# Wire Type and Gauge Question



## speedre9 (Mar 9, 2014)

I need to extend the wiring, and don't know what type, and gauge of wire to use, beyond the fact it should be stranded. 
One, is the wires for a heater cartridge on a 3d printer head ?. 
Two, is the wires leading to a thermistor on a heated 3d printer MK 1 plate ?. 
The general way is to tape (Kapton) the thermistor, to the center of the plate, solder, and extend the wires to the PID. 
So what gauge of wire do I use to do this?


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## Tony Wells (Mar 9, 2014)

Size generally will be fairly small, like 24 or 26 gauge, but distance can throw the calibration off if extreme. Of particular concern is the type of insulation, and that depends on the environment. It is primarily temperature dependent.  

http://www.omega.com/pptst/EXGG-2CU_3CU_WIRE.html


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## therbig (Mar 10, 2014)

Your choice of wire depends on a few considerations:
*1. Voltage.*  I assume that these are low-voltage heaters (<110V), so you can use what's called "hook-up wire," which generally has thinner insulation than high-voltage wire
*2. Current.*  This will set the gauge of the wire.  More on that below
*3. Temperature.*  If the wire is near hot areas, use Teflon-insulated wire.  It costs more, but it will save you a lot of trouble
*4. Mechanical.*  If the wires have to move, you must use stranded wire.  Stranded is also easier to solder

How do you select the gauge of the wire?  This depends on the current that the wire has to carry as well as its length.  The reason is very simple: the wire is a not a perfect conductor but rather a resistor - just like your heater.  The resistance of the wire *decreases* with larger wire size (lower AWG) and *increases* with the length of the wire.  You want to make sure that the resistance of your wire is much smaller than the resistance of your heater.  

*Example calculation*

Here is a made-up example.  Let's say that you have:

Power of heater: P = 60 Watts
Voltage: V = 12 Volts
Distance from power supply to heater: d = 5 feet (accounting for the routing of the wires)

First let's figure out the *resistance of your heater*, R[SUB]H[/SUB], which is calculated as:R[SUB]H[/SUB] = V[SUP]2[/SUP]/P
   R[SUB]H[/SUB] = (12[SUP]2[/SUP])/60 Ohms = 2.4 Ohms​
As for the *resistance of the wire*, R[SUB]W[/SUB], I would keep it at 1% of the heater's resistance (don't go above 5% if you want to push things).  This would mean:R[SUB]W[/SUB] = 0.01 * R[SUB]H[/SUB]
   R[SUB]W[/SUB] = 0.01 * 2.4 Ohms = 0.024 Ohms​
The *resistivity of wire*, r[SUB]W[/SUB], is given as Ohms/1000 feet for standard gauge wire.  The *total wire length* l isl = 2 * d, because we need to count the wire from the power supply to the heater and back
   l = 2 * 5 feet = 10 feet​
This means that the *maximum resistivity of the wire* r[SUB]W[/SUB] we can allow isr[SUB]W,max[/SUB] = R[SUB]W[/SUB]/l
   r[SUB]W,max[/SUB] = 0.024 Ohms/10 feet = 0.0024 Ohms/foot = 2.4 Ohms/1000 feet​
Now look at the American Wire Gauge Table (column 4) to find the right *wire gauge*.  The results is:*
AWG 13* - because its resistivity of 2.003 Ohms/1000 feet is lower than our limit of 2.4 Ohms/1000 feet​
AWG 14 might be just fine as well at 2.525 Ohms/1000 feet, but AWG 13 is the one that works in a formal sense (in this example).  Obviously, you will have different values of P, V, and d, so your results will be different.  But I hope that this can at least point you in the right direction.

Tom


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## Tony Wells (Mar 10, 2014)

Thanks for that, Tom. I didn't catch that he needed help with selection of wire for the heater(s) as well. I was too focused on the thermistor.


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## speedre9 (Mar 10, 2014)

The results therbig gave is way above my attention span,L.O.L. but I get it. What I've seen concerning wiring the thermistor generally says use hook up wire, 24 thru 18 gauges. But, when it comes to extending the heater cartridge wires, there are little or no ideas shown. I guess many users simply use Teflon coated wire, or they are not building stand alone systems. I think I forgot to mention that in my initial post. My heat bed system is stand alone, I am using Mach 3 to control the movement of the machine, but, using a manually controlled heated bed system. I am not exactly sure that 3d printer software that slices the stl. files will control it. So, because I will be using many different filaments, using different bed temps, I chose to go stand alone and manual. I'm not sue that is a clear synopsis of what I'm doing but, I guess I got the information I needed, Thank You guys.


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