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Thread: Resistance of splices

  1. #1
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    Resistance of splices

    How does the resistance of wire nut splices compare to splices with terminal blocks?? Working on 24v fire alarm equipment that is semi sensitive to resistance and wondering which one would create less resistance in your opinion?? Or looking for documented evidence to support either.

  2. #2
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    Seems pretty easy to solve. Just pull out your multimeter and set it on ohms.

  3. #3
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    Quote Originally Posted by Coppersmith View Post
    Seems pretty easy to solve. Just pull out your multimeter and set it on ohms.
    He is going to need a milli or micro-ohm meter.

    I would suggest making some samples with the materials you are thinking of using, then connect a load & do a fall of potential test.
    Advise is a dangerous gift, even from the wise to the wise.

  4. #4
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    Gregg38:

    Very simple. Do as SG-1 suggested. Both will be a very low resistance.

    Make two samples. Use your normal small wire, probably #22 or thereabouts. Have bare wire extend about 1" from the joint. Create 1 A thru the joint. Use a meter that can resolve 10 microvolts. With 1 A thru the joint measure the voltage between the two sides of the joint with the voltmeter probes contacting the bare wire 1/2" from where the joint starts.

    10 V and a 10 ohm resistor will provide 1 A. The resistor needs to be 10 W or more.

    Joint resistance will be equal in magnitude to the voltage.

  5. #5
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    Using tinned #22 stranded copper wire my results were at 1 A DC:

    Gray wire nut .............. 1.8 millivolt
    300 V AB terminal block 2.0 millivolt

    .

  6. #6
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    Last night I ran the experiment as I described in post #4, 1" of wire before the joint. Number 22 copper is 16 ohms per 1000 ft, or 0.0013 ohms per 1".

    Today I put the probes as close to the joint as was possible.

    Results from last night with 1" of wire:

    Gray wire nut .............. 1.8 millivolt
    300 V AB terminal block 2.0 millivolt

    So about 1.3 millivolts of these measurement are from the wire before the joint.


    With the probes as close as possible to the joint the results were:

    Gray wire nut .............. 0.9 millivolt
    300 V AB terminal block 0.96 millivolt

    Voltage drop across the AB square tube bar:

    300 V AB terminal block 0.03 millivolt

    .

  7. #7
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    Next I took two pieces of the same wire and twisted them together for 1", then soldered the 1" length with 63-37 tin-lead solder. Put the probes as close to the joint as possible.

    Across joint .................................................. ................. 0.6 millivolts.
    From one side of joint to far end of twist ........................... 0.3 millivolts, as expected.
    From one side of joint to on the solder close to start of joint 0.2 millivolt, might have guessed 0.3 millivolts.

    .

  8. #8
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    Quote Originally Posted by Gregg38 View Post
    How does the resistance of wire nut splices compare to splices with terminal blocks?? Working on 24v fire alarm equipment that is semi sensitive to resistance and wondering which one would create less resistance in your opinion?? Or looking for documented evidence to support either.
    20 AWG with blue wire nut, stranded wire, not tinned, 34 milliohms. Some wire resistance in that measurement as sense point on wire at edge of wire nut.

    10 AWG THHN in heavy tinned hydraulic crimped terminal to alodine 600 aluminum plate with torqued NAS603 screw and lock nut = 360 microohms.

    Big difference percentage wise. Those are the only specific ones I remember from testing over the years (IIRC). Also recall the rotating field winding of a 3 MW 11 KV diesel gen set is 84 microohms, remember that as had to measure that once aboard ship off Adak, Alaska.

    Alodined 2 in sq connector backshell to alodine plate with 4 ea #10 screws = 66 micorohms, After 20 years in weather exposure, 64 microohms. Polysulfide sealant on all edges of connector flanges.

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