Cable Heating

> From: Tom Price <tprice at u_washington.edu>
> Perhaps what David is thinking of is the case of power transmission
> lines, where a required amount of electrical power is transmitted more
> efficiently at high voltage than at low voltage.  (I just looked up an
> example in my freshman Physics book; a sure sign that this thread has
> gotten out of hand).

Oh, that does make sense, even though it would support the theory that
HIGHER voltages are better for carrying power. :)  That was sort of what I
was getting at yesterday: we actually have to go out of our way to get
equipment (wires, transformers, etc) to run low-voltage heaters that isn't
necessary for high-voltage units.  (See below)

> I think the reason that lower voltage is more attractive for undergravel
> heating cables is that it allows for more reasonable cable dimensions to
> be used.  To generate 100W of power, cables subjected to 120 volts
> require a resistance 25 times greater than cables subjected to 24
> volts.  When we are talking about using continous cable to develop the
> resistance (like in UG heating cables), the cables have to be pretty
> thin or really long to get enough resistance.  The upshot is that
> reasonable size wire and shorter lengths can be used at 24V DC than at
> 110V AC.  

See, I'm not sure about that.  Consider Nichrome wire, for instance.  It's
a great heating wire, because the metal (alloy?) has an inherently higher
resistance than copper or silver, so you can make reasonable lengths of
wire into a heater at 120 volts.  A 24-volt Nichrome wire heater is very
thick (I have some, and may someday build a little heater out of it).  I
think you can probably (given a Dupla-like pool of resources) fabricate a
wire of any length and wire cross-section to have the appropriate
resistance for any voltage.  It's just us hobbyists who have to use things
like (yikes) wire-wrap wire or nichrome raided from the UW Chem stores.

By the way, I pretty much agree with Steven Pushak's post; if you're not
exactly sure what you're doing, just buy a registered device, and for
chrissakes, don't try to build it. I don't think Dupla cables are UL
listed, though. :) 

   - Erik

PS: Thanks for the many responses to my "What's this plant?" post last
week.  The mystery plant is indeed Hygrophila difformis, as it is now
beginning to produce the traditional submerged leaves.

Erik D. Olson					         amazingly, at home
eriko at wrq_com