Inground Conduit Box for Driveway Snow Melting

Question

I'm installing snow melting cables into a concrete and stone paver driveway. It requires 5 mats rated at 25 Amps and 6000 watts. I also need a 6th small run of heating wire to cover the full area. The cold leads (non-resistive conductor cable) are 10/3 and 20 feet long.

So to get it to my basement I need an in-ground conduit box to run the 6 cables into then splice them with 10/3 NMWU in the box and run those through one or 2 conduits to the house panel.

So my first questions are about the conduit sizes and pulling the wires. For the cold leads to the conduit box ...

Will 1/2" PVC conduit be to hard to pull through (3 to 15 feet, 2x 90 elbows)?

Is 3/4" overkill?

Can I get 6x - 10/3 wires easily through one 2" conduit? (3x 90 elbows @ 15-20 feet).

Fill table says 63 max for 10/3 conductors and I will have 6x3 =18 plus ground and sheathing.

Ontario, Canada

Answer 1

If in only 1 pipe is there a derate for the number of current carrying conductors? In the U.S. I would be using thhn/thwn wire, but with 25 amp actual draw I would need to upsize the wire to #8 because 125% of 25 is 31.25a . 1-1/2 rigid pvc would allow 18 #8 , i would not try to pull multi conductor cables through conduit it costs more and is not needed since you are running conduit. Thhn/thwn wire is rated for wet locations and would cost less than NMWU. And as Isherwood mentioned since the splices are going to be below grade they need to be listed for wet / direct burial. On a side note are you going to put insulation under the mats? It will really help with power consumption that is going to be an expensive way to melt the ice and snow.

Answer 2

Six 30A circuits, eh?

And it requires neutral? Are you sure? There's no logical reason to need neutral, and if their cold cord is 10/3, remember this is cordage and in cordage, the /3 number includes ground. Regardless, it doesn't change what I'm about to say about conduit derate. In a split-phase circuit, neutral is not counted for wire count derate.

Here's how the math works with conduit derate (at least on the south side of the 49th parallel). You look at the wire capacity of the wire at its maximum temperature - 90 degrees C for THWN -2, THHN or NM -B. Multiply by 80% for 3-6 wires, 70% for 7-9 wires or 50% for 10-20 wires. And in split-phase supply, all circuits have 2 wires that count. So with 3 circuits, #10 wire is good for 32A, with 4 circuits 28A (which you can still get away with) and 5-10 circuits only 20A (no good at all).

So I recommend 2 conduits with 3 circuits per conduit.

Each conduit only needs 1 ground wire. You don't need to design for each circuit to have a ground fault at the same time!

That's either 7 or 10 wires depending on if you need neutral. Both are allowed in 3/4" conduit, but 10 wires in 3/4" is the statutory limit, and that means a very challenging pull.

However if you are very committed to 6 circuits in 1 conduit, you must derate 50%. You need 30A service, so you need wire capable of 60A at its highest operating temperature. For THWN-2, that's #6 copper or #4 aluminum, which allow 75A which you derate to 37.5A. Assuming 13 wires (12 conductors + 1 ground), you'll want 2" conduit for #6Cu or 2-1/2" conduit for #4Al, to keep the pulling effort "merely brutal" instead of totally impossible. If neutrals are required go up another 1/2".

Upside, with #4, remarking is now allowed, so you can buy all black and tape it white or green for neutral and ground. Downside, you need to provide large cavities for wire splices with consideration for bending radius.