PBHQ recommends using the PSU Y adaptor and connecting to 2 rails on the PSU. typically ATX PSUs will run 2 parallel 20AWG wires to each 12V rail connector. read on why using 1 rail alone will create a Firebot...
My PB+ draws about 16A at 11.1V when everything is running (heat bed, extruder and motors). that is 180W so call it 200W to be safe. a wire typically has a specific thickness or gauge. the most common measurement is AWG. this is the i use. the wires from my PSU and to the PBrd are 20AWG as supplied from PBHQ. 20AWG is rated for 11A. so doubled up you get theoretical 22A of capacity. however the doubled up wires are close to each other and so they will heat up because if my PB is drawing 16A then thats 8A per wire (2 wires) and the rating is 11A - 8A is very close to the rating so the wires will get warm. put one warm thing next to another warm thing and the ability of each to dissipate heat is reduced (eg. gerbil pile). and so doubled up 20AWG wiring expected to take 16A total is likely going to burn at some point.
The best thing is to use more parallel wires or better yet a single thicker wire rated well higher than what you expect to use. on my PB i have the 2 12V rails of the PSU joined together to a single XT60 plug. that means i have a total of 4 20AWG wires on each + and - for a capacity of 4 x 11A = 44A. they still get warm on full bore but at least not nearly warm enough to melt. but you can see, even 4 20AWG wires with a draw of 16A are still going to get warm. if you want them to run cold you have to basically go with 4x the current you are drawing in amperage capacity. 16A x 4 = 48A = 10AWG. that is one thick wire!
to the heat bed again the stock config is 2x 20AWG. now if you have these bundled together or in a covered sheath (which can include wrapping of any kind), then the ampacity will be significantly reduced. here is a showing open air vs enclosed ampacity. you can see the 20AWG wire is reduced to 7.5A when enclosed. so doubled up thats 15A. you might as well call two wires sitting next to each other running across a table "enclosed" so 16A through 2 wires with a total rating of 15A... poof.
even 16A through 4 rated for 30A is still going to get warm but at least we are far enough away from max that hey wont burn.
connectors are also a concern and have a max current rating as well. so you have to consider that. on my PB I ran 14AWG silicone insulated wire from the PBrd to an XT60 jack on the back of the Printrbot. then i bundled up the 4 + and - wires from the PSU to another XT60. XT60 connectors are rated for 60A. this way i can easily disconnect the main power to the PB. externally i have 30A total of wire current capacity and internally its 32A with 60A connectors. i don't want these overload problems.
to my heat bed i removed the paired up wires and used 14AWG wire again to an XT60 connector going to a mating XT60 connector on the PBrd. when you are dealing with power you have to ensure all your wiring and connectors are well over typical operating currents because unless you are around 4x over operating currents with open air wire, the wires are going to still get warm. the heat reduced the current capacity further, further heating them, further reducing capacity, etc etc until
Final note on insulation... PVC is typically used and is flamable at lower temps. i prefer to use silicone wire. silicone has a much higher flash temp and so will prevent fire. i choose a PSU that uses silicone wire on the outside. silicone coated wire is a littler harder to find but its worth it.
Wire: RetireeJay points out that stranded wire with a high strand count (more individual wires bundled in the conductor) is preferable to wire with less strand count (and thicker individual wires). high strand count provides more flexibility in the wire which is important for wires that will incur movement and stress due to motion, such as those connected to the heat bed and the X axis. This where I get my wire:
http://www.hobbyking.com/hobbyking/stor ... Guard.html . their silicone wire is pretty decent and has the thinner conductors i mentioned. its mainly used in radio controlled models which are always vibrating and crashing so the wire has to stand up to this.
Please don't compromise on these things and if you feel your Printrbot is in danger of becoming a Firebot, replace all the high current wiring with at least 14AWG or more silicone insulated wire and upgrade your connectors.
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