8 space subpanel (no main breaker)
  • 12 space subpanel (no main breaker)
  • 12 space main panel (main breaker)

You might be able to take the guts of this 12 space Square D Homeline panel and replace the corresponding parts of your panel. That would cost you $ 65 in "parts" (the entire subpanel). You might find that you can get just the guts from a Square D distributor (not Home Depot/Lowes/etc. but rather an electrical supply house) and if they will sell it you (i.e., if it is available and if they will sell it to you knowing full well you are a DIY and not an electrician) then you might save a few $.

But once you are doing that, you should seriously consider putting in a bigger panel as that really won't cost much more. Currently at Home Depot (not necessarily the best choice, but easy to find pricing online) a 20 space is only $ 10 more! Or a 30 space with several "free" breakers for $ 129 total. That's just looking at the same brand/type so that you can use all of your existing breakers.

But the easiest solution is half size breakers. You can replace the 3 20A single breakers with 2 x 20A half-size breaker pairs (so 4 20A circuits total, including one new circuit) and have two regular spaces available for your new 240V circuit. Home Depot has the 20A breakers:

Square D 20A tandem

for about $ 16 each.

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    Thank you so much! Exactly the info I needed. I think I will go with the double half-size breaker. Seems like the easiest option, and in-stock at my local home depot! Commented Nov 5, 2023 at 3:53
  • Also, this makes it much easier to do it legally. I just bought the homeowners electrical permit from my county, and have triple checked the current electrical code. I didn't know about tandem breakers before, but now I do. Thanks!!! 😎 Commented Nov 5, 2023 at 4:40
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    You are in a good situation here - Square D Homeline isn't the greatest but it is current and breakers are inexpensive. A lot of times we see people with older panels who are overdoing the number of tandem breakers (a lot of older panels had limits for reasons) or where the panel is so old that it never was allowed to have tandems (even if they happen to fit) or they mix tandems from one brand with a panel from another brand. You've got it easy here. Commented Nov 5, 2023 at 4:46
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    Update: With a homeowners permit in hand, I moved two of the 20A to a tandem breaker, added the 40A breaker for the Chargepoint Home Flex charger, connected it with 8 gauge wire in schedule 40 1/2" conduit. It works, and passed inspection. The inspector only asked that I put a larger "40A Max" label on the charger, so a future homeowner doesn't accidentally set it higher in software. He also said I could go bigger in this panel, but he agreed I probably didn't need to. Fun part - he drove up in the same electric vehicle I own. 😎 Commented Nov 14, 2023 at 1:55

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