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Best way to wire multiple heatpads?

Best way to wire multiple heatpads?

Postby gekosin » Fri Mar 15, 2013 9:14 am

Morning all i am still busy with planning my tanks and next step is to plan the wiring and lighting etc, currently i have 1 thermostat which is controlling 2 heat pads (it is an 800w thermostat and currently only has 28w connected to it).

My thought was to hook up the outgoing end of the thermostat to a "14 socket multiplug" this would allow me to wire each heat pad to a 3 pin plug, place it in a multiplug socket and then run everything off the 1 thermostat.

Was thinking of doing something similar for the lights but they will just go directly into the multiplug and ill use the 4w LED lighbulbs.

I am no electrician so this might be totally crazy and insane. LOTS of advice needed/ welcome :)

Thanks!
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Re: Best way to wire multiple heatpads?

Postby Primogen » Fri Mar 15, 2013 9:23 am

Multiple tanks on one thermostat is fine as long as the tanks have basically identical setups as in same position of heatpads, same wattage of heatpads and same cage dimensions.
The probe will need to be in the higher tank as hot air rises.
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Re: Best way to wire multiple heatpads?

Postby dobby7 » Fri Mar 15, 2013 1:37 pm

I agree with primogen. The tanks will need to be exactly the same size and have the same wattage heat pads. The placement of the pads will have to match aswell.
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Re: Best way to wire multiple heatpads?

Postby gekosin » Fri Mar 15, 2013 1:43 pm

Ok great will have to look at it again for this current build then! And the idea with the multiplug do you think that will be ok if I build again with all tanks the same size etc?
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Re: Best way to wire multiple heatpads?

Postby Primogen » Fri Mar 15, 2013 2:31 pm

the adapter is fine just make sure to never exceed to the max wattage of the thermostat or circuit breaker.
Try get fused plugs for the heatpads and stat so if anything goes wrong, like a heatcable short and meltdown it will simply fuse off instead of bursting into flames.
You might have to change the default fuses in the plugs from 16 amps to lower ones depending on you heat pads. I use 1 amp fuses for every 100watts per plug.
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Re: Best way to wire multiple heatpads?

Postby Bushviper » Fri Mar 15, 2013 6:06 pm

Check the resistance of each heating pad with a multimeter. I tested 30 last week and two were twice as "hot" as 27 others and one was less "hot" ie less resistance. Do not trust the salesman.
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Re: Best way to wire multiple heatpads?

Postby froot » Mon Mar 18, 2013 8:22 am

The less the resistance the more current goes through it, the hotter it gets. Resistance is inversely proportional to power dissipated. There's an easy equation you can use to find the actual power dissipated by a heat pad:

Power = 48400/resistance

This equation is valid for 220VAC mains supply only.
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Re: Best way to wire multiple heatpads?

Postby Westley Price » Mon Mar 18, 2013 8:24 am

I thought it's the opposite; the higher the resistance, the higher the temps?
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Re: Best way to wire multiple heatpads?

Postby froot » Mon Mar 18, 2013 11:46 am

Nope it's the other way round.
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Re: Best way to wire multiple heatpads?

Postby Westley Price » Mon Mar 18, 2013 12:12 pm

I haven't done this type of calculation in literally 8 years, but if I recall first year physics, one can use a formula to calculate power (heat):

P=I x (V1-V2)

Where V1 is the ingoing voltage and V2 is the outgoing voltage from the resistor (element). So the larger the voltage drop, the larger the resistance.

And the voltage drop is directly proportional to the power according to the above formula, so that would mean that the higher the resistance (ie. voltage drop) the higher the power and hence the heat output, not so?

This was for direct current though, so AC may be different, and like I said, I'm very rusty so just trying to brush up a bit.
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Re: Best way to wire multiple heatpads?

Postby froot » Mon Mar 18, 2013 9:02 pm

That equation would be true for 2 resistors in series and you were finding the power dissipated by one of them. In the circuit in question the supply voltage is applied to a single resistor, ie the heating pad. If you raise the supply voltage, DC or AC on this type of resistance, true, the power dissipated will increase. But in this case the supply voltage is a constant not a variable. So...

Equation a) Power(W) = voltage (V) x current (I)
Equation b) resistance(ohms) = voltage(V) / current(I)

From eqn a. We can see that an increase in either V or I will result in higher W.
From eqn b. we can see that the lower the value of I the higher the resistance would be.

Manipulate eqn. b.
R = V/I
I x R = V
I = V / R
Insert into eqn. a.

W = V x (V/R)
= V^2 / R
= (220 ^2) / R
= 48400/R

I hope I'm making sense here. Basically at a given voltage, the higher the current that goes into a circuit, the more power the circuit dissipates and it is resistance that restricts the flow of current.
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Re: Best way to wire multiple heatpads?

Postby Westley Price » Mon Mar 18, 2013 9:15 pm

Ahh, thanx froot. Helps a lot!
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