Roof De-Icing Cable Numbers

Having recently kibitzed on a project using de-icing cables (with some success) to soften PVC pipe for bending, herewith the useful numbers.

Data printed on the original cable:

  • 100 ft length
  • 120 VAC
  • 800 W

Derived values:

  • 6.7 A = 800 W / 120 V
  • 8 W/ft = 800 W / 100 ft
  • 1.2 V/ft = 120 V / 100 ft
  • 18 Ω = (120 V)2 / 800 W
  • 180 mΩ/ft = 18 Ω / 100 ft

The starting point was a 62 ft length of the cable, as I’d long ago converted the end into a heated bed for starting plants early in the spring. That presented a resistance of 11 Ω, drew a current of 11 A, and dissipated 1.3 kW at 21 W/ft. A kilowatt-class dimmer handled the load, but adjoining sections of the cable got hot enough to melt the insulation and terminate the experiment.

A shorter length of cable might be suitable for a cheap laptop brick power supply. To keep the dissipation under, say, 10 W/ft, we have:

  • 7.5 A = sqrt( 10 W/ft / 180 mΩ/ft )
  • 1.3 V/ft = 7.5 A * 180 mΩ/ft

The Dell D220P-01 brick on the M2 provides 12 V at 18 A (!) and costs under $20 on eBay:

  • 9 ft = 12 V / 1.3 V/ft
  • 90 W = 12 V * 7.5 A
  • 1.6 Ω = 9 ft * 180 mΩ/ft

You could run two 9 ft lengths cables in parallel from the same hulking brick. Whether that’s enough to soften a length of PVC pipe from the inside, without having the insulation get all melty, that’s another question…

10 thoughts on “Roof De-Icing Cable Numbers

  1. I have two questions on how the wiring is set up. I had a couple of lengths on the roof before I re-shingled, but had nicked both cables in clearing the ice jams. I didn’t trust my silicone caulk patch, so the old lengths were shelved. I haven’t cut the cable (further) to examine it, but assumed a resistive bulk material between bare feed wires. Any refinements beyond this? Any idea of what the bulk material is?

    1. Arggh, posted before actually paying attention to your data. [groan] The resistive material must be in series, not in parallel. So, I’d guess it’s a copper wire and a resistive “wire” on the other leg. How is the connection to the resistive side handled?

      1. a copper wire and a resistive “wire”

        As nearly as I can tell, the two wires are identical and equally resistive, so it’s by-and-large just high-resistance zip cord. The conductors are (each) a single wire that’s much stiffer / springier than ordinary copper, so it’s probably an alloy somewhere along the nichrome axis.

        1. Thanks! Have to figure a use for the lengths I have. I tell Julie that it’s not hoarding if there’s actually a use for it*, and when the suppliers are a long ways away, the heap of gubbage can be essential.

          * A fine line at times, though I try to keep the warehouse section of the shop from taking over…

    1. Having a semi-solid rod inside keeps the tube from collapsing when it’s all bendy!

      1. I normally fill PVC with sand before bending it for that reason.

        1. A well-used Greenlee (!) PVC heating pad has entered the fray and I spy a sandbox heaving into view over the horizon…

  2. I seem to remember reading that swimming pool installers use truck exhaust gases to heat PVC for bending.

    1. What’s a little diesel soot among friends? Besides, carbon monoxide gives you nice bright red blood! [koff choke wheeze]

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