Bending radius for shielded cable

[jhdeakle]

Hello,
I was wondering if someone could explain why there is a difference in the bending radius for shielded between types TC and MC cable.

For MC cable, NEC 2008 section 330.24(C) says (7) times the overall cable size and for TC cable, NEC 2008 section 336.24 says (12) times the overall cable size. Even with the typically larger diameter MC cable the radius difference is significantly smaller than TC.

The bending radius for non-shielded TC cable (4-6 times OD) is smaller then the non-shielded MC cable (7 times for corrugated). Only in this one case does the bending radius of TC exceed the radius of MC.

The only thing I can think of is there is a difference in the construction of the cables but there is no explanation given.

Thanks

 

[physis]

It's actually pretty simple. The bend radius is somehow conjuered by the CMP's and ideally it's based on what the cable can safely endure. I don't have any idea what information or tests, if any, are used to make these deteminations. Though I am sure there are some.

I do know that I can safely bend a cable far in excess of the code allowance. Not that I ever have, or will, (lie), or have installed equipment with UL stamps and listed for the conductor or cable sizes where it's a physical impossiblity to meet these allowances under code. If you have five inches of panel space and code requires six inches for something it's listed to handle, I'ts either forget the code or forget the listings. Can the lable supercede the code? If so, well, the listings have the codes blessings. Is the code then agreeing that the bend allowance is unessicarily stringent?

This leaves me with only two possible rationals. Preventing just plain incompitance from damaging the cable, like a code can defeat incompitance, those people wouldn't even know of this, code. The other possiblity is the tolerances of conductors and cables are so sloppy that they can't be expected to perform within spec. Not that I mind builing things 50 or a 100% over spec.

Anyway, to answer your quetion, I can't explain where any bend radius numbers come from, only that we're expected to comply.