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…
The Smell of Molten Projects in the Morning 
Comments
10 responses to “Roof De-Icing Cable Numbers”
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?
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?
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.
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…
…why would you want to bend PVC from the *inside*?
Having a semi-solid rod inside keeps the tube from collapsing when it’s all bendy!
I normally fill PVC with sand before bending it for that reason.
A well-used Greenlee (!) PVC heating pad has entered the fray and I spy a sandbox heaving into view over the horizon…
I seem to remember reading that swimming pool installers use truck exhaust gases to heat PVC for bending.
What’s a little diesel soot among friends? Besides, carbon monoxide gives you nice bright red blood! [koff choke wheeze]