A split bus panel is an electrical panel with two separate, non-connected buses. The bus is the metal bar or bars in the panel that carry the electricity, and to which the individual breakers attach. (The bus bars are represented by the red and blue lines in the diagrams below.) A split-bus panel is different from other panels because other panels have buses that run the full length of the panel, while a split-bus panel has one bus in the upper part of the panel, and a second bus in the lower part of the panel.
Let me explain a little further. Electric panels that are not split bus have only one main breaker. (NOTE: Some older panels which are not up to current codes do not have main breakers.) This one main breaker can be used to shut off the power to the entire panel, and thus to the entire home. On a split-bus panel, it can require up to six breakers to turn off the power to the home.
The diagram below shows how the breaker (shown as a black rectangle) in the upper part of the panel supplies power to the lower bus in the panel.
Photos of Split Bus Panels
A typical split bus panel with the cover still on it
A typical split bus panel with the cover still on it.
The same panel with the cover removed.
How Does a Split Bus Panel Work?
In the photo on the right, you can see six double-pole breakers in the upper part of the panel. Turning off all six of these breakers will turn off power to the entire home.
Now, take a look at the lower left breaker in the upper half of the panel (labeled #5 in the photo). If you follow the two large wires (marked with green dots) that are attached to this breaker, you will see that they loop around and under the small breakers in the lower half of the panel. This is where they attach to the lower bus. These two large wires take the power from the upper bus, through breaker #5, to the lower bus in order to provide power to all the breakers in the lower half of the panel. So, turning off breaker #5 will turn off the power to the lower bus.
The breakers in the lower bus supply power to the lights, small appliances, and receptacles in the home. The other breakers in the upper bus (labeled 1 – 6), supply power to the big loads: air conditioner, furnace, water heater, clothes dryer, oven, etc.
The Six Throw Rule
For years, the National Electric Code (NEC) required that all power in the home be able to be shut off with no more than six hand movements or by “throwing” no more than six breakers. This means that as long as a panel has no more than six main breakers that can shut off all of the power in the home, then this panel meets code. In the example panel above, there are many more than six breakers, but by moving or throwing only six of them to the “OFF” position, you are turning off the power to the entire home, so this panel meets code.
History of NEC Codes Regulating Split Bus Panels –
In 1965, a statement in the National Electric Code (NEC) said the following; “individual protection for lighting and appliance branch circuit panelboards is not required where such panelboards are used as service equipment in supplying an individual residential occupancy and where any bus supplying 15 or 20 ampere circuits is protected on the supply side of the service.” This rule was an exception to a previous rule that allowed no more than two main circuit breakers. Hence, in 1965, the split-bus electric panel was born.
In 1978, the restriction on 15- and 20-amp circuits was removed. This restriction had prohibited 15- and 20-amp breakers in the upper half in the panel. With the removal of the restriction, it was theoretically possible to install small, single-pole breakers in the upper half of the panel.
In 1981, the NEC restricted the section allowing split-bus panels to “existing installations.” This meant that no new split bus panels could be installed after 1981, but existing ones could remain in place.
Are Split Bus Panels Dangerous?
No. Split-bus panels are not inherently dangerous. They do not have any of the same inherent dangers of some other panels such as Federal Pacific panels or Zinsco brand panels.
I am aware of two potential issues with split-bus panels. First, they are going to be at least 40 years old. Older breakers are less likely to function properly (trip on an overcurrent condition) than newer breakers are. This potential issue is no more of a concern with a split-bus panel than it is on any other panel of the same age.
The second issue is that it is possible to add some single-pole breakers to the upper part of the panel if all the slots are not filled with double-poled breakers. Doing this could require more than six breakers to be thrown in order to shut off the power to the entire home. In my opinion, this is a very minor issue because, worst case, it may add one or two seconds to the time required to shut off the power to the home in the event of an emergency.
I don’t believe that either of these situations make a split-bus panel more dangerous than other panels of the same age.
© 2020 Mike Morgan
This article was written by Mike Morgan, the owner of Morgan Inspection Services. Morgan Inspection Services has been providing home, septic and well inspection services throughout the central Texas area since 2002. He can be reached at 325-998-4663 or at firstname.lastname@example.org. No article, or portion thereof, may be reproduced or copied without prior written consent of Mike Morgan.