I'm in the middle of building a D-Bot printer, and have run into a bit of an issue when it comes to the heat bed wires. The heated bed is an aluminum plate with a silicon heater attached to it, and the heater wires are not long enough to make it through the drag chain when the Z-axis is fully extended.

The silicon pad is 120 V AC / 750 W and will be turned on/off by a Fotek SSR. The heater wires are cloth-covered and are probably 22-24 AWG. (Gauge is not labelled)

I suspect I'll need to extend the wires by putting in some sort of coupler at the top of the drag chain, but I'm not certain if there are specific requirements for the wires for an AC powered heat bed.

To this end, I was wondering:

  • Is there a specific wire gauge that I should use for the heater wires, and should it have a specific cladding?
  • What type of connector would be best for connecting the wires together securely in this case?

Thanks in advance!

  • $\begingroup$ Remember to set your firmware to operate that SSR either in bang-bang mode, or with a PWM frequency lower than 5 Hz if you use PID control. SSR often operate at zero-crossing, therefore you can toggle switch them up to 120 times a second. With 5 Hz PWM the resolution of the power setting would be 120/5=24 power steps. $\endgroup$ – FarO Nov 11 '20 at 17:05
  • $\begingroup$ Actually half that value, since you have to toggle them back: 5 Hz = 12 power levels $\endgroup$ – FarO Nov 11 '20 at 17:19
  • $\begingroup$ Would it be possible to completely replace the wires? You don't really want a soldered or crimped joint in the part which is subject to repeated flexing in the drag chain if you can avoid it. $\endgroup$ – Andrew Morton Nov 12 '20 at 12:36

750 W at 120 V is 6.3 A. 22-24 AWG is on the thin side for this. I would recommend 18 AWG or thicker. You don't need a specific style of insulation for this (other than something that is rated for the voltage and temperature the wire will need to withstand, but most commonly found wire should be good).

A good way of connecting the wires would be to solder them. If you do not want to solder, there are many products on the market for connecting wires. A butt connector that you crimp could be a good option, or you could use a WAGO clamp. Whatever option you end up using, be sure to provide adequate strain relief as the connection point (be it soldered or with a connector) is more likely to fail from fatigue.

  • $\begingroup$ Comparing the wire with a 20 G wire, the core seems about the same size, so I might actually be OK there. (I don't have any 18 gauge wire handy right now for comparison). Would a 1-2" length of 20 G wire pose a problem? (I suspect it'll still be a bottleneck, but maybe if I minimize the length before going to a larger gauge, it won't be as bad. Unfortunately, I''m not certain I can get at the contacts in the pad without destroying it.) $\endgroup$ – Bluetopia Jun 14 '20 at 23:40

750 W at 120 V is around 6.3 A.

You can use a voltage drop calculator to find out how much power is lost in the wires, and therefore their temperature increase.

I did it for you. With 10 feet (2x5 feet) 20AWG wire and 6 A current, 7 W are dissipated on the wires.

It's 0.7 W/foot which is ok for silicone wires, also because the bed doesn't operate at 100% except for the initial heating up (and then the heat goes to the air around, which is fine anyway).

If you use another website, you can see that 20 AWG with 6 A current produces, in free air, around 10 degrees temperature increase. Silicone insulation will worsen that, but silicone holds well over 100 °C...

Use thicker wires if you can, or go on if you cannot.

The most important thing is to very effectively join the two, so that no extra resistance is introduced. I would place them next to each other and physically solder them. Then heat shrink tube around the joint.

See for info what not to use for the joining


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