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Discussion Starter · #1 ·
Looking to add a subpanel next to my current 200 Amp / 42 space Square D QO breaker panel and trying to find the stab limits for this panel.

Link to panel here

I'm having trouble locating the stab limit info on the interior panel stickers. This is the only sticker that might be the stab limit information, but I'm unsure if I'm reading it right. In the picture below, is this the bus stab limit?

breakerinfo.png
 

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That’s not it. It should be on a label on the cover possibly. On average the buss stab rating is around 125 amp. You add the breaker rating, not the load, also.
 

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You're on the right label, but you're not showing us the right part of it.

What you circled is the allowable (more likely: available) sizes of breakers. For instance it's saying QOT (twin/tandem/double-stuff) breakers are only allowed up to 20A.
 

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Discussion Starter · #4 ·
This is the other half of that label. I'm not entirely sure what "Tenant Circuit Breaker" is. Is the QO-VH (125) under "Max A" the bus stab limit?
breakerinfo2.png
 

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Discussion Starter · #5 ·
Ohhhh figured it out. I couldn't find the "Bus Bar Rating" on any of the labels because it doesn't exist! Per Square D site, if the "Bus Bar Rating" is different than the "Mains Max" rating, THEN it will be listed. Otherwise, it will ONLY list the "Mains Max". In other words, the bus stab limit for my Square D QO panel is 225A.

My box:
stablimit.png

Square D site instructions:
 

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Bus bar and bus stab are 2 diffent things. I believe the bus stab limits are 125A.

Go back to the Square D site. You will find the biggest plug-in breaker you can get is 200A. Now look at the spec. You will see the breaker actually has 4 stab sockets on it and takes up 4 spaces. Thats because each stab/socket is only rated for 125A.

Also, have a look at the stab physically. Note the size. Now look at a piece of 2/0 wire which is rated for 200A. Do you think that little piece of copper that makes the stab could carry 200A?

You really should call them and verify.

If you don't mind my asking, what is it you are trying to accomplish? Install a 100A breaker for the subpanel across from another large breaker, a range perhaps? You should be able to relocate breakers to mitigate that if you need.
 

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Ohhhh figured it out. I couldn't find the "Bus Bar Rating" on any of the labels because it doesn't exist! Per Square D site, if the "Bus Bar Rating" is different than the "Mains Max" rating, THEN it will be listed. Otherwise, it will ONLY list the "Mains Max". In other words, the bus stab limit for my Square D QO panel is 225A.
Yeah, that's as I'd expect from QO. They use copper bus bars and they're fairly thick. Copper is very dense, so that makes it a good conductor by cross-sectional area which works great for bus bars.
 

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Red Seal Electrician
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I don't know the answer.... but I do remember the Q12100/Q12125 breakers that are double-long and prevent you from using the stab with another breaker.

I have a lot of QO stuff... I'll see what other info I can dig up.
 

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As discussed, the limit per space is limited by the fact that there are no available breakers that exceed 125 Amps without being double tapped per pole.
 

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I found this:

Most Square D QO or HOMELINE panels are supplied with "double row" bus
bar construction. This means that the bus bars will have branch circuit
breakers plugged onto the bus bars from the left and from the right.
The "bus stab", "bus finger" or even "connector finger" is the point on
the bus bar that the branch circuit breakers electrically connect to the
bus bar. For instance, a 40 space panel will have 20 "bus stabs". The
bus stabs accept circuit breakers from both sides, providing the
capability to plug on 40 1-pole circuit breakers.

If your panel is restricted to 125 ampere per bus stab, that means that
you cannot put two breakers onto that bus stab (one from each side)
that exceed 125 amperes between them. For instance, you could place two
60 ampere breakers (60A+60A=120A) across from one another on the same
bus stab(s), but you could not place two 70 ampere breakers
(70A+70A=140A) across from one another on the same bus stab(s).

This restriction is not applied to all QO or HOMELINE panels. The
restrictions are applied only when we must limit the total amperage on a
stab to pass applicable UL heat rise tests.

I hope that this has satisfactorily answered your questions. Please let
me know if you need anything further on this subject.

Regards,
Rick Snapp
Product Technical Support Group
Square D Company - Lexington Plant
 

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Discussion Starter · #11 ·
Based on that response from the Square D rep, it sounds like not all QO panels have a bus stab restriction - only limited by what breaker sizes you can get / fit into panel.

I will call Square D next week and confirm one way or the other regarding my panel.

@joe-nwt , That's exactly what I'm planning out - 100 A subpanel next to main panel. Realizing how quickly 42-spaces get filled factoring in all of the items required by code, then a bunch of double pole breakers like small subpanel for garage, AC, stove, dryer, generator interlock, etc.
 

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Red Seal Electrician
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My interpretation of the rep's info is that what be believe to be max size breaker restriction is actually a stab constraint. I think the labels should be worded better if that's the case.
 

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My interpretation of the rep's info is that what be believe to be max size breaker restriction is actually a stab constraint. I think the labels should be worded better if that's the case.
Even if you look at the specs it's not spelled out. I'm not sure why that would be. I'm sure they're asked that question a lot.
 

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Discussion Starter · #14 ·
Called Square D and as @ joe-nwt said, there is no spec for bus stab rating on this panel.

For this panel, the "Mains max" is 225A, which is also the "Bus bar" rating (which is not the bus stab rating).

Between two tech supports that I talked to, the only breaker restrictions listed about this panel is that the max branch breaker size is 200A. The only other restriction they could find was that the branch breaker size should not exceed the main breaker size.

Not sure what to make of this other than using some common sense when installing breakers to reduce load on each bus stab regardless of what the panel allows.

Other thoughts:
  1. Largest single pole breaker is 70A
    • So 70A on left and 70A on right for 140A total on a bus stab
    • This would be allowed
  2. Largest double pole 2-space breaker is 125A
    • So 125A on left and 125A on right would be 250A on a bus stab
    • I'm thinking this would also technically be allowed per the panel if the load calc for these (2) 125A breakers comes out to less than the 225A bus bar rating?
  3. Largest double pole 4-space breaker is 200A
    • Not sure how to count this, but I'm figuring each pole on the breaker touches at two bus stabs, which would mean 100A at each bus stab for each pole on each side of the panel
    • So 100A on left and 100A on right would be 200A on a bus stab
    • Also thinking this is technically allowed per the panel
 
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