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This is my main panel cabinet. 2

My original plan was to put the subpanel in front of main panel inside the cabinet by just letting it stand naturally (because it would be distracting or not good to the look to connect it to the wall outside the cabinet). For those who have tried this. What problems have you encountered? This is why I need to limit the Siemens sub load center to 12 space 18" height so I can still access the main panel.

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This is my main panel with 16 2-pole breakers US style. All use GE (General Electric) plug-in breakers.

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The breaker on top is a 125A 2-pole. Are all 125A breakers really that big?

I'm replacing six of the original GE 2-pole breakers with six Siemens GFCI 2-pole breakers. Since I can't directly plug the 6 pcs due to bus nub mismatch. I need to use a subpanel. Now my question:

My 6 pcs of Siemens 2-pole GFCI breakers are rated at 60A 2 pcs, 30A, 4 pcs.

Can I connect them or the 6 branch subpanel to just one of the main breaker output say rated at 70A or even 100A? In a panel, How many ampere can a breaker bus support provided the whole panel is rated at 100A.. is a single breaker bus also 100A? If not, what is the rule of the connections generally?

I can't replace the entire panel because it's difficult to get it out of the concrete wall and put another one fit right for it.

What's the usual way to add subpanel? What did you do?

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  • 225A breakers had to be that big. 125A breakers had to be interchangeable with the 225. (i.e. have the same form-factor).
    – Harper - Reinstate Monica
    Commented Dec 17, 2020 at 21:27

2 Answers 2

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In the US, part of the panels ratings are the maximum current for a single bus stab. Many panels say on the label not to exceed a certain current level on a single bus stab. Most residential panels are double row so a single bus stab feeds two breakers, one on the left, one on the right. The sum current these breakers supply can't exceed the bus stab rating.

There are other ways to feed a subpanel. If your panel has or can be fitted for feed through lugs, you can feed the subpanel from those lugs.

The NEC has feeder tap rules that allow you to tap the feeders for a subpanel without overcurrent protection - normally a big no-no - within certain limits. The feeder tap rules outline these limits - distance limits, running in conduit, etc.

Another option may be to change out the panel "guts" with a retrofit interior kit. With these kits you remove and replace the bus, lugs, etc. If Siemens makes a retrofit kit with a bus compatible with your breakers, you could replace the guts and use your Siemens breakers. However you'd have to replace all the breakers in the panel with Siemens breakers as well.

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  • For example the panel is rated at 100 ampere. What is usually the allowable current level on on a single bus stab? Can you give actual example? I'd like to know if it can reach 70A.
    – Samzun
    Commented Nov 9, 2018 at 13:24
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    Here's an example which doesn't tell us anything about your panel but this Siemens 125A panel has bus stabs limited to 110A images.homedepot-static.com/catalog/pdfImages/fb/…
    – batsplatsterson
    Commented Nov 9, 2018 at 21:26
  • @samzun - also edited answer with another option
    – batsplatsterson
    Commented Nov 9, 2018 at 21:27
  • Thanks for the link above. It's the same model I ordered and cancelled because I thought it was not compatible to the Siemens GFCI breakers.. but I saw the QPF listed so I think it's compatible. But what I'll order now is the one with the built in 100A breaker pdf.lowes.com/useandcareguides/040892635836_use.pdf Do you know what is meant by "Short Circuit Current Rating" and why it differs depending on what breakers are installed.. for example if the QPF (the Siemens GFCI) were installed the SCCR would be 65,000A?
    – Samzun
    Commented Nov 9, 2018 at 23:47
  • batsplatsterson.. the passage " Sum of QT breaker rating is not to exceed 110 amps per branch circuit bus stab." refers to the QT breakers which are "Siemens type QT dual circuit breakers are 1 inch wide and consist of (2) 1-pole breakers, in varying combinations of ampere ratings from 15 to 30 am".. so for normal breakers.. the rating may be the full capacity of the panel, isn't it? (usually for other brands)
    – Samzun
    Commented Nov 10, 2018 at 12:41
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Most of the time I find the max breaker size at 100- 125 amp for creating a feeder.

Looking on GE's web site they do go up to 200 but we would need to know the model number of the panel (many panels have the same bus and just the main is swapped out for different sizes. As blatsplaterson said you could tap your panel directly run a short piece of conduit to the sub with a main breaker and move your loads you want GFCI protected to the sub.

You have lugs just under the main breaker that could be used to power your sub. That panel looks heavily loaded for a 125 amp main. I would see if the main breaker could be upsized (based on the model number) and if the conduit is large enough to up size the feed from your meter. The wire ampacity from the service only needs to be 83% of the main breaker listing. I would be surprised if that bus was only listed for 125 amp you may find it is much larger but the model # or listing information on the panel will give us that information.

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  • The screws under the main breaker are no lugs.. just connector between bus. See picture at imageshack.com/a/img922/8772/WTQZbk.jpg The panel is locally made generic (it's not GE brand). Any idea how to splice the main wire into two? Also are 125A breakers this big... can I buy smaller version of it? Or the simplest way is just to add 100A breaker to one of the main so I can use this as feed to the subpanel.. example of the main bus bar is: imageshack.com/a/img921/9610/YsAHaa.jpg
    – Samzun
    Commented Nov 9, 2018 at 15:33
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    Since its not a listed panel it would be a crap shoot. The branch circuit breakers snap into the buss just like your double pole, on most of the larger ones the ones I use take 4 positions 2 positions wide and 2 positions tall and that breaker has screws to hold it in the panel. But it depends on the panel. I did look at the photo and the 2 bolts that connect the breaker to the buss would be a place to attach the lugs that could feed a sub panel using a screw type chair lug or a compression lug , the chair lug would be best for a home owner as the hand crimper I use cost ~750.00$ .
    – Ed Beal
    Commented Nov 9, 2018 at 16:03
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    I always use wire nuts if you only use 2 wires you can go with smaller ones like yellow I am not sure if the B+O blue and orange can take 2# 12 wires but know the yellow ones can, I strip the wires hold them even and slide the nut over then twist until the wires have twisted around each other 2 times I have not had problems with them comming loose but on industrial equipment that vibrates a lot I will usually put some tape on them.
    – Ed Beal
    Commented Nov 10, 2018 at 0:19
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    I was talking about wire nuts , ideal brand are a type with a hard plastic shell the colors identify the size,orange, yellow, red, blue are small to large. Another brand scotch lock orange& blue, tan and red, blue and gray small to large for TBE 2 brands I use most. I would have to check the rating 2 #10 may work on a yellow, but definitely will work on a red or tan& red
    – Ed Beal
    Commented Nov 10, 2018 at 17:17
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    Just twisting and electrical tape is not safe unless you use the western union standard of 7 wraps. I was trained on how to do this in the 70's for knob and tube repairs on historical homes and to tell the truth I would not do it even though I know how unless using the old type of solid copper in a very specific location where they want to maintain the historic value. The t taps and y splices if not done correctly can be a point of failure where a few wire nuts are safer in my opinion with today's soft copper not the old cold drawen hard copper.
    – Ed Beal
    Commented Nov 11, 2018 at 21:22

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