Reducing Trench Minimum Cover with Concrete Slab - Direct Buried

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AKSolar

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All,

Contractor is having a difficult time installing the DC trenching for a large solar plant, and has proposed an alternative. I am not sure it is NEC compliant.

Is it possible to interpret Table 300.50, Column 4 as allowing a contractor to pour a 4" thick concrete slurry slab above a trench in order to lower the cover requirement? Or is the spirit of this section only applicable to buildings, equipment pads, etc.

Follow-up: if this is allowed, can it be permitted with direct-burial cables? Column 4 specifically states "Raceway" and I don't think direct-burial is technically a raceway, but am uncertain.

Finally, General Note 3 allows a reduction in depth per every 2" of concrete cover, but only for the undefined "industrial establishment." Do you think an enclosed PV solar plant can be twisted into being considered industrial?

Thanks!
 
AKSolar said:
All,

Contractor is having a difficult time installing the DC trenching for a large solar plant, and has proposed an alternative. I am not sure it is NEC compliant.

Is it possible to interpret Table 300.50, Column 4 as allowing a contractor to pour a 4" thick concrete slurry slab above a trench in order to lower the cover requirement? Or is the spirit of this section only applicable to buildings, equipment pads, etc.

Follow-up: if this is allowed, can it be permitted with direct-burial cables? Column 4 specifically states "Raceway" and I don't think direct-burial is technically a raceway, but am uncertain.

Finally, General Note 3 allows a reduction in depth per every 2" of concrete cover, but only for the undefined "industrial establishment." Do you think an enclosed PV solar plant can be twisted into being considered industrial?

Thanks!
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Frozen ground or stony cover?
 
I think it is cleared that you can reduce the cover by 6" for every 2" of concrete. So 4" thick slab can have a reduction in depth of 1 foot. That's how I see it
 
As far as industrial location, that's not defined in the NEC, perhaps we know what it is by what it isn't.
Does your location have restricted access? Is it a big solar farm? Seems industrial to me, same as a power plant.
 
AKSolar said:
All,

Contractor is having a difficult time installing the DC trenching for a large solar plant, and has proposed an alternative. I am not sure it is NEC compliant.

Is it possible to interpret Table 300.50, Column 4 as allowing a contractor to pour a 4" thick concrete slurry slab above a trench in order to lower the cover requirement? Or is the spirit of this section only applicable to buildings, equipment pads, etc.

Follow-up: if this is allowed, can it be permitted with direct-burial cables? Column 4 specifically states "Raceway" and I don't think direct-burial is technically a raceway, but am uncertain.

Finally, General Note 3 allows a reduction in depth per every 2" of concrete cover, but only for the undefined "industrial establishment." Do you think an enclosed PV solar plant can be twisted into being considered industrial?

Thanks!
Click to expand...

To answer directly, in the 2014 edition the column is titled "Raceways Under Buildings or Exterior Concrete Slabs, 100 mm (4 in.) Minimum Thickness." You need to use "raceway" but other than that you're fine.

I don't think you can use direct burial cable with this provision. See 300.50 (A)(3). Which will also let you get away with 3" of concrete.

Are you trying to get relief from the 30" requirement in column 1?
 
Thanks everyone! I appreciate the various perspectives.

Yes, this is 1500V DC and we are trying to get away from the 30" depth for direct-burial. Conduit for the lengths/quantity we are talking about would be prohibitively expensive. I am not sure on the exact nature of the trenching problem, but think it relates to the narrow available space between rows of panels.

It sounds like Column 4 is out the window, as the cables are not in a raceway.

I don't think it's too much of a stretch to call this an industrial facility. It is close to 800 acres, fully fenced in with security. I'll probably move forward with the reduction allowances under Note 3.

Thanks again!
 
AKSolar said:
Thanks everyone! I appreciate the various perspectives.

Yes, this is 1500V DC and we are trying to get away from the 30" depth for direct-burial. Conduit for the lengths/quantity we are talking about would be prohibitively expensive. I am not sure on the exact nature of the trenching problem, but think it relates to the narrow available space between rows of panels.

It sounds like Column 4 is out the window, as the cables are not in a raceway.

I don't think it's too much of a stretch to call this an industrial facility. It is close to 800 acres, fully fenced in with security. I'll probably move forward with the reduction allowances under Note 3.

Thanks again!
Click to expand...

I think all the concrete, the material and labor is far from trivial. I have a hard time seeing how it would be more economical to use the concrete than dig a bit deeper, unless there is bedrock.
 
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