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Heating Cable Sources

>This is done totally off the top of my head, and I may have missed
something, so please correct me if I'm mistaken. The point I am trying to
make is that plain wire is unlikely to work in this application without
something to increase the resistivity of the conductor. Therefore, I'd keep
looking for the special wire that Bill Wichers mentioned. <

Please take a moment to review the archives before jumping in to the
discussion.  This topic has been covered in INCREDIBLE detail several times.
Your numbers appear to be OK but you are working backwards.  First determine
the wattage you require.  From your dimensions, I assume you have a 70
gallon (265 liter) tank.  A 50 Watt cable will provide about 0.2 W/l which
is about middle range for a continuous system.  50 Watts at 12 volts
requires about 2.9 ohms resistance.  28 AWG wire wrap wire has about 0.217
ohms/meter resistance meaning you need about 13.4 meters (44 feet) of wire.
This will require about 4.1 amps current.  A transformer that delivers 5
amps would be acceptable but 6 or more would be better.  The wire will get
extremely hot if not kept in the water.  Be sure to double - TRIPLE check
your calculations AND measure the actual resistance of your system with a
good meter.
A good source for transformers are at home improvement stores like Home
Depot.  They sell low voltage outdoor lighting transformers with built-in
timers that are cheaper than you can build them yourself.

The WHOLE story that got me started on the DIY kick was in the March 1995
Aquarium Fish magazine.  The text of that article can be found at

Lyndle Schenck