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Re: substrate heater design

Date: Wed, 30 Dec 1998 13:32:48 -0800

> Date: Wed, 30 Dec 1998 13:32:48 -0800
> From: Mortimer Snerd
> Subject: Substrate heating cables
> I asked a similar question earlier, and only got one (very helpful)
> response.  I now have a couple of other questions, in addition to the
> original, and would really appreciate some additional opinions.  I was
> asking about heating cable wattages, and the electrical requirements of
> them.  I was looking over George Booth's website, and noticed that in
> his 100 gallon tanks he uses cables as low as 150w, and as high as
> 200w.  I'm leaning pretty heavily towards the 150w cables (Dupla's 500s,
> I think), run by a 24v, 7 amp transformer, as I think it would allow the
> cables to be on more.  Any suggestions?
> Also, in The Optimum Aquarium, Horst and Kipper say that .3-.5 w/gal is
> ideal, yet they are now selling cables that run 1-1.5 w/gal.  Anyone
> have any ideas as to why the change occurred?
> Finally, I need to get a transformer for the cables, and am a little
> confused.  I was under the impression that the cables would run hotter
> if you put more amps through them.  I am now wondering if this is the
> case.  Do the cables develop only so much heat, and the rest of the
> power put out by the transformer is unused, or will the transformer only
> put out as much power as the cables need to develop a particular
> wattage?  Or (as seems to happen with frightening frequency :-) am I
> completely wrong?
> All help is greatly appreciated,
> Justin Collins

The amount of power dissipated in the heating wires is governed by the current and
voltage at the ends of the wire. The transformer will put out 24 Volts, but the
current will be equal to the voltage divided by the resistance of the heating cable.
Since the power is the Voltage times the current ( in Volts and Amps, repectively), it
is also equal to the Voltage squared (multiplied by itself) divided by the wire
The heating wire will only take as much power as the applied Voltage and its
resistance determine. The remaining capacity of the transformer goes unused.
It is important to note that the transformer will also be dissipating power in its
secondary winding that is the output Voltage squared divided by its secondary winding
resistance. The transformer can run quite hot.
150 Watts may be just fine in a warm climate, but if a 200 W water column heater
couldn't keep up with heat losses on my 90 gallon tank here in  Connecticut, I expect
that a substrate heater will not either. I currently run 400 W in my 90 to keep it at
82 degrees with a 60 - 65 degree environment.
If you need more help with the electrical design of the substrate heater let me know.
I would be glad to generate some graphs showing the relation of wire length to total
Watts for a specific voltage.
Harvey Schneider
(harvsch at earthlink_net)