Heated Bed SSR Wiring Size

  • I know the Duet can handle up to an 18A load for the bed heater. I'm curious what the actual power draw might be if I used a Crydom D2410 to switch a mains powered heater so that I might size my wire appropriately. Any ideas?

    Edit: Looks like the minimum input current for the SSR is is 7 mA and the maximum is 12 mA.

  • https://www.mouser.com/ProductDetail/Crydom/D2410?qs=KoN42VEC627o%2bYDs0hA8Nw%3D%3D

    That's the data sheet. Input current range says 7-12ma so you should be fine using as thin of wire as you want.

  • http://www.paigewire.com/pumpWireCalc.aspx

    16AWG I think would be good from mains to the SSR. From the Duet to the SSR can be much lower gauge. 22AWG for instance.

  • After seeing the datasheet, I see now. Luckily the bed heater already has it's wires attached so I shouldn't need to worry about that. I was hoping I could go with 22AWG as it's what I have.

    Also a question about the VIN to the Duet, I'm using an external brick style power supply and the connector has 4 pins, 2 that supply 24VDC and 2 Ground.

    A 22AWG wire from each of the 24VDC pins and a wire from each of the Ground to the Duet should be sufficient correct?

  • I would recommend using 18 or 16 gauge from the PSU to the duet, and 14 or 16 from the wall to the SSR, and 22-32 from the Duet to SSR.

  • Edit: Found the power supply manufacturer suggests 18AWG for wiring the connector.

  • Hi,

    Current capacity is different for single strand and for multi-strand wire. Current carrying capacity is also mitigated by how much heat the insulation covering the wire can endure before breakdown. I personally perfer to use PTFE 'Teflon' insulated silver plated copper multi-stranded wire for everything I wire. It gives a high-tech appearance, carries a higher current with less weight, it solders wonderfully, and takes years to oxidize but it is more expensive. There are vendors on eBay who supply this type insulated multi-strand wire via surplus sources.

    Included below is a useful( PVC) wire gauge/capacity table for determining basic current carrying capacity derived from the circular mil diameters of individual unplated copper strands. Sounds complicated, but its actually not, however, it is an engineering strategy to determine the correct wire gauge size for a specific current requirement.

    • Determine the amount current to be carried...the 'load',
    • Allow 5 or 7 Amperes wiggle room (add this to the maximum current to be carried),
    • Determine length of wire to be used, (long runs drop current, keep the runs as short as practical),
    • Locate supplier for multi-strand wire, ask for the circular mil Ampere current carrying formula (better than relying on tables),
    • Calculate wire gauge based upon current carrying capacity requirement or use a table

    A PVC insulated wire, circular mil table is listed here:

    Same size wire (circular mils) carries different current depending upon insulation covering, PVC for instance does not carry as
    much current because the insulation will breakdown at lower wire temperature, while PTFE 'Teflon' insulated wire can carry more current because the insulation breaks-down at much higher wire temperature.

    For PTFE silver plated copper wire, my favorite wire, can be found below, although there are also many other vendors around, (incidentally I am not affiliated with any of these other than I buy wire from them occassionally).

    Here is a source for Mil-Spec Wire (PTFE) 'Teflon' Stranded Silver Plated Copper which I use:
    eBay vendor: "SkipMalley", has many different (all surplus) gauge sizes and colors,
    enter this number
    for 20Ga: "292417410713" via eBay search field
    for 16Ga: "291102551795" via eBay search field

    For Alpha 5858 PTFE Wire information, see page 11,

    Here is a useful current capacity and Voltage drop calculator:

    Please don't make the mistake of using only one color wire for power wires, use at least two different colors. It is both useful and wise to create an accurate drawing of your wiring configuration, (a little picture of your SSR showing the color of the wires and where they are routed to and from, relying of DC42's layout drawings of the controller board for the other end reference locations, etc) and put the drawing in your printer document drawer for future reference.

    Good luck (and please, don't burn down your house),


  • I would stay away from the PTFE coated stuff. I tried to use it and it was an absolute pain in thee ass to strip with wire strippers because it was so slippery and it wouldn't bend at all. I would just go with some silicone wire from amazon made by BNTECHGO. Nice and flexible and will carry plenty of current.

  • @surgikill

    I use a Sears stripping tool, which cost around $12 US to strip my PTFE insulated wire. Good materials require good tools too.

    I use the PTFE because it is superior material. Its great.


  • @3mm said in Heated Bed SSR Wiring Size:


    I use a Sears stripping tool, which cost around $12 US to strip my PTFE insulated wire. Good materials require good tools too.

    I use the PTFE because it is superior material. Its great.


    I use a $40 wire stripper to strip the wires, and it just glides over the surface and won't bite the PTFE........ I also can't grab onto it because it's PTFE and super slippery. The only thing it's good fir is making wiring harneses that won't flex. The silicone wire is pretty much better in every regard for a hobbyist.

  • @surgikill

    I'm not intending to be insulting, but rather, share my experience with the good stuff. It is true that many stripping tools do not work well with Teflon type insulated wire, however, I searched around for tools that do work well with Teflon, in fact I went to my tool lock and got out the four tools that I use here at home to get the part numbers for y'all, two of which I bought at Sears and one online. I'll list them below with their URLs and prices. These work well with PTFE, Teflon, PVC, HHTN, HU, TVNH, eg, all of them including high-voltage silicon. I imaged the two tools that I use the most for your perusal.

    Here are the strippers I use:
    Strips PVC, Teflon & Silicone wires - Platinum Tools #PN15005 - $13


    Another is Sears Craftsman - Strips PVC, Teflon & Silicone wires perfectly
    #73573 (Itm: 00973573000) - 12Ga ~ 16Ga - $14

    #73574 (Itm: ) 32Ga ~ 22Ga - $13

    Although not my favorite tool as this one is a bit more cumbersome to use, it is a more traditional style Teflon type stripper tool and it does strip Teflon type insulation perfectly and is fairly inexpensive for its type of tool, as for instance AMP charges around $400 for these type tools.

    Sears Tool: #200-003 (Itm: SPM7057351107) - 8Ga ~ 22Ga - $14

    I use the PTFE because it is a superior material and it doesn't shrink when ya solder it and it exhibits a very long oxidization cycle, so crimps do not become Ohmic after a couple of years, etc.

    Left - Sears & Right - Platinum Tools

    Here's a shot of the DUET-E Controller wired with PTFE, Silicon & PVC Wire
    0_1547591509384_DUET-E Controller w Teflon Wire_rszd.jpg


  • @blt3dp

    The two terminals for the DC Return (aka 'ground'), SHALL be the same size gauge as the power-wires supplying the controller Vin. If your full load current is 18 Amperes, I would recommend adding at least 5 Amperes for load variations and as electronic components age the Amperes invariably increases as components become less efficient, 23 Amperes is likely a good number.

    22Ga is ok for small miliAmpere curents but not 10+ Ampere currents.

    There are several ways to wire the circuit. I suspect that the simplest is to use one of the control-outputs (as I did) to drive the SSR's control terminals using 22Ga or 24Ga, etc small gauge wire, which will handle the 12 mA SSR control current flow.

    When searching for a better priced solid-state-relay (SSR), pay close attention to the specs. There are essentially two types of SSRs, those that switch AC loads and those that switch DC loads. Depending on how the load is powered will mandate the type, DC or AC. Also pay attention to the input Control Voltage rating, as many SSRs will only trigger at 4,5Vdc and above, while the DUET controllers supply 3,3Vdc control Voltage.

    There are a few SSRs that trigger at 3,0Vdc upto around 30Vdc and is what you should use if you don't use the DUET MOSFET power-output to control the external SSR which will involve more complicated wiring. The SSR load current rating should be rated at least 50% higher than the actual load current.

    Here is the SSR that I've used over the years in numerous projects, it is inexpensive (this forum has also recommended its usage in the past) and it is a reliable product, in my opinion. Shipping cost might be an issue if not shipped in the continental US? This SSR also provides a quite handy red LED that illuminates when the SSR is triggered.

    Auber Instruments SRDD-100 SSR
    0_1547618675272_SSR_Aubur SRDD-100 CloseUp_rszd.jpg

    See Auber Instruments at: http://www.auberins.com/index.php?main_page=product_info&cPath=2_30&products_id=288
    This Auber Instruments 'SRDD-100' SSR is rated at 3,0Vdc ~ 32Vdc Control Voltage which switches 5Vdc to 100 Vdc @ 100 Amperes.

    The SSR will need to be heat-sinked. I used the switching power-supply aluminum case and used a heat-sink compound to sink off the heat from my 30A load 100 Ampere rated SSR as depicted below.
    SSR Mounted on Top of Power-Supply
    0_1547618794833_SSR_Auber SRDD-100_PwrSup HtSnk_rszd.jpg


  • I use one of the auto wire strippers. I can't deal with the ones where you have to select the wire size and then hope it doesn't nick part of the wire. I've had the best luck with silicone. Doesn't shrink, doesn't melt, and is easy to handle. Never had an issue with oxidation on anything either.

  • @3mm said in Heated Bed SSR Wiring Size:

    See Auber Instruments at: http://www.auberins.com/index.php?main_page=product_info&cPath=2_30&products_id=288

    Thanks for that. I've been looking for a new SSR to replace my suspect Fotek SSR.

  • @surgikill

    I've not actually not worked much with silicon insulated wire, sounds good, I'll give it try. Thanks.


  • I do have some cable size recommendations in this post

  • @pro3d

    The PA20 crimper is fairly good tool, it is expensive considering that it likely cost 50cents to make or less. But, it is a kewl tool as it crimps many of the popular connector pins hobbyiest use.



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