Thread: Buss Stab Rating

Thought I would post this for all you home inspectors that would like to know. This would be related to John Kogels posts about the panel with a 70 amp maximum branch circuit breaker restriction. A lot of panels will have labeling which states something like " sum of branch circuit breakers not to exceed xxx amps per bus stab" .Sometime ago I inquired about this with Square d to get clarification. So I thought I would post that communication for future reference to those interested.

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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Some panels are 110A per bus stab, this is why reading the panel label is so important to know the manufacturers instructions & limitations.

Originally Posted by Rollie Meyers

Some panels are 110A per bus stab, this is why reading the panel label is so important to know the manufacturers instructions & limitations.

Hi Rollie

How are you this evening? Anybody cut their fingers off over on Sawmill Creek..

Yes panels have different bus stab ratings. Some have no restrictions at all as explained by Mr.Snapp.

My old shop had a 200 amp challenger panel with 125 amp bus stab ratings.

Roger,

Thank you for that information. I consider myself pretty good at electrical inspections, but I did not know that. I look forward to putting my new knowledge to the test.

Originally Posted by Gunnar Alquist

Roger,

Thank you for that information. I consider myself pretty good at electrical inspections, but I did not know that. I look forward to putting my new knowledge to the test.

Your welcome

And your not alone I probably talked to a half dozen electricians I knew when I was communicating with square d over this subject that had no idea what I was talking about...

EDIT: Wanted to say you probably will not see 2 single pole breakers that will sum up to over the bus stab rating. Usually it will be a double pole across from single poles or two double poles across from each other or a single double pole

Originally Posted by Roger Frazee

Hi Rollie

How are you this evening? Anybody cut their fingers off over on Sawmill Creek..

Yes panels have different bus stab ratings. Some have no restrictions at all as explained by Mr.Snapp.

My old shop had a 200 amp challenger panel with 125 amp bus stab ratings.

I only post at Sawmill Creek once in a while,as I prefer old machinery over shiny cheap Chinese peanut butter cast iron like Grizzly.

BTW, Challenger is just a later incarnation of Zinsco, Zinsco was bought by GTE in 1973 & later named GTE/Sylvania which became the Challenger Electrical Equipment Corp. until it was bought by Westinghouse, who relabeled their circuit breakers in Challenger gear, such as the Bryant/Westinghouse BR frame breakers & Series C industrial breakers to provide a couple of examples. (Have a Challenger bolt-on panelboard that with out the label you could not tell the difference from a Westinghouse).

R.F.,

Are these the pictures from the post you were referring to? (with reported 100 amp breaker?) If so I have a question for you, regarding 7,8.

HG

Yes that is the panel.

It is not listing a bus stab rating but a maximum branch circuit breaker size of 70 amps .

The two tandem breakers in spaces 7 & 8 would appear to be incorrectly located. Is that what your seeing?

I'm unfamiliar with that panel but as I see things the bottom right 100 amp 2 pole breaker (if it was allowed) occupying single pole tandem locations/spaces 11-12 & 15-16 should be where the two tandems in 7 & 8 are located and then those tandems moved to where the double pole is currently located for a correct configuration with the wiring diagram..

Whether or not the tandems were modified to fit in those locations would depend on the bus stabs and any rejection feature on the tandems that would have to be defeated. Or it is possible that the tandems are NCL.

Originally Posted by Roger Frazee

HG

Yes that is the panel.

It is not listing a bus stab rating but a maximum branch circuit breaker size of 70 amps .

The two tandem breakers in spaces 7 & 8 would appear to be incorrectly located. Is that what your seeing?

I'm unfamiliar with that panel but as I see things the bottom right 100 amp 2 pole breaker (if it was allowed) occupying single pole tandem locations/spaces 11-12 & 15-16 should be where the two tandems in 7 & 8 are located and then those tandems moved to where the double pole is currently located for a correct configuration with the wiring diagram..

Whether or not the tandems were modified to fit in those locations would depend on the bus stabs and any rejection feature on the tandems that would have to be defeated. Or it is possible that the tandems are NCL.

Thanks, Roger. I was pondering that part of the schematic as well.
What is NCL?
The good news - My clients intend to upgrade the service to 200 amps, so this panel is destined for the local metal merchant. We hope. I am recommending they downsize the breaker for the remote panel to 60 or 70 amps, but the electrician would see to that anyway. We hope.
Finally, I suspect much of this work was done without a permit and have notified my client of that likelihood. I can't believe the seller's story at this point.

R.F.,

Yes, that was exactly my concern, at least the way I read the panel diagram.

Thank you for the confirmation. I noticed in the two threads J.K. created on the same subject that wasn't mentioned on either, and was curious why that might have been overlooked. IIRC, the story was that both the electrical authority inspector and an electrician had recently "approved" the installation, after J.K.'s inspection report; found that difficult to digest (in the wee hours).

Started to wonder if I was having a "senior" moment myself, looking at that panel diagram.

I thought the red color handles were 20 amps up north on those zinsco style breakers, and the blues 15s. I can't make out details in the picture of the breakers other than those colors (old eyes, poor photo editing skills); think I see green up by 1,2....

Originally Posted by H.G. Watson, Sr.

R.F.,

Yes, that was exactly my concern, at least the way I read the panel diagram.

Thank you for the confirmation. I noticed in the two threads J.K. created on the same subject that wasn't mentioned on either, and was curious why that might have been overlooked. IIRC, the story was that both the electrical authority inspector and an electrician had recently "approved" the installation, after J.K.'s inspection report; found that difficult to digest (in the wee hours).

Started to wonder if I was having a "senior" moment myself, looking at that panel diagram.

I thought the red color handles were 20 amps up north on those zinsco style breakers, and the blues 15s. I can't make out details in the picture of the breakers other than those colors (old eyes, poor photo editing skills); think I see green up by 1,2....

I actually didn't think past what John was asking in the other threads and didn't pay attention to the rest of what was going on in that panel. I'm glad you brought that up and it appears John also was wondering .. so good catch on where those tandems were located.

The panel numbering and layout is a bit eye crossing. It took me a bit to actually see what was going on . Once you get it figured out it is not all that complicated just have never seen a panel like that so it took me a second or two.

I never can remember the color scheme for those breakers, especially GE ones. I can't tell if they are actually GE tandems or not. . They use 1/2 size THQP in modern panels in the US.

Strained my eyes also looking at that panel labeling and layout....

Originally Posted by H.G. Watson, Sr.

R.F.,

Yes, that was exactly my concern, at least the way I read the panel diagram.

Thank you for the confirmation. I noticed in the two threads J.K. created on the same subject that wasn't mentioned on either, and was curious why that might have been overlooked. IIRC, the story was that both the electrical authority inspector and an electrician had recently "approved" the installation, after J.K.'s inspection report; found that difficult to digest (in the wee hours).

No this work was supposedly done long before I arrived. As I said elsewhere, I am doubting that an authority came anywhere near this mess, and no paper evidence has been presented at this point. Thanks to all for helping with this report.

John

Some breakers ..tandems especially.. are CL and NCL.

CL = Circuit limiting and will have a rejection feature where the breaker clips onto the bus stab. If the stab is not designed for a CL tandem the rejection tab will not allow the breaker to install on the stab.

NCL = Non-Circuit-Limiting these breakers will not have the rejection feature and can be installed on stabs not intended for tandems. The caution here is that you could exceed the circuit limit on the panel listing by using NCL tandem breakers.

I put this together to show the differences

I should add that just because the breaker is NCL does not give permission to install it on a stab not shown for a tandem...that is a practice to avoid.

What some electricians do is breakout the rejection tab (defeat its purpose) so that a CL tandem will fit the stab or finger. This to replace a full size single pole breaker and gain a branch circuit.

R.F.,

Thanks again. That makes sense. I figured between you on one of the posts, and Jerry Peck on the other, between both of you, one would have mentioned it normally, which is why I was second guessing myself (in the wee hours) that I was indeed reading the diagram correctly and seeing what I thought I was seeing in the photos! (resorted to cleaning the monitor and pulling out a magnifying glass)

Appreciate your thoughtful and insightful replies.

H.G.

P.S. I'm thinking the non-allignment of the switch handles for the two subject tandem breakers incorrectly located, hint at the type breakers having been installed (and/or altered) and the type bus (as does the vintage of the panel). Nice graphic btw.

P.S. I'm thinking the non-allignment of the switch handles for the two subject tandem breakers incorrectly located, hint at the type breakers having been installed (and/or altered) and the type bus (as does the vintage of the panel). Nice graphic btw.

Yes the misaligned handles can be a good indication that something is amiss. It is one of the first things I key in on when I suspect breakers that are not listed for a panel.

Most of the graphics I use are mine that I generate with drawing software. Sometimes I may copy graphics from public sites and modify with text but will always give credit to the origin of the graphic.

Roger,
What drawing software do you use?

Darrel Hood
DILIGENT PROPERTY SERVICES
(936)827-7664

Originally Posted by Darrel Hood

Roger,
What drawing software do you use?

Darrel Hood
DILIGENT PROPERTY SERVICES
(936)827-7664

Hi Darrel

I run an UBUNTU linux based system where some very powerful drawing softwares are free downloads. I'm pretty sure the one posted here was a drawing program called OpenOfficeDraw or it might have been Inkscape. Another good one is GIMP. best thing to do is google UBUNTU drawing software and you will get a long list many of which are downloadable to several systems and many are free....not sure if free outside Linux but I think there are several free ones for windows..