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From a 100 amp main panel (load center) in a residential house U.S. a circuit or space goes to a subpanel within the basement. The subpanel (going from memory) is about 12" or 14" square and I think has 8 or 10 spaces, and is 50A or 60A, and was installed many years ago. Something like 6 of those spaces have 15A breakers, the remaining spaces were marked with magic marker "do not use". This subpanel has a large main breaker in it, that when opened it disconnects power to all the 15A breakers in it.

  • Has there, or is there, a limitation on the number of breakers that can be in a sub panel? And has that changed over the years?
  • Why did someone write "do not use" on the remaining used spaces in the sub panel?
  • What reason would prohibit using one of unused spaces in the subpanel, using a 15A breaker, to wire some LED ceiling light fixtures in an unfinished basement? Or to anything else?
  • If you buy a sub panel with however many spaces, can you always make use of every space in that sub panel, If not why not?

update: no main breaker in sub panel, see pic below. It is a 50A breaker feeding this from the main panel. Sub panel doesn't have a main breaker in it, my mistake when I said it did. This was done 20+ years ago. Is this that rule of 6 thing, and does that still apply can it fill this box with breakers?

enter image description here

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    It could be the person who installed it, thought you could only use breakers that added up to the main breaker. Most breakers are not used to their full amp rating, most only use a few amps. It might also be a panel design so a picture of the panel will help.
    – crip659
    May 4, 2023 at 20:21
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    Could be that the guy who installed it was an electrician (or got the panel from an electrician buddy) and reused a partially damaged panel. He marked the spots "do not use" because he knows the bus is damaged. Of course, this is pure speculation. Pics of the panel (without the dead front, so we can see what the inside looks like) and pics of all the labels on the sub panel are the only way we have a chance of knowing.
    – FreeMan
    May 4, 2023 at 20:45
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    You'd have to ask them. Not that it matters; some BS written in sharpie doesn't count for shit. "can you always make use of every space in a panel?" ... that is a viable question, although I can't imagine the answer to it being a no.
    – Mazura
    May 5, 2023 at 0:58
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    Can you post photos of the panel, including the labeling on the inside of the door, please? May 5, 2023 at 2:52
  • photo tomorrow when i get a chance
    – ron
    May 5, 2023 at 3:46

3 Answers 3

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Many possible reasons.

One is that the sub-panel may only be fed from a 120V breaker connected to one bus, and the other bus may not be powered at all. You mention "a circuit or space" not a double-circuit or two spaces, which would lead this direction.

Others have been mentioned in other answers. You need to provide far more detail for anything beyond a guess.

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For a long time the NEC specified maximum 42 switches in a "lighting" panelboards. Panelboards were built and Listed compliant to the NEC. The NEC has changed, but panels are subject to the testing and marking on the panels that indicate the max. Labeling is part of the Listing, and changing of the rules does not provide approval beyond the what they were tested for. For instance if it is labeled as a "30 space 40 circuit panel" then the 20 spaces shown on the labeling as full space breaker are only allowed to have full size breakers and the 10 spaces that are marked for tandems are allowed tandems. Most of the time tandems do not properly fit where not indicated.

You are not completely clear about the "do not use" spaces, beyond physical damage there are two other typical situations. One is some manufacturers make 16 and 20 space panels that fit in the same cabinet and they make only one 20 space cover for both both versions, but there is no bus behind some of the cover knockouts. Another situation is "stab rating", often back-to-back breakers are mounted to the same tab, and that tab may have a rating listed on the panel cover that is less than the maximum breaker size that physically fit. A maximum size breaker may be installed on one side necessitating the other side be left empty.

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If you buy a sub panel with however many spaces, can you always make use of every space in that sub panel, If not why not?

If you buy a 6-space subpanel like a BR612, would you say you are entitled to use 6 spaces?

Well, the panel has a stamped cover with twist-outs. Suppose to keep costs down, the manufacturer uses the same cover on both the BR612 and the more costly BR816 8-space panel. If you see 8 twist-outs on the cover of the 6-space panel that you paid for, does that change your answer any? :)

Has there, or is there, a limitation on the number of breakers that can be in a sub panel? And has that changed over the years?

Not in the range you are concerned with. The 42-circuit limit was abolished, but that doesn't affect a 6-space panel.

Why did someone write "do not use" on the remaining used spaces in the sub panel?

Probably for the reason mentioned above, they bought the 6-space panel and it came with the shared cover with the 8-space.

Or, there were breaker spaces there, and they were damaged, e.g. burned up by using an off-type breaker.

What reason would prohibit using one of unused spaces in the subpanel, using a 15A breaker, to wire some LED ceiling light fixtures in an unfinished basement? Or to anything else?

No breaker space there, or not usable.

Take the cover off and find out for sure.

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