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120V Cable Heating: Why not?


  1. substrate heating cables
    by (Wed, 19 Apr 1995)
  2. Elec. and Water and a Question
    by (Sat, 22 Apr 1995)
  3. Heating Cable voltage: Physics Lesson :)
    by Erik Olson <(e-mail)> (Thu, 20 Apr 1995)
  4. Cooner Wire
    by (Karen A Randall) (Tue, 5 Mar 1996)
  5. Under gravel Heat Cables
    by "Schenck, Lyndle" <lschenck/> (Tue, 7 Nov 2000)
  6. Under gravel Heat Cables
    by George Booth <booth/> (Thu, 09 Nov 2000)
  7. Aquatic Plants Digest V4 #1388-RE M3 heat cables
    by "S. Hieber" <shieber/> (Wed, 7 Nov 2001)
  8. Questions on Under Gravel Heat systems
    by "James Purchase" <jpurch/> (Thu, 21 Sep 2000)

substrate heating cables

Date: Wed, 19 Apr 1995

    > From: (Len Trigg)
    > A couple of weeks ago I was in our local fish shop and mentioned
    > to the owner about using substrate heating cables for plant
    > tanks, and from the back of one of his shelves he pulled out a
    > 25W heating cable made by an outfit called Renacor (or something
    > like that - I can look it up when I get home).  He said he'd had
    > it sitting there for ages. ...
    > The main difference between these cables and the Dupla/Dennerle
    > systems is these use mains voltage rather than low voltage (which
    > in turn means lower current than Dupla/Dennerle).  I'm not sure
    > what this means safety-wise (I think that lower currents are
    > less likely to kill), but I plan to use a GFI for safety anyway.


    I *would not* put 120VAC heating cables into an aquarium, GFCI or not.
    Yeah, the current is the bullet that kills you, but it's the voltage
    that pulls the trigger.  Example -- a car battery can dump a kiloamp
    with ease, but it doesn't have enough voltage to push that current
    through your body resistance, so you can safely pick up the charged
    battery by its bare terminals without lighting up your life.  If you
    could find a *really* high-wattage 120VAC cable, like a kilowatt or
    so, you might be able to get a decent amount of heat from it using a
    24V transformer or some other low-voltage device, but I doubt that
    you'll find such a cable.

    -- Keith

    | Keith Brummett            Ofc: 614-860-3187         AT&T, Room 3B202 |
    |      Fax: 614-868-4106        6200 E. Broad St. |
    |         R,DW,HAHB!          Columbus, OH 43213 |

Elec. and Water and a Question

Date: Sat, 22 Apr 1995

I have been subscribed to the list for a couple of weeks now.  I have been
following the thread on substrate heating and would like to offer this

To support my aquatic planting I work as a lighting designer for a very large
theme park firm and am quite familiar with electricity in water.  It is our
policy to use only low voltage lighting underwater.  Yes, it's possible to
legally use line (mains) voltage with GFI protection but we just won't do it
if we can avoid it at all.  This goes double for Europe and 220volts (pun
intended). Glass tube heaters aside, I would not build my own substrate
heater on anything more than 24volts.  Its not worth it.

That speech over, I would like to ask if anyone can tell me the actual name
for a plant sold to me as an "Octopus Plant"?  Like the name implys it has
many wild/thin tubular arms and make an interesting focus piece.  


David Taylor

Heating Cable voltage: Physics Lesson :)

by Erik Olson <(e-mail)>
Date: Thu, 20 Apr 1995/1996

Len Trigg writes (regarding heating cable voltages):

> Well, I did think about this before I bought them, but I figured these
> cables would be no more dangerous than a standard aquarium heater which
> also uses mains voltage.  As long as I take appropriate precautions
> (GFI, fuses, turning the appliances off before putting arms and legs in,
> keeping oscars and other chew-happy fish out of the tank), I'm willing to
> take the risk.  But I'm also interested in hearing some more detailed 
> explanations on exactly how much more dangerous the 240V,0.3A option
> is over the 12V,6.25A option.

Sure thing.  Let us say that the water in your tank has a finite 
resistance R.  Then if there is a break in your heating cable, it will 
allow voltage V to flow directly through the water.  Ohm's law tells us:

  V = I * R

Since the water has a constant resistance, the current through the water 
will be proportional to the voltage V.  The key here is not the current 
rating of the CABLES, but rather how much current you can have if the 
mains voltage will become exposed.  The 240 volt system allows TEN TIMES 
as much current to run through the tank as a 24 volt system.

But that's not the end of it!  There's also the issue of what happens when
you merely expose the mains leads to the water (a more likely scenario). 
Let's look at the resistance of the heating element itself.  Let us consider
comparable 24 volt and 240 volt heating elements that must provide similar
power P (wattage).  Again, electronics tells us: 

      P = I * V = V * V / R         or   R = V * V / P

The resistance of a wire providing the same wattage goes as the intended 
voltage squared.  The point of this is that the 240 volt wire is 100 
times more resistive than the 24 volt one.  

Now what happens if, say, the insulation doesn't completely work between
your power cord and the heating cable.  The mains voltage is now 
exposed directly to the water.  The current now has a choice...
continue to go through the cable, or through the water.  With the 24 volt
cable, most of the current will continue to pass through the cable, because
of its low resistance.  With a 240 volt cable, most of the current will pass
through the WATER! 

Some other thoughts:  Yes, you sort of run the same risk with your normal 
tube heater.  However, the tube heater is easier to detect some failures
(ie, broken glass, a half-filled tube, etc) and certainly easier to 
replace.  Heating cables are inherently more delicate.  GFCs and breakers 
will help reduce the shock risk in either situation.

- ---
Dr. Erik D. Olson                       licensed and bonded to teach High School
(e-mail)               Physics over e-mail. :)

Cooner Wire

by (Karen A Randall)
Date: Tue, 5 Mar 1996

>  Well, I received today in the mail two samples of wire from Coo
> Wire. I read about them in the Krib/Plants/Tech series of articl
> DIY substrate heating so I gave them a call. The gentleman I've 
> with was extremely helpful and sent me samples of the silicone r
> coated and the PVC coated one, both in 36 gauge. Unfortunately t
> are very, very thin (O.D. .0258, it doesn't say if that's inches
> mm). I was hoping they would be similar to the EKG hook-up wire 
> have at work, which is a thick 1/8" silicone coated. The Dupla o
> are 1/8" too so, until I setup the big tank, bye-bye substrate h

Just a note on thin coated wire for DIY substrate heating.  Our 
local club decided to use one of my tanks as a "Guinea Pig" for 
DIY substrate heating cables.  The only wire that the person who 
put together the system could find was quite fine.  When the 
cables were first installed, they were checked with a continuity 
tester(? Remember, I graduated from Gary Larson's school for the 
mechanically declined, so this may not be quite right<g>) and it 
seemed fine.  Somehow, in the next couple of weeks, something 
happened to the cable, and it started to electrify the tank.  Of 
course the voltage was stepped way down with a transformer, so 
Ididn't electrocute myself, (besides, it's not April 1 yet;-) but 
I had some _very_ unhappy fish until I figured out what had 

A couple of the fish started acting _extremely_ jumpy, and I went 
to check the pH, thinking, perhaps, that something was wrong with 
the CO2 system. (I know, probably stupid, but I told you I'm not 
very mechanically inclined)  When I went to fill the test tube 
with water, a cut on my finger hit the water, and YEOW!  I 
couldn't feel it with my other fingers at all.  I ended up losing 
my female Krib the next day, but all the other fish have returned 
to normal.

So the tank is again without substrate heating. (oh well, it's 
done fine without it for years) after this experience, I'd say if 
your gut feeling is that the wire is too fragile, don't use it.  
Someone has suggested installing 2 sets of cables at the time the 
tank is set up, that way if one set fails, you can hook up the 
other set.

If I were to do it again, I'd place a UGF plate underneath so that 
if the cables failed, I could go back to the UGF/heater system... 
I _know_ that works reliably!<g>  The other good thing about the 
UGF/heater system is that the electrical part is easily accessible 
if the need arises to replace it.

Under gravel Heat Cables

by "Schenck, Lyndle" <lschenck/>
Date: Tue, 7 Nov 2000

Monolith Marine Monsters has Under gravel Heat coils cheaper than you can
make them.  They sell the Azoo cables that run directly on line (115 VAC).
I was apprehensive about that but the cables seem to be well made and I have
a good Ground Fault Circuit protection.

Pros:  	The cables are inexpensive ($60 for a 300 Watt cable).
	115 VAC system permits variable heat control with X-10 dimmer

Cons:  	115 VAC may be a problem.  They appear to be well made but use a
	The power cord is too short to form a drip loop so an extension cord
is needed.
	A temperature controller may be required with a higher wattage

Since I already have computer control of my home lights I have connected my
coils to a X-10 lamp dimmer that lets me vary the intensity automatically.
I'm still experimenting but they are initially set for 3/4 intensity at
night and 1/2 intensity during the day.  They are set to OFF if the
temperature exceeds 78 degrees F.

I have made 4 DIY 12 VDC systems and the hassle, time, and money do not
always outweigh the benefits.  This is another option.

The tank was just filled this weekend so I do not have any long term
observations.  If anyone is interested let me know and I will post updates.

Under gravel Heat Cables

by George Booth <booth/>
Date: Thu, 09 Nov 2000

>Date: Tue, 7 Nov 2000 11:04:32 -0500
>From: "Schenck, Lyndle" <>
>Monolith Marine Monsters has Under gravel Heat coils cheaper than you can
>make them.  They sell the Azoo cables that run directly on line (115 VAC).
>I was apprehensive about that but the cables seem to be well made and I have
>a good Ground Fault Circuit protection.

I wouldn't worry too much about 115v cables. People use 115v heaters 
(enclosed in glass!) all the time. 115v powerheads are very popular. When 
is the last time you herd of someone killed by a broken heater or powerhead?

George Booth in Ft. Collins, Colorado (
"The web site for Aquatic Gardeners by Aquatic Gardeners"

Aquatic Plants Digest V4 #1388-RE M3 heat cables

by "S. Hieber" <shieber/>
Date: Wed, 7 Nov 2001

"Natural Aquariums" <> asked:
". . . does anyone have any experience with the M3
> substrate
> heating cables?  They seem very reasonably priced.  They also have a
> either
> an analog or LCD heat controller.  Has anyone used these?  Which, do
> they
> prefer?  I am a high tech gadget guy who is looking for low
> maintenance.

Some humble opinons:
The cables are indeed reasonably priced -- jsut keep looking around!
The insulation is thinner than that found on some other much more
expensive cables and includes no ground wire, unlike the FishVet brand
sold by, for example Pet Warehouse ("Pet" with no "s" meaning not
suing).  But lack of ground wire should not be an issue provided that
you connect the cables through a GFCI and have a grounding probe in
your tank -- both are very good ideas in any event!  I have used a set
Ed's M3 cables for almost 2 years without undesireable results.  The
substrate reaches a maximum temperature next to the cables of about 90
degrees F after running all night -- I was afraid it might be much
higher than that.  They run cool enough not to melt the epoxy coating
on, for example, TopFin and other similar packaged gravels -- if that's
what you have.  In terms of wattage per foot, these are more like Dupla
(higher watts/ft.) than the Dennerle (low watts/ft.)  So they are
likely to cycle on and off in most tanks, given the heat from the
lights, etc. Some prefer the cycling and actually use higher wattage
cables with timers to prevent the substrate from becoming evenly heated
and thereby not producing the desired convection currents  -- but
that's another story -- see prior postings by George Booth on this

The digital controller is used by many manufcturers of heating and
chilling equipment of various kinds -- not just in the hobbyist aquatic
market but also in the industrial and farm markets -- need to keep
those chickens and dairy cows warm in the winter and cool in the
summer.   :-)  The controller is a standard in several industries and
very reliable -- but I think it's maker recommends that the spread
between trigger-on and trigger-off be at least two degrees, which some
might think is high -- but you can set it to less.   Ed sells it for a
very good price -- I haven't seen it for less but I have seen it for
much more.

I am not familiar with the analogue unit at M3 but I recall i0t being
an Azoo, the same as the cables as probably as good and reliable as the
Otto (see below).

A cheaper but usable alternative is the Ultralife Reef Products digital
heat controller.  I have found these to be reliable also, although less
versatile than M3 -- they are for heat control only.  I use them with
glass tube heaters so that the water temperature is controlled by a
thermostat separated from the heater coils.  If all you want to do is
control a heater, it's a pretty good controller.  Even cheaper but
practical is the Otto analgue heat controller.  Again, a reliable
product but you set the temp with an analogue dial instead of a digital
readout, so it takes a little longer until the setting is just where
you want it.  With the digital models the display shows just what
temperatrue you have set the controller to and what the tank temp is. 
The analogue dials have to be checked against a thermometer until you
get the setting jsut right.

You can get Ultralife and Otto equipment at
and probably other places.

Good Luck,
Scott H.

Questions on Under Gravel Heat systems

by "James Purchase" <jpurch/>
Date: Thu, 21 Sep 2000

Lyndle asked about the cable heaters sold by M3 and Pet Warehouse.

M3 sells substrate heaters by AZOO. They operate off of mains voltage and
Edward makes it very clear that they should be plugged into a circuit which
has GFI protection (he even sells a GFI unit). I bought one of the systems +
controllers from him in the spring and haven't been electrocuted yet! As
usual, I received excellent advice and service from Edward when I was trying
to decide what I should buy and I'm more than happy with the unit that he
sold me.

Now, that being said.......summertime in an non-air-conditioned apartment is
not the correct time of year to assess how useful a substrate heater is.
Mine has rarely even switched itself on yet, but as the room temperature
drops below 80F with the approach of autumn, I expect that to change.

The cable itself was well made and came with enough holdfasts to keep it
buried securely (they are almost identical to the holdfasts from Dupla), and
the controller I got thru Edward was an Aqua Logic model which is widely
used among marine hobbyists.

I did look at the system sold by Pet Warehouse, and corresponded with them
about it. They are from Aquarium Landscapes (Fish Vet) and they are
grounded, while the AZOO models aren't, if safety is something you are
worried about. Shawn Prescott from Aquarium Landscapes is on the APD and I'm
sure that he could give you more info regarding them.

Good luck,

James Purchase

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