# Re: Transformer for Dupla heating cables

```At 3:48 AM -0400 10/9/97,  Neil Schneider <pacneil at home_com> wrote:

>Ohms law says: P(ower in watts)=E(volts) x I(current) so a 250W cable at
>42 volts draws 5.95 amps of current. Driven by a 24 volt transformer the
>same cable would draw 10.42 amps. I doubt the voltage is crititical for
>something like a heating cable. You will need to supply a transformer
>that
>can supply sufficient current. I would recommend something capable of 15
>Amp
>minimum, preferably 20 Amps for a safety margin. As always Your mileage
>may
>vary.
>

You are absolutely right that P=EI. The conclusion that the current will go
up as the voltage decreases is like saying that as the water pressure goes
down, you can get more water through the same size pipe. The resistance of
the heating cable is constant.

You first need to figure the resistance of the cable using the formula R =
E^2/P where E and P are the rated voltage and power ratings of the cable.
If it's rated for 250 watts at 42 volts, then the resistance is 7.056 ohms.

WIth a resistance of 7.056 ohms at 24 volts, the cable will deliver P=E^2/R
which is 81 watts at 24 volts.

I suggest that you measure the resistance of the cable with an ohmmeter,
and verify the rated voltage and power.

Paul

Paul Nicholson               Electronic Imaging Systems, Inc.
TEL 805 532 1068             5148 Commerce Avenue, Unit F
FAX 805 532 1065             Moorpark, CA 93021 U.S.A.
paul at eisusa_com		     http://www.eisusa.com

```