What is a split bus electric panel?
Friday, June 29, 2018
A split bus panel can be a little puzzling when you first encounter one because, unlike today’s main panels, there is no single main disconnect breaker at the center of the top of the panel. We see them in homes that were built from the early 1950s to about 1970 that still have the original electric panel in place, like in the photo above.
The split bus panel design took advantage of a NEC (National Electric Code) allowance that a main service panel could have up to six switch-throws to shut off all power. The top cluster of breakers, usually the top two or three in each column for a total of four or six breakers, are each marked at the side as a “main breaker”. At the back of the panel box, the two bus bars (vertical strips of metal that connect to each breaker to supply power to the wiring) are separated in the middle, with all of the main breakers connected to the top bus sections.
One of the top main 240-volt breakers feeds the bottom bus bars, which is typically 120-volt household lighting and receptacle circuits. Shutting off that breaker leaves all the bottom breakers dead, but the other mains at the top cluster that feed major appliances such as the water heater, range, and air conditioner will remain live and must be shut off individually.
Here’s the same panel with the mains, a total of four in this case, boxed in red:
Note the tiny stickers between them that say “MAIN BREAKER.” To understand a little better how this panel design works, lets look at the same panel with the deadfront cover plate removed:
The red rectangle shows the one main breaker that controls all the bottom breakers (enclosed in blue), and the white lines show the route of the wires that go behind the top 240-volt breakers to electrify the bottom bus bars.
The two defects we commonly see with split bus panels that are still in service and now 40 to 60 years old are:
1) Not all the main breakers are clearly marked. The little stickers have fallen off over time.
2) A panel with six main breakers, each of which is 240-volt, has had one or more 240-volt breaker changed out for two 120-volt breakers, pushing up the number of necessary switch-throws in the top cluster to shut off all power to more than the maximum six allowed. The panel shown below takes 9 switch-throws to shut off all power.
Also, all of the original circuit breakers are now past the their estimated 40 year functional lifespan. So, if you have a split bus panel in your home and are not planning on replacing it anytime soon, consider at least replacing the original breakers as a safety upgrade. Arc Fault Circuit Interrupter (AFCI) breakers are now code-required for most replacement circuits, and will provide an extra level of fire protection—which is especially important in a home with older wiring. They detect any arcing or sparking in wiring that become frayed, damaged, or deteriorated, and trip to cut off the circuit—on top of the normal circuit breaker duty of tripping when too much current is flowing through the wires.
The manufacture of split bus panels was discontinued in the mid-1980s and they are now just an interesting bit of electric wiring history. Up to six main disconnects grouped together in the same main panel are allowed, but we rarely see more than one.
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