7

I'm having trouble identifying a split bus panel. I design and install Solar systems, and lately (after Mr Wiley gave a few seminars to local inspectors) the local authority having jurisdiction (AHJ) are refusing to allow a second feed to a service panel if the utility feed is in the middle of the bus (center). Citing if the panel is a split bus, the 120% rule doesn't apply.

Can anyone confirm what a split bus is and how to identify. is it simply when the feed is in the middle of the bus in the panel?

14

Imagine a 2-phase sub-panel: 2 bus bars down the middle, 2 lugs at the top to connecting the incoming power from the main panel. (No main breaker here because this is a sub-panel; the disconnect breaker for this type of panel lives in the upstream panel.)

Now imagine a sub-panel that contains 2 completely independent sets of bus bars. This is a split bus panel.

Here's a picture:

normal panel vs. split bus panel

Typically, in a split bus panel, the wires delivering the power to the panel are connected directly to the lugs for the upper bus. Then a double-pole breaker is installed on the upper bus and wired to the lugs on the lower bus.

split bus panel wiring diagram

This is allowed because of the "6 breaker rule", which says that a panel must de-energize all of its circuits by switching off at most 6 breakers. Provided that the upper bus has at most 6 breakers installed (say all double-throw breakers for big appliances) and one of those 6 is the double-pole powering the lower bus, then it takes at most 6 breakers to de-energize all of the attached circuits for the entire panel.

So you can identify a split bus panel by looking in the middle for a (ahem) split bus. It should be obvious because there are lugs buried between the breakers, and those lugs are likely wired to a breaker above.

(In looking for pictures or diagrams to help me explain this, I came across a website from Charles Buell, a home inspector, that explains a lot about split sub panels, including some mistakes he sees, plus some things that people claim are mistakes but are not. It's a good read and it might help you.)

  • 2
    actually no, it doesn't require a disconnect breaker upstream. The whole point of the Rule of Six/split bus panels was that fat 100-150-200A main breakers were prohibitively expensive, and they wanted to do the job with six much cheaper 20-70A breakers. (even today, 20-70A are like $9 and 100+A are $60+++++.) These panels are hot-wired straight to the meter. The six are the disconnect. – Harper Feb 15 '18 at 19:10
  • 1
    Note that in situations where the "rule of six" is not applicable, split-bus configurations are still Code-legal, and still made by Siemens, primarily for "all-in-one" main/standby applications which integrate a main panel, transfer switch, and standby subpanel into a single enclosure – ThreePhaseEel Mar 26 at 23:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.