TC Cable

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augie47

augie47

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State Electrical Inspector (Retired)
This should be a simple answer, but somehow, in reading 336, I can't see the forest for the trees.
I inspected a campsite where the contractor had used TC cable, rated for direct burial, underground (sleeved with PVC at each end).
Violation??
 
Tray cable is permitted to be used in raceways as per 336.10 and it is not permitted to be direct buried unless identified for the use. Looks like it would work in this installation.
 
I've never seen a buried cable tray. :shock:

Seriously (hey, it could happen!), why would TC need to be rated for direct burial?
 
The technical violation is 336.12(2) since it doesn't fully meet the requirements of 336.10(7). If it is also TC-ER (very likely nowdays) it SHOULD be acceptable and I would excercise my 90.4 "Special Permission" powers, if I felt it were necessary.
 
Some additional info from a current Proposal:
7-81 Log #2701 NEC-P07 Action: Reject
(336.10(7))
_______________________________________________________________________________________________
Submitter: Dorothy Kellogg, American Chemistry Council
Panel Meeting Action: Reject
Panel Statement: TC cables are designed for installation in cable trays. They have nonmetallic outer jackets to facilitate installation without damage to the cabled insulated conductors within. The cables are not designed for exposed installations. A specific construction of TC was developed with enhanced overall cable mechanical performance to
enable the cable to meet the crush and impact requirements of MC cable. The enhanced cable is permitted for specific applications to be installed outside of a cable tray. Expansion of the use of TC beyond those specific applications will expose the cable to damage that it is not designed to withstand. The differences in the physical properties nonmetallic and metallic outer coverings are very different. The tensile strength of steel and aluminum is typically 72,000 and 42,000 psi, respectively. The tensile strength of a nonmetallic covering is typically 2,000 psi. Simply meeting a crush and impact requirement does not encompass the abilities of a metallic outer covering on the cable.
Click to expand...

I don't have the full proposal, but the I can tell you the folks who oppose TC-ER are the raceway and MC cable manufacturers (even those that also make TC-ER). The tensile strength (BTW it is the shear strength that is important) is irrelevant. If the test standard for MC cable is too low RAISE THE STANDARD; otherwise TC-ER should be accepted.
 
For the purposes of full disclosure.

The crush and impact tests for Type MC cable permit a certain number of failures. TC-ER cables may occasionally fail but usually well within the limits; however, MC cable virtually never fails.

To me, the proper approach would to be to recognize the reduced failure rates of MC cables and make them the "minimum" standard rather than using some of the bogus arguments like "Type TC is designed for installation in trays (except when we let you direct bury it)"

It may eliminate one of my favorite cable types but it is the right way to do it.
 
The differences in the physical properties nonmetallic and metallic outer coverings are very different.
Click to expand...

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I better stop. :roll:
 
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