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Archive for July 10th, 2013

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…

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