Derating NM

[Eddie702]

334.12(B) allows you to sleeve NM for protection against physical damage. So I drop down into a finished garage ceiling (split level house). Instead of putting running boards on the garage ceiling I want to pull (4) 12/2s through a pvc sleeve to get over near the panel. The pvc will not attach to the panel and will have TAs and plastic bushings on the ends.

I have to derate to 70% (8cc conductors). Because NM has 90C conductors do I use the 90C table or (I hope not) start the derate at 60C because that is all NM is good for?


Also, 334.12 B allows a sleeve why is 312.5 C so restrictive when going into a panel (which I am not in this case)? You can only go in the top, can't penetrate a structural ceiling, raceway sealed or plugged at the outer end, minimum raceway 18" max 10' and this is all to avoid having to secure the cable to the Panel. Doesn't make sense to me.

 

[wwhitney]

I have to derate to 70% (8cc conductors). Because NM has 90C conductors do I use the 90C table

Yes.

Also, 334.12 B allows a sleeve why is 312.5 C so restrictive when going into a panel (which I am not in this case)?

Dunno, but the restriction only apply when you want relief from the 312.5(C) requirement to secure the cable to the enclosure. Figure out how to do that, and you are not subject to the restriction in 312.5(C) Exception.

Also, if you bring your NM into an enclosure and secure the cable to that enclosure, then route the NM through a complete conduit system from enclosure to enclosure, the NM doesn't need securing to the second enclosure.

Cheers, Wayne

 

[Dennis Alwon]

334.12(B) allows you to sleeve NM for protection against physical damage. So I drop down into a finished garage ceiling (split level house). Instead of putting running boards on the garage ceiling I want to pull (4) 12/2s through a pvc sleeve to get over near the panel. The pvc will not attach to the panel and will have TAs and plastic bushings on the ends.

I have to derate to 70% (8cc conductors). Because NM has 90C conductors do I use the 90C table or (I hope not) start the derate at 60C because that is all NM is good for?


Also, 334.12 B allows a sleeve why is 312.5 C so restrictive when going into a panel (which I am not in this case)? You can only go in the top, can't penetrate a structural ceiling, raceway sealed or plugged at the outer end, minimum raceway 18" max 10' and this is all to avoid having to secure the cable to the Panel. Doesn't make sense to me.

Is your sleeve more than 24"?

Actually it doesn't matter anyway because NM is rated at 30 amps for 90C so you derate from that amperage. 30 x .7 = 24 amps so you can still use a 20 amp breaker

 

[infinity]

How long is the sleeve? Often the exception to 310.15(A)(2) allows you to eliminate the derating altogether.

 

[Dennis Alwon]

Also, 334.12 B allows a sleeve why is 312.5 C so restrictive when going into a panel (which I am not in this case)? You can only go in the top, can't penetrate a structural ceiling, raceway sealed or plugged at the outer end, minimum raceway 18" max 10' and this is all to avoid having to secure the cable to the Panel. Doesn't make sense to me.

I have tried to write a code change on this but I didn't get much help. I am assuming that if arcing happens in the panel it would be hard for sparks to enter a conduit and travel 18" to cause a fire.

 

[Eddie702]

Sleeve is about18' with 1 pvc sweep. Happy about the de rate that's what I assumed it would be.

So with 312.5C multiple cables coming out of a panel you would have to secure them to the panel then go into a sleeve to avoid all there restrictions in the exemption.

TY

 

[Dennis Alwon]

If you are going into the panel with nm cable connectors and not a sleeve then you need to fasten the nm as you normally would.