What is a split bus panel, and how do I recognize it in the field?

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?

-
See also this question, which has pictures of an actual split-bus panel. – BlueRaja - Danny Pflughoeft Sep 17 '13 at 21:36

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:

Typically, in a split bus panel, the wires delivering the power to the panel are connected directly to the lugs for the upper bus. (Obviously this requires a disconnect breaker to be located upstream.) Then a double-pole breaker is installed on the upper bus and wired to the lugs on the lower bus.

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.)

-
Great answer and great pics...I now understand! – user15387 Oct 2 '13 at 8:38