UG concrete encased PVC conduit Q's

[TinCanFury]

I'm looking at the thermal de-rating of cables for a 480V system that we are running in PVC conduit encased in concrete where the desire is for the top of the concrete is to be at grade and will be covered in 6" of crushed stone.

The formulas I've seen, and software like AmpCalc, both do not allow for 0" of earth above the concrete.

Table 300.5 does not address this situation. Column 3 lists nonmetallic raceways, but without concrete encasement. 300.50 addresses 600V and up, but not below. I cannot find anything that looks to reference minimum coverage for concrete encased PVC.

Is there any code requirements for conduit encased in concrete?

Does anyone know of a method for calculating thermal de-rating of cables in such a situation?

Thanks!

 

[GoldDigger]

The formulas I've seen, and software like AmpCalc, both do not allow for 0" of earth above the concrete.

Just as a kludge, will it allow you to input .1" or some other small enough value that it does not matter?
Also, I am not sure at first thought whether the heat dissipation will be better or worse with crushed stone compared to open air.

 

[TinCanFury]

Just as a kludge, will it allow you to input .1" or some other small enough value that it does not matter?
Also, I am not sure at first thought whether the heat dissipation will be better or worse with crushed stone compared to open air.

sadly no, it requires >=24". My initial thought was to put 6" as well, but that won't work, and AmpCalc also does not let you have a RHO for the surrounding earth and a separate RHO for what is above it, so I can't cheat and give the Earth thermal RHO value for "crushed stone".

another thought I had was set the thermal RHO for Earth to that of Air, and increase the concrete dimensions below and to the sides of the conduit to approximate the high thermal RHO of Earth, but that leaves me in the same uncertainty limbo as any other method as to the validity of the calculation.

So that leads me to require another formula/method to calculate the de-rating of the cables.

thanks!

 

[petersonra]

it seems like it ought to be a fairly straightforward insulation heat transfer calculation. why not just handle it that way and see what you come up with.

 

[TinCanFury]

it seems like it ought to be a fairly straightforward insulation heat transfer calculation. why not just handle it that way and see what you come up with.

Your understanding of Neher-McGrath must be well beyond that of the average electric power engineer. I don't see how performing the calculation for 10 conduits in an offset 3x4 grid could be considered "straightforward". Possible, yes, but I can't imagine it being all that straight forward.

If you have some resources on how to properly set up and perform this calculation by hand that would lead you to say something like that, I'd be very interested in reading it.

 

[jusme123]

I'm looking at the thermal de-rating of cables for a 480V system that we are running in PVC conduit encased in concrete where the desire is for the top of the concrete is to be at grade and will be covered in 6" of crushed stone.

The formulas I've seen, and software like AmpCalc, both do not allow for 0" of earth above the concrete.

Table 300.5 does not address this situation. Column 3 lists nonmetallic raceways, but without concrete encasement. 300.50 addresses 600V and up, but not below. I cannot find anything that looks to reference minimum coverage for concrete encased PVC.

Is there any code requirements for conduit encased in concrete?

Does anyone know of a method for calculating thermal de-rating of cables in such a situation?

Thanks!

.......

 

[petersonra]

Your understanding of Neher-McGrath must be well beyond that of the average electric power engineer. I don't see how performing the calculation for 10 conduits in an offset 3x4 grid could be considered "straightforward". Possible, yes, but I can't imagine it being all that straight forward.

If you have some resources on how to properly set up and perform this calculation by hand that would lead you to say something like that, I'd be very interested in reading it.

never tried it. just not my deal. it seems like one ought to be able to model the heat produced inside each conduit and model the insulation around it and see what happens.

Maybe this is a lot harder than it looks at first glance.

I have seen some strange looking insulation/heat transfer calculations for multiple layers of different kinds of insulation in layers with heat being generated in the different layers that seems similar. to me it looked like the same set of equations over and over again.