Ampacity, power and bed wiring and max gauge



  • Hello,
    I looked at this page in the documentation including the picture:
    https://duet3d.com/wiki/Power_wiring
    I will use a 24V, 300W silicon heated bed and thus will need 12.5A at max power
    Nice because I will be able to draw that directly from the board in my 1m50 high delta I decided to build the electronic on top, because I also plan to extrude pasts and liquids occasionally. So the Duet will be ~1m80 away from the bed.
    Looking at various AWG-Ampacity sources on the internet I found that gauge 10-12 AWG is probably desirable in this case:
    http://www.cerrowire.com/ampacity-charts

    What I am not sure is what is the max AWG including ferule that the connectors would allow.
    AWG 10 is about 2.5mm diameter
    and
    AWG 12 is about 2mm diameter
    So will the 10 fit after crimping with a ferule the one in the pictures seems about maximum size.

    Thank you for your comments, help or else.


  • administrators

    Hi sga

    Big printer! The VIN and Bed power terminals are the same as these:
    http://uk.farnell.com/metz-connect/31701102/terminal-block-wire-to-brd-2pos/dp/2434409

    So they take 10-26AWG wire according to specification with a 4mm^2 conductor area

    Cheers

    Tony



  • Thank you Tony,

    So AWG10 wire is the limit.
    I see a 3.6mm dimension in the height but it seems to be only the plastic hole the metal would be smaller.
    I guess it's always possible to squish the ferule a bit higher than wide and it shall fit.
    I'll try my chances.

    Cheers


  • administrators

    The ferrules I use (Farnell 9972242) are about 2.2mm inside diameter. However, the terminal block would accommodate a larger ferrule if you have the right crimping tool that crushes it into a square shape. The internal width of the metal part is about 3.2mm by my measurement, and 3mm according to the datasheet.



  • FWIW, even 14ga would be plenty for 12.5A.



  • you are surely right for short length however I used this now:
    http://www.rapidtables.com/calc/wire/voltage-drop-calculator.htm

    with 2x 2m = 4m back and forth to the board and copper AWG 14 24V@12.5A. I lose 3.4% 0.8V in the wire which seems a bit more than I am happy with…
    That's about 10W out of 300W,
    Now as a bonus, consider that I have both wire heating inside one vertical alu profile of my delta how much does this tower length increase 😉

    Select wire type:
    Or enter resistivity:
    1.72e-8
    Ω·m
    Enter wire diameter size:
    14

    Enter wire length:
    4

    Select current type:
    Enter voltage in volts:
    24
    V
    Enter current in amps:
    12.5
    A
    Calculate Reset
    Voltage drop in volts:
    0.826562
    V
    Percentage of voltage drop:
    3.44401
    %
    Wire resistance:
    0.0661250
    Ω



  • 14ga wire is rated for 15 amps buried in wall insulation, an aluminum profile would be considerably less insulated.

    As for the 0.8v voltage drop, just raise the power supply voltage 0.4v to make up the difference between 10awg and 14awg.



  • Just curious, is it essential to use ferrules with these connectors? I wired my duet0.8.5 with no ferrules on the 12v, nor the bed output. Should I use ferrules for the DuetWifi? Which ones?



  • @elmoret:

    14ga wire is rated for 15 amps buried in wall insulation, an aluminum profile would be considerably less insulated.

    As for the 0.8v voltage drop, just raise the power supply voltage 0.4v to make up the difference between 10awg and 14awg.

    Too be updated once numbers are rechecked… again 🙂



  • Respectfully, a few things appear to be incorrect with your analysis. Here goes:

    @Aussiephil:

    2M = 4M loop resistance of 14awg is 0.41 Ohms

    0.033 ohms, actually. Source

    @Aussiephil:

    This makes the total series resistance = 2.33 Ohms
    a 24V supply will deliver 10.3A into this resistance
    The heater has 24V x 10.3A = 247W

    This is incorrect, because of the above.

    All that matters is the difference in resistance times the (desired) current, to determine voltage drop on each of the theoretical conductors.

    12awg is 0.020 ohms per 4m
    14awg is 0.033 ohms per 4m

    0.02012.5 = 0.25 volts
    0.023
    12.5 = 0.42 volts

    Of course to achieve 12.5amps you'd need 24v at the heater terminals, so 24.25 vs 24.42v respectively for 12awg vs 14awg.

    So actually the difference is less than I thought, it seems OP's calculator is off. If you want to push 12.5 amps, the difference in wire size contributes less than 0.2v.

    Your analysis is also flawed in that it does not compare one wire size to another, it compares theoretical heater output (zero wire resistance) against voltage increase needed with 14ga wire. When you say "the heater has" 24v, this isn't actually correct, as it see less than that after the voltage drop. Apples to apples, and all that.

    You should only be looking at the different voltage drops across the wire, and two tenths of a volt is trivial to make up for on the power supply end if it matters so much.

    Also as a sanity check, if 4m of wire resulted in 0.4 ohms, that's be 6 volts of drop at the breaker rated current in my house, just 4m from the breaker. I have some runs that are 20m - so I'd be down under 90v. That just isn't the case.



  • Damn Spreadsheets….. picked up the wrong field.... now that's embarassing.

    Last post fully deleted as My spreadsheet numbers are wrong....



  • Hi guys,

    Sorry never thought it would generate so much controversy.
    My only point is that I don't want to dissipate more than about 2% in the wire leading to the heatbed.
    Call it efficiency or stupidity but since power supply are now in the ~80-90%+ power efficiency It seems a waste to lose 5% in the wires.
    So since P=RI^2 and I is constant (more or less as bed will heat up and change resistance), it's just that I want Rwires<0.02*Rbed.
    Bed is around 2 Ohms so R wires should be max 0.04 Ohms so you are right AWG 14 is good enough at 0.033 Ohms.
    I found there was a factor 2 difference between your and my calculation because the calculator I used already had the factor 2x for return wire included 😉
    http://www.rapidtables.com/calc/wire/voltage-drop-calculator.htm

    2x2m awg14 =
    Voltage drop in volts:
    0.413281 V (but I don't really care if the bed sees 24 or 23.6
    Percentage of voltage drop:
    1.72200 % (good enough)
    Wire resistance:
    0.0330625 Ω (same as yours)



  • Another question about power supply,
    I was contemplating the luxurius HEP600-24 and HEP600-30 from Mean Well with 96% efficiency and remote OFF… (if anyone can have a 50% discount on them please raise your voice).
    It so happens that the 24V version has voltage adjustment from 20-25 wheras the 30V version goes from 25.5 to 30V.
    So I went to check the duet wifi rated power supply and read here:
    https://duet3d.com/wiki/Hardware_differences_from_the_Duet_0.8.5
    that the max voltage was reduced from 30-25 V due to the new drivers.
    So just went to check the drivers see if they could survive 25.5V
    http://www.trinamic.com/products/integrated-circuits/stepper-power-driver/tmc2660
    and they seem to accept 30 V so are there other drivers somewhere or other components which would fail?
    Any voltage stress test done yet?
    Thanks



  • 25v rating on the Duet gives some headroom to avoid running into the 30v rating of the TMC2660s. Things like stepper motor decel can cause bus voltage to rise above nominal.


  • administrators

    Hi sga, yeah is you want to run your bed at 30V, then better to switch that externally and stick to 24V max on the DuetWifi. the reason for this is exactly as Tim says, to reduce the possibility of back emf form the motors causing issues.


  • administrators

    As Tim says, the limitation is the stepper drivers, which are rated at 30V when the motors are powered, and motors pump energy into the supply rail when they are decelerating. The other components are rated at 35V or more. A future revision of the firmware will disable the drivers if the bus voltage exceeds about 29V.

    Is there a reason why you want to use more than 25V?

    The remote power on/off facility on the Meanwell PSUs appears to be compatible with the implementation on ATX PSUs, so it ought to be work with the Duet WiFi PS_ON output.



  • I think he was asking because of the PSU selection he had, one could only be driven as low as 25.5V and he was worried/wondering if that would damage the drivers.



  • Thanks,
    I read you all and this fully answer my question. I guess 25V shall be fine.
    What is a typical pumping voltage when breaking up to 3-4 volts?
    I guess it depends on how well the powersupply regulation electronics can sink reverse current and of the stepper inertia and inductance.
    We had a problem like this on one of our electronics and we had to change the regulator to cope with it.

    Good thing about the psu on!

    @dc42:

    As Tim says, the limitation is the stepper drivers, which are rated at 30V when the motors are powered, and motors pump energy into the supply rail when they are decelerating. The other components are rated at 35V or more. A future revision of the firmware will disable the drivers if the bus voltage exceeds about 29V.

    Is there a reason why you want to use more than 25V?

    The remote power on/off facility on the Meanwell PSUs appears to be compatible with the implementation on ATX PSUs, so it ought to be work with the Duet WiFi PS_ON output.


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