NM through bored holes

[Greentagger]

In regards to NM through bored holes sealed with thermal insulation, what is exactly the reference to “and the provisions of 310.14(A)(2), Exception, shall not apply.”? Is there a difference when there is a bored hole in wood with thermal insulation, and just being installed in contact with thermal insulation, as mentioned in 3rd paragraph of 334.80? I understand the concept of derating after 9 -#12 CCC on 20A circuit , just trying to wrap my head around the 310.14(A)(2) part. Thanks in advance.

 

[wwhitney]

310.14(A)(2) Exception often lets you ignore the lower ampacity of short sections of conductors. The authors of 334.80 don't want to let you do that for NM cable through a bored hole with insulation. So the language in 334.80 specifically rules that out.

For example, multiple NM cables through a wood double top plate 3" thick, with the hole insulated. Before the exclusion of 310.14(A)(2) Exception was added to 334.80, if the cables proceed another 30" down the stud bay while maintaining spacing, you would get to ignore the reduced ampacity along that 3" length of cable through the double top plate.

Cheers, Wayne

 

[infinity]

When this was first introduced (2005 NEC) there was no prohibition of applying the 310.14(A)(2)Exception so it was basically unenforceable. The next code cycle (2008) corrected the error.

 

[jimport]

310.14(A)(2) Exception often lets you ignore the lower ampacity of short sections of conductors. The authors of 334.80 don't want to let you do that for NM cable through a bored hole with insulation. So the language in 334.80 specifically rules that out.

For example, multiple NM cables through a wood double top plate 3" thick, with the hole insulated. Before the exclusion of 310.14(A)(2) Exception was added to 334.80, if the cables proceed another 30" down the stud bay while maintaining spacing, you would get to ignore the reduced ampacity along that 3" length of cable through the double top plate.

Cheers, Wayne

Any reason they were so concerned about 3 inches of conductor?

 

[infinity]

Any reason they were so concerned about 3 inches of conductor?

If I remember correctly there was some testing done where they put more than 2 cables through the hole and filled it up. Then they applied something close to the maximum current that the cables could normally carry and by doing so the temperature within the hole exceeded the temperature rating of the conductors. IMO not a very real world test.

 

[jimport]

If I remember correctly there was some testing done where they put more than 2 cables through the hole and filled it up. Then they applied something close to the maximum current that the cables could normally carry and by doing so the temperature within the hole exceeded the temperature rating of the conductors. IMO not a very real world test.

Doesn't seem much different that encased in spray foam between studs.