PVC Coated MC in Concrete Trench

[647Lapua]

I am working on a multi-story post-tension deck resi-building. There have been island hops (smurf from wall to island in-slab) that have been filled with concrete, and I must find a way to get power to the island. The floors do not have drop ceiling on the floor below, so I can't core drill in two places and call it good. The floor material is also 5/16" vinyl.

So, with the PT deck, if a trench is to be used it must be 1" deep or less. My plan is to use PVC coated MC from wall to island in this shallow trench.

I do not think 300.5 is really applicable in this case because my issue is not dealing with dirt. 2" of concrete and a 2 3/4" deep trench is needed if we go this route, but it's just too deep in the deck. This is the reference my inspector is looking at.

My issue is that I need to find in the NEC where it is acceptable to use 1/2" of concrete fill over this MC to make my inspector happy, but I sure can't find it.

Another thought is using flat cable, but with the floor material being so thin I think I'm hosed with this option.

I am open to some thoughts here.

Ultimately, I would really like an NEC reference for the 1/2" concrete fill... but maybe it's not there...

Thanks for help!

 

[synchro]

Can you use 1" square steel tubing and have the top of it flush with the surface of the slab? Then use it as a sleeve for the MC. Such steel tubing is available in various wall thicknesses. This should be structurally robust, and it would protect the MC from damage if the vinyl floor covering is removed and replaced in the future.

 

[letgomywago]

This has been discussed before in concrete is not burried its a damp location but not burried so no burial depth is needed. You can cut it and epoxy patch it no concerns

 

[infinity]

Ultimately, I would really like an NEC reference for the 1/2" concrete fill... but maybe it's not there...

You won't find this reference because the 1/2" cover is not required. Coated MC is buried in concrete and in chases all of the time. The trench or chase only needs to be deep enough to lay in the MC and to cover it with patching material. In a dry location you can use regular steel MC cable embedded in the concrete fill. {330.10(A)(10)}

 

[don_resqcapt19]

I would not do anything without the written approval of the structural design engineer. The post-tension deck is structural.

 

[SceneryDriver]

Can you use 1" square steel tubing and have the top of it flush with the surface of the slab? Then use it as a sleeve for the MC. Such steel tubing is available in various wall thicknesses. This should be structurally robust, and it would protect the MC from damage if the vinyl floor covering is removed and replaced in the future.

This is a very good idea. It allows someone to replace a damaged cable, or re-pull one with more conductors in the future without tearing up the floor.

Alternatively, can you not "trench" in 1/2" EMT? That's well under 1" in diameter.

SceneryDriver

 

[647Lapua]

Can you use 1" square steel tubing and have the top of it flush with the surface of the slab? Then use it as a sleeve for the MC. Such steel tubing is available in various wall thicknesses. This should be structurally robust, and it would protect the MC from damage if the vinyl floor covering is removed and replaced in the future.

That's genius

 

[647Lapua]

This is a very good idea. It allows someone to replace a damaged cable, or re-pull one with more conductors in the future without tearing up the floor.

Alternatively, can you not "trench" in 1/2" EMT? That's well under 1" in diameter.

SceneryDriver

I would have to continue the EMT up, and I really don't want to do that... I want to stick with MC on both ends.

 

[647Lapua]

You won't find this reference because the 1/2" cover is not required. Coated MC is buried in concrete and in chases all of the time. The trench or chase only needs to be deep enough to lay in the MC and to cover it with patching material. In a dry location you can use regular steel MC cable embedded in the concrete fill. {330.10(A)(10)}

Understood. Thank you! Hopefully my inspector will see it this way. If not, I've got some great alternatives!

 

[647Lapua]

I would not do anything without the written approval of the structural design engineer. The post-tension deck is structural.

True that. The engineer already approved the trench and MC method, but it was my inspector who was concerned with it.

 

[infinity]

I would have to continue the EMT up, and I really don't want to do that... I want to stick with MC on both ends.

Understood. Thank you! Hopefully my inspector will see it this way. If not, I've got some great alternatives!

Nothing wrong with embedding MC cable in a shallow chase.

 

[SceneryDriver]

I would have to continue the EMT up, and I really don't want to do that... I want to stick with MC on both ends.

Use the 1/2" EMT as a sleeve, and pull the MC through. Or use the 1" sq. steel box tube idea. You can set that in your trench with epoxy rather han mortar.


SceneryDriver

 

[wwhitney]

Ultimately, I would really like an NEC reference for the 1/2" concrete fill... but maybe it's not there...

300.4(F) would cover this installation. 1/2" concrete would not comply, you'd need a 1/16" steel plate. Or synchro's idea as long as the wall thickness of the tube is 1/16". Or the EMT sleeve.

Cheers, Wayne

 

[don_resqcapt19]

300.4(F) would cover this installation. 1/2" concrete would not comply, you'd need a 1/16" steel plate. Or synchro's idea as long as the wall thickness of the tube is 1/16". Or the EMT sleeve.

Cheers, Wayne

I don't see 1/2" of concrete as being even remotely similar to "wallboard, siding, paneling, carpeting", and see a 1/2" concrete as providing more protection than a 1/16" nail plate.
For sure once again, we are not going to agree

 

[infinity]

300.4(F) would cover this installation. 1/2" concrete would not comply, you'd need a 1/16" steel plate. Or synchro's idea as long as the wall thickness of the tube is 1/16". Or the EMT sleeve.

Cheers, Wayne

Patched with concrete is not on the list so it is not applicable.

 

[LarryFine]

So, what's the minimum depth for conduit embedded in a concrete slab?

 

[infinity]

So, what's the minimum depth for conduit embedded in a concrete slab?

0". It also can be run one the slab itself.

 

[wwhitney]

I don't see 1/2" of concrete as being even remotely similar to "wallboard, siding, paneling, carpeting", and see a 1/2" concrete as providing more protection than a 1/16" nail plate.

Most importantly for the OP, 300.4(F)'s discussion of installations like that proposed provides a basis to demonstrate to the inspector it is compliant (if adequately protected). More generally:

300.4 is about protection against physical damage. 300.4(A), for example, is about bored holes in wood members, and it requires steel plate protection if the hole is less than 1-1/4" clear from the face of the structure.

(F) Cables and Raceways Installed in Shallow Grooves. Cable or raceway-type wiring methods installed in a groove, to be covered by wallboard, siding, paneling, carpeting, or similar finish, shall be protected by 1.6 mm (1∕16 in.) thick steel plate, sleeve, or equivalent or by not less than 32-mm (11∕4-in.) free space for the full length of the groove in which the cable or raceway is installed.

Exception No. 1: Steel plates, sleeves, or the equivalent shall not be required to protect rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, or electrical metallic tubing.

Exception No. 2: A listed and marked steel plate less than 1.6 mm (1∕16 in.) thick that provides equal or better protection against nail or screw penetration shall be permitted.

Exception No. 2 demonstrates that the concern in 300.4(F) is about screw or nail penetration. The mention of carpet in 300.4(F) demonstrates it applies to floors. Certainly 5/16" vinyl flooring is comparable to carpet.

1/2" of cementitious patching material (too thin for concrete with large aggregate) is not going to stop a screw or nail when one is expecting concrete and using an appropriate fastener. 1/16" steel plate has a chance to do that, which is why it is called out.

So while there are some judgement calls here, if the AHJ says 300.4(F) requires either 1-1/4" of separation or a 1/16" steel plate, that's 100% defensible. And seems in line with the rest of 300.4.

Cheers, Wayne

 

[wwhitney]

0". It also can be run one the slab itself.

Given that 300.4(F) is worded about grooves it does not seem to apply to conduits cast in place. Still, seems like if you are using something other than "rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, or electrical metallic tubing" it would be wise to comply with 300.4(F) one way or the other.

Cheers, Wayne

 

[infinity]

Given that 300.4(F) is worded about grooves it does not seem to apply to conduits cast in place. Still, seems like if you are using something other than "rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, or electrical metallic tubing" it would be wise to comply with 300.4(F) one way or the other.

Wise yes, required no. I think that we agree that the wording is poor, and it seems to imply that what's on the list is not making any reference to a groove in a floor. Everything on that list including carpet could pertain to walls. If floors were to be included it should say floors.