Okay next question: Is it acceptable to install Type MC 12/2 (metal clad) cables below ceiling joists where they are stapled to the bottom of the joist?

Why in the world would you attempt to run MC BELOW ceiling joists?

http://www.wisegeek.org/what-is-a-ceiling-joist.htm

Do you mean rafters?

I assume you mean unfinished spaces?
Interesting catch. There does not seem to be an equivalent to 334.15(C) for MC cable. You can put in MC "monkey bars" if you want ... I guess.
I might start looking at an extreme example as "subject to physical damage"

IMHO, if you ran the MC under a roof rafter in an attic in the corner down low, I don't see any problem with it.
The same would go for running the cables on top of the ceiling joists again if it was tucked in the corner where you can't walk on it.

The B O T T O M of ceiling joists is where sheetrock is mounted -- usually.

Here and there, you'll see open joists (at the bottom) -- for 'the look', in which case, the ceiling is mounted 'high and low' between the joist beams and their bellies.

'Unfinished joists' suggests F L O O R joists -- and the unfinished space would face a bottom cavity.

Unfinished ceiling joists -- that's a new one for me.

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As for Harold's notions: that's the way I've always run Romex and MC in wood structures.

Another location, which is what the OP might have had in mind, would be PURLINS.

http://en.wikipedia.org/wiki/Purlin

The underbelly of purlins are used for Romex and MC runs all the time -- consistent with the NEC. It is extremely common for such purlins to run bare.

I was thinking about unfinished places like a single story garage or shed. Perhaps "joist" is not exactly the correct term but some of us are not engineers

These days I would expect to see the bottom chord of trusses but the issue is the same.
If you did put in furring for drywall you need to maintain the 1.25" separation from the edge of the member.

I took the question to mean a basement where the floor joist of the upper level would be the ceiling of the basement. Question is kinda murky. In either case based on the responses, the issue seems to boil down to physical protection or subject to damage and I guess that's going to be how I respond. Not much in the code about it. Mainly addresses NM cable running below joist.

Arlington has a nice new product out for running NM wire along floor joists and it still protects from damage.

http://www.aifittings.com/catalog/n...ts/cableway-cable-runway-support-system/

You might have to copy that web page to get there. It is like running boards for NM under the floor.