I put 12V DC through a pair of twisted wires of a 50 meters (~160 feet) UTP cat.5 cable and noticed that there was a voltage drop at the other end of the cable. It only had 5V. If I'm feeding the same 12V through two wires that aren't paired (twisted), I don't get that voltage drop. What's the phenomenon that causes the voltage drop?

  • \$\begingroup\$ Before you create new tags, look to see if there are any that are close enough to work for you. Also only create ones that you think might be used by a fair number of people. \$\endgroup\$
    – Kellenjb
    Aug 8, 2011 at 12:29
  • \$\begingroup\$ Thanks, I'll be more careful next time. I wasn't aware I created a new tag. \$\endgroup\$
    – m.Alin
    Aug 8, 2011 at 12:38
  • 2
    \$\begingroup\$ Any per wire resistance measurements? Will be of interest. \$\endgroup\$
    – Russell McMahon
    Aug 9, 2011 at 11:12
  • \$\begingroup\$ I hadn't had the chance to do any resistance measurements. I changed the power to be on two wires that are not pairs/twisted and it worked. No more voltage drop. I suspected that the twisted wires were the cause; it might have been just a coincidence. \$\endgroup\$
    – m.Alin
    Aug 9, 2011 at 12:56

3 Answers 3



  • It sounds highly likely that you have got a sample of 'known to exist' Asian sourced knock-off clones of genuine produts, which use variable conductor sizes and resistances within the same cable and copper clad aluminum.

  • Applied brand names and labelling can be the same as genuine products.

  • Look at the wires closely - if you have copper aluminum it's a cheap fake.

  • Measure wire by wire resistances - see below. All should be the same and all should be under 20 ohms per 100m loop resistance. (For exact value - see below).

But first - just in case:

  1. I'd measure end to end resistance of each wire. As will be seen below, you may get 6:1 (or worse of different) resistances within the same cable.

  2. You don't say what load you are placing on the cable when getting this result - either in terms of load resistance or current drawn.

    If you measure with a voltmeter and with no load the result should be essentially 12 volts. If not then you are doing something wrong or something starange is happening. Voltage drop is caused by current flowiing in a resistance - a voltmeter alone draws ~= zero current so there will be ~= zero voltage drop.

With a say 10 ohm loop resistance (see below), to get 5V out from 12v in the load resistor would have to be 10 * 5/(12-5) =~7 ohms. Less if your loop resistance was less. If your load is > 7 ohms them summat aglae. Read on ...

What you should see:

Cat 5 cable can be

  • Westpenn 20, 22 or 24 gauge. Loop resistances per 100 metres are 7 / 11 / 18 ohms

  • AMP Cat5e 24 gauge, 9.4 ohms/100m

  • Wikipedia 19 ohms.


Low quality clones

  • Uh Oh !!! Electronicspoint 20 + 40 + 120 ohms,

    • Whilst doing some dc loop resistance tests on some 'certified' cat5e cable I found that green and orange pairs had 2x the expected resistance, and the blue and brown pairs had 6x the expected resistance. Close inspection revealed cooper clad aluminium with the copper thickness varying depending on the pair....

And other people comment

  • Section of this document http://www.commsalliance.com.au/__da.../S008_2010.pdf gives the Australian requirements for approved telecommunications cables. Other than coax, which is permitted to use a copper-clad aluminium centre conductor - provided the centre conductor is at least 2mm in diameter, all other cables must be either plain copper or plated (tin) copper. Table 3 gives the resistance requirements.


  • That cable is definitely a Chinese knock-off and is not UL certified.

  • \$\begingroup\$ Actually, copper-clad aluminium doesn't have to be a bad thing, thinking of skin effect. Well, copper thickness should still be more than 2\$\mu\$... \$\endgroup\$
    – stevenvh
    Aug 8, 2011 at 13:31
  • 1
    \$\begingroup\$ The point was that, according to the documents I referenced (reader to decide merit) (1) Copper clad Al is not allowed by formal spec so its presence indicates "a problem". (2) Plus clone cables have different resistances on different pairs whereas spec does not allow this. \$\endgroup\$
    – Russell McMahon
    Aug 9, 2011 at 2:07
  • \$\begingroup\$ @stevenvh - cable meeting or exceeding the spec, when it comes to wire gauge, is very important for proper operation of PoE (802.3af/at). \$\endgroup\$
    – Les
    May 31, 2015 at 18:12

It is caused by the resistance of the wire. Cat5 wire gauge is usually 24 AWG which would be 2.567 ohms per 100 feet. Your run is 160' * 2 = 320' 320/100 = 3.2 * 2.567 = 8.21 ohms total loss in the wire run. The small wire gauge is acting like a resistor in series with your load.

I am assuming the cable end is terminated into a load of course.

What gauge wire of the untwisted pair did you use to test with? I'm guessing it was a larger wire size that had less resistance losses. If you terminate your cat5 with baluns, or loads you will likely see the loss of voltage.


If you have a voltage drop that large it can hardly be caused just by the cable's internal resistance (a Cat5 cable has a loop resistance of less than 2\$\Omega\$/10m). If you connect a 10M\$\Omega\$ DMM at the end you have a voltage drop of 10\$\mu\$V over the cable without other load.
My guess is that you have load resistors inside the connectors. This would also explain why you don't get the voltage drop on non-paired wires: the load resistor is placed between paired wires only.

  • \$\begingroup\$ Actually, It wasn't a fixed voltage drop. The voltage drop increased with the distance. For example, at 10 meters I would have 9V, at 25 meters I would have 7V and so on.. I know it has something to do with the length of the cable.. \$\endgroup\$
    – m.Alin
    Aug 8, 2011 at 12:01
  • \$\begingroup\$ @m.Alin - How did you get these figures? Did you cut the cable? \$\endgroup\$
    – stevenvh
    Aug 8, 2011 at 12:06
  • \$\begingroup\$ Also, do you have some load on the line or are you just hooking a volt meter up to it? \$\endgroup\$
    – Kellenjb
    Aug 8, 2011 at 12:08
  • \$\begingroup\$ Yes, I cut the cable at different distances. I had loads on the cable. I tried to power a few CCTV cameras over a pair of wires. I should also mention that I used baluns. \$\endgroup\$
    – m.Alin
    Aug 8, 2011 at 12:45
  • \$\begingroup\$ But the strange thing is that if I apply 12V to two wires that are not twisted, I don't get the voltage drop! \$\endgroup\$
    – m.Alin
    Aug 8, 2011 at 12:47

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