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Wire gauge and a near serious accident

Posted by AndrewBCN 
Wire gauge and a near serious accident
April 06, 2015 08:40AM
Hi,

I am posting this as a safety warning and a suggestion regarding best practices for wiring the heatbed.

Yesterday while testing a MK2 heatbed for the first time I had the "magic smoke" coming out of one of my printer builds. sad smiley

It turned out not to be PSU, not the RAMPS, not the heatbed either: the wiring from the RAMPS to the heatbed had overheated and melted.

The accident took place in less than one minute, showing things can go from normal to really bad in just a few seconds. I turned on the heatbed, 40 seconds later I noticed some smoke coming out of the printer, and by the time I reached for the stop switch I had already breathed in a good amount of toxic fumes. sad smiley I am guessing a few seconds more and things could have been much, much worse.

Here is the piece of charred, melted cables:



Here is the same cable, stripped, showing the amount of copper, clearly insufficient to carry 10~11A, even when doubled.



This is the cable I am replacing this with, noticed the much larger amount of copper:



Lesson learned? I hope so. I'll be taking the heatbed wiring more seriously from now on.

A few guidelines for myself and others:

  1. Always test the heatbed wiring for resistance. Remember, at 10A, 0.1Ohms means 1V voltage drop means 10W dissipated by the wiring!
  2. Always test a new heatbed.
  3. When testing a new heatbed, stay close to the printer and keep your hand close to the power switch.
  4. Use a thick wire gauge for the heatbed, and verify the amount of copper by stripping a good bit of wire.
  5. Make sure whatever connectors you are using are properly tightened and secured. A bad contact means overheating and things melting, burning and eventually causing a serious accident.

Re: Wire gauge and a near serious accident
April 13, 2015 11:26AM
I always overbuild stuff. My 6A bed is being run from 8 gauge super flexible ptfe core car audio wire and it is soldered directly on the controller. Obviously such a massive wire gauge is overkill, but is is super bendy and it stays stone cold. Good luck with your wiring and be careful!
Re: Wire gauge and a near serious accident
April 13, 2015 03:08PM
Quote
iamdarkyoshi
I always overbuild stuff. My 6A bed is being run from 8 gauge super flexible ptfe core car audio wire and it is soldered directly on the controller. Obviously such a massive wire gauge is overkill, but is is super bendy and it stays stone cold. Good luck with your wiring and be careful!

That's very good advice, thanks! Point taken and I'll be extra careful from now on! thumbs up
Re: Wire gauge and a near serious accident
April 13, 2015 05:23PM
I've used some left-over cable from my R/C models in my printers - it's very flexible, has a very high current carrying capacity, and has a silicone rubber sleeving that can resist up to around 200C. My delta is wired up with 16AWG silicone cable that has a capacity of 22A - more than enough for my needs, considering that the heat-bed will take around 11A or so (wired separately from the RAMPS/Arduino and its stepper motors + hot-end). Not too expensive either.

Here's a UK supplier - Component Shop - but any model shop that sells make-it-yourself electric radio-controlled models will stock it.

Edited 2 time(s). Last edit at 04/13/2015 05:25PM by David J.
Re: Wire gauge and a near serious accident
April 16, 2015 06:54PM
What gauge wire was the one that burned up?

And what size are you using now?

I am about to buy or take apart electronics for to find the wiring I need to hook up a 310mm onyx heatbed, and all the power. Making sure I don't have this happen.
Re: Wire gauge and a near serious accident
April 16, 2015 09:10PM
Check the suggestions above by iamdarkyoshi and David J. In my case, I believe I am using 14 gauge speaker wire right now, that seems to be good enough for the approx. 10A MK2B heatbed I am using.
Re: Wire gauge and a near serious accident
May 12, 2015 03:05AM
This is a usefull website to check which gauge wire you should be using.
AWG Calculator
Re: Wire gauge and a near serious accident
May 14, 2015 02:17PM
Ohm does not like his law being violated.

Heated beds are big current eaters. Mine is 34 amps when running. This requires serious copper - running 10 gauge stranded feeder from the power supply.
Re: Wire gauge and a near serious accident
May 14, 2015 03:39PM
Quote
itchytweed
Ohm does not like his law being violated.

Heated beds are big current eaters. Mine is 34 amps when running. This requires serious copper - running 10 gauge stranded feeder from the power supply.

For that sort of power, IMO a mains powered heated bed is generally a better option. That is what I have on my large delta. Of course, using mains voltage to heat the bed brings an additional set of safety concerns, especially on a machine with a moving bed.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Wire gauge and a near serious accident
May 19, 2015 10:54AM
Quote
dc42
Quote
itchytweed
Ohm does not like his law being violated.

Heated beds are big current eaters. Mine is 34 amps when running. This requires serious copper - running 10 gauge stranded feeder from the power supply.

For that sort of power, IMO a mains powered heated bed is generally a better option. That is what I have on my large delta. Of course, using mains voltage to heat the bed brings an additional set of safety concerns, especially on a machine with a moving bed.

Mains voltage on a heated bed? That brings it own huge bag of issues with it. Going from low voltage to high voltage ( > 48 volts ) and the attendant issues of arc flash protection, different wiring practices, etc. Even though the current is higher, I will stick with the 14.2 volts that runs the whole show.
Re: Wire gauge and a near serious accident
May 19, 2015 03:09PM
Quote
itchytweed
Quote
dc42
Quote
itchytweed
Ohm does not like his law being violated.

Heated beds are big current eaters. Mine is 34 amps when running. This requires serious copper - running 10 gauge stranded feeder from the power supply.

For that sort of power, IMO a mains powered heated bed is generally a better option. That is what I have on my large delta. Of course, using mains voltage to heat the bed brings an additional set of safety concerns, especially on a machine with a moving bed.

Mains voltage on a heated bed? That brings it own huge bag of issues with it. Going from low voltage to high voltage ( > 48 volts ) and the attendant issues of arc flash protection, different wiring practices, etc. Even though the current is higher, I will stick with the 14.2 volts that runs the whole show.

Mains voltage does indeed bring additional safety concerns such as protective grounding, insulation, and preferably RCD protection; but it avoids the need for huge power supplies, switches to control high currents, and very heavy gauge wires. Delta printers are particularly suited to mains-powered heated beds, because the bed is fixed, and all the mains wiring (including the PSU for the rest of the printer) can be put under the bed and made inaccessible to prying fingers.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Wire gauge and a near serious accident
July 20, 2015 03:44AM
May I know what is your wire gauge, sleeve/insulation type, length and resistance?
Re: Wire gauge and a near serious accident
July 20, 2015 05:11AM
Quote
sarf2k4
May I know what is your wire gauge, sleeve/insulation type, length and resistance?

If you are referring to my mains powered heated bed, then the bed heater was supplied with a 2 core double insulated silicone lead. For the mains wiring under the bed, I used the cores from 3 core 5A mains cable. See [miscsolutions.wordpress.com] for photos.

A moving mains powered heated bed would require extra flexible cable with a suitable voltage rating, such as the type of cable used for multimeter test leads, with strain relief at both ends.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Wire gauge and a near serious accident
July 21, 2015 08:18AM
I bought a 16awg wire with silicone sleeving rated around 100'c, 5 meters for a pair of them from aliexpress. I didn't cut them to my required minimum length yet so the wire resistance are rated at 0.4ohm at 5 meters.

This wire will be used to connect from the psu to the board. I still haven't measure the current stock wire that I have now on my gtech prusa printer
Re: Wire gauge and a near serious accident
July 22, 2015 08:02AM
Quote
sarf2k4
I bought a 16awg wire with silicone sleeving rated around 100'c, 5 meters for a pair of them from aliexpress. I didn't cut them to my required minimum length yet so the wire resistance are rated at 0.4ohm at 5 meters.

This wire will be used to connect from the psu to the board. I still haven't measure the current stock wire that I have now on my gtech prusa printer

Here is a chart of resistance per meter for various copper wire gauge:

[www.daycounter.com]

So either there is a very big error in your measurement or your copper wire gauge is incorrect.

Personally I have settled on inexpensive 14 gauge (1.5mm^2 here in Europe) multi strand speaker cable. It does the job for an MK2B heatbed at or around 12V/11A and costs less than 1€ per meter. Also check the other recommendations above. And as dc42 wrote, for a moving bed some sort of strain relief is recommended at both ends
Re: Wire gauge and a near serious accident
July 22, 2015 08:01PM
Not another awg resistance reference again angry smiley
Re: Wire gauge and a near serious accident
May 03, 2016 12:53PM
Thanks for sharing this..

Nice gentle reminder to improve wiring before it's too late.
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