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#197037 - 11/05/10 10:19 PM Split buss panelboards  
wewire2  Offline
Joined: Nov 2002
Posts: 244
Just a little warning about split buss panelboards. Some of you may not have run into these. These are panels where the top half and the bottom half of the busses are separate and fed independently. A lot of times the voltage will be checked at the feeder lugs at the top and if it is a split bus the bottom half could unknowingly be live and present a shock hazard. I've run across two of these in the last few months. The second one had the line lugs at the bottom, right behind the neutral buss. It also explained why they had such a substantial shield over the neutral buss. Also, there was no labeling to indicate it was a split bus panel.

Arc Flash PPE Clothing, LOTO & Insulated Tools

#197039 - 11/05/10 10:57 PM Re: Split buss panelboards [Re: wewire2]  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
That's the style of 50 years ago. High amperage circuit breakers really went at a premium so the engineers came up with the 6 circuit rule and the split bus.

If top fed, the largest loads would be ganged there -- a five double-pole maximum -- with the sixth position leading off to the daughter bus directly below.

The daughter bus could have far more breakers -- overwhelmingly 15A single poles.


One must be on the look-out for bottom fed split bus panels. Obviously, the scheme is turned upside-down.


Beyond that, the panel designs from the 1950s are truly scary.

Very tight quarters for our field conductors.

Lots of live exposed bussing -- typically two straight copper rails. This layout makes 240V circuits or running 14-3 very odd. To get to the other phase one must cross over to the other side of the breaker array! It also means that you don't have a chance in hell of working a two-pole handle tie!

When the budget permits these panels need to be retired.

In the mean time work on them cold and meg them before power-up.


#197047 - 11/06/10 11:50 AM Re: Split buss panelboards [Re: Tesla]  
renosteinke  Offline
Cat Servant
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
Not to let the air out of a 'blame the lazy, greedy manufacturers' theory, but the split-buss panel is the only legit way to get single pole breakers in a three-phase panel with a delta feed.

Absent a split-buss, you need to feed a separate panel to have single-pole circuits.

Code requirement? Not directly. If you look to the fine print in the lables on the panels, you will see this restriction where they discuss the feeds for which the panel is approved. It's therefore a 'listing and labeling' requirement.

#197048 - 11/06/10 12:01 PM Re: Split buss panelboards [Re: renosteinke]  
ghost307  Offline
Joined: May 2005
Posts: 921
Chicago Illinois USA
I use these quite a bit when I have a number of outdoor site lighting circuits and don't want the hassle of the usual multipole lighting contactor. I run my normal circuits from the upper bus, my lighting circuits from the lower bus, and feed the lower bus via a contactor inside the panelboard.
It sure neatens up the installation IMHO.


#197146 - 11/11/10 10:43 PM Re: Split buss panelboards [Re: ghost307]  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA

I've seen central-tapped neutrals on delta 240 all over my area.

The inspectors have permitted single pole breakers all over the 240 3-phase panel -- as long as they skip the stinger phase -- it being well marked.

Just what are you proposing ?

Mother, daughter 3-phase/ 1-phase boards ?


#197152 - 11/12/10 09:16 AM Re: Split buss panelboards [Re: Tesla]  
renosteinke  Offline
Cat Servant
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
Tesla, that's exactly what I believe the 'fine print' would require: that a two-pole breaker feed a separate panel.

Don't infer that I am 'proposing' anything, though. I'm not sure I agree with the restriction on the listing / labeling. After all, ours is a 'skilled' trade, and making things ever more 'idiot proof' only inflates the egos of the untrained ... who only think they know what they're doing.

#197157 - 11/12/10 01:29 PM Re: Split buss panelboards [Re: renosteinke]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,142
I have seen single phase loads in red leg panels. Like Tesla says, you just have to be careful to stay off the red leg and to only load the real transformers when you have delta vee.
A lot of 240v load across the missing transformer will make you some strange phase balancing. That can get tricky when a 3 phase machine has single phase loads.

Greg Fretwell

#197438 - 11/30/10 07:31 AM Re: Split buss panelboards [Re: gfretwell]  
sparky  Offline
Joined: Oct 2000
Posts: 5,316
On theother hand, i gotta job from a home inspector who claimed the 'panel had no main breaker'

and hey, who am i to dis him?


#197441 - 11/30/10 09:12 AM Re: Split buss panelboards [Re: sparky]  
renosteinke  Offline
Cat Servant
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
HI's ... you gotta love 'em. Now I understand why they put 'keep out' stickers on panels laugh

"Just be careful and don't use the stinger" certainly works, and I've seen this more times than I can count. Yet, speaking academically, doing so is a violation of the listing and labeling - at least, if you read the fine print on the sticker inside the door.

#197450 - 11/30/10 07:45 PM Re: Split buss panelboards [Re: renosteinke]  
tdhorne  Offline
Joined: Mar 2001
Posts: 345
Maryland, USA
Not all of those that I have seen have single phase loads prohibited by the listing and labeling, although perhaps they should have been so restricted. I have never seen one were there was a three phase buss arrangement and a single phase buss arrangement in the same cabinet. Do such panels actually exist. I would love to see a picture of one.

Last edited by tdhorne; 11/30/10 07:46 PM.

Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison

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