I want to re-wire an AC to DC power adapter for an ONT (Optical Network Terminal - AKA Fibre Box mounted on a wall) and replace the adapter with a standard DC plug. This is because I want to connect it to a UPS (uninterrupted Power Supply).
How do I tell which one of the two wires is the live or positive wire in this picture?

Click to Embiggen
picture of cord

I am from South Africa, if that is at all applicable. This is the cable connected to an ONT.
Click to Embiggen
picture of cord

This is the standard DC pin Jack I want to use:
Click to Embiggen
picture of cord

  • 1
    A photo of the plug blades may help. In US the smaller blade is hot
    – Kris
    Commented Jun 30, 2022 at 12:31
  • 2
    What is a "standard DC plug"? Do you have DC wall service there?
    – isherwood
    Commented Jun 30, 2022 at 12:37
  • 7
    Best bet is to get a multimeter that reads DC voltage. Simple touching of output pin will tell which is positive/negative. Will need the same test on the UPS. Hoping the pins are hooked up the same is foolish.
    – crip659
    Commented Jun 30, 2022 at 15:26
  • 5
    Why not simply plug the white "wall wort" (in pic #2) directly into your UPS? That's how all of my home electronics are protected by my UPSs, including my ONT, router, etc.
    – FreeMan
    Commented Jun 30, 2022 at 15:53
  • 1
    @Criggie, I found a PoE Splitter that takes a standard DC plug/pin as input and provides 12V DC as output over an ethernet cable. I am considering to hook up the PoE Splitter to my mini UPS and use the ethernet output from the PoE as power for the ONT. My question is: would the ONT be able to function with its main power supply left unplug and receive power through 1 ethernet cable while providing internet access through another?
    – Romans
    Commented Jul 2, 2022 at 12:44

3 Answers 3


You don't have a voltmeter. Do you have a potato?

With the power adapter unplugged from your electrical outlet, cut the wires, strip a little insulation from the ends, twist the strands of each wire into a point. Do not allow the bare wires to touch each other from this point on.

Cut the potato in half. Take one half, and poke both wires into the cut face of the potato about 2 cm apart. Plug in the power adapter to the wall outlet. In a short time, the potato around one of the wires will turn green. That is the positive wire.

Unplug the adapter and clip off the ends of the wires so you have clean wire for soldering your new plug.

  • 6
    Oh my I have never heard of this , I would put the wires in the potato before plugging in. And on an insulating surface, on dc I could see it work and since 10v is finger safe it would act like a plating process got to give a + lol
    – Ed Beal
    Commented Jun 30, 2022 at 16:21
  • 7
    This works because the wires are copper, not (as I thought) because of the potato. Which is the case in this example, but that's not completely universal.
    – wizzwizz4
    Commented Jun 30, 2022 at 21:12
  • 11
    "Cut the potato in half. Poke both wires into the cut face of the potato about 2 cm apart." Note that this means, "both wires should be poked into the same half of the potato, not one into each half (with the halves held close together). Commented Jul 1, 2022 at 15:11
  • 6
    In the UK, in some towns from around 1900 to the 1930s or later, municipal authorities provided 110v or 220v DC, and the standard way of deciding polarity involved a potato. This was important for 'AC/DC' vacuum tube radios of the time. Commented Jul 1, 2022 at 19:27
  • 1
    Is it safe to touch/handle the potato after plugging it in? Does the potato need to be placed on an insulator like a rubber sheet?
    – WackGet
    Commented Nov 25, 2022 at 4:20

From personal experience: don't rely on the wire marking in regard to polarity.

I had once the boring task of shortening some power brick wires from a single batch.

I ended up swapping some bricks vs some jacks and - surprise! - about 20% of them failed to work.

Luckilly, the devices were tolerant for wrong polarity so I only lost ~2 hour of work.

Since then, I always use voltmeter.

About 70% of these are marked wire negative, but the other 30% are marked wire positive.

  • Umm... if you were shortening the leads, wouldn't you have simply reconnected "marked" to "marked" and "unmarked" to "unmarked" and not cared one bit which was positive and which negative?
    – FreeMan
    Commented Jul 1, 2022 at 15:38
  • 2
    Good... until you cut all of them to size first and then mix them later.
    – fraxinus
    Commented Jul 1, 2022 at 15:52
  • Ah! It was a process error, not necessarily a marking error. ;)
    – FreeMan
    Commented Jul 1, 2022 at 15:55

In North America the marked (striped or ribbed) wire is neutral (or negative, when it's actually DC). It also has a wider blade on the plug or connects to the outside of a barrel. The conductor sometimes has aluminum blended in with the copper.

I don't know South African standards, but I've never had a universal or international cord that was otherwise.

  • 6
    >Connects to the outside of a barrel. Not if it's a guitar pedal power supply, in which case it's center negative and I have no idea why Commented Jun 30, 2022 at 14:27
  • All of it is copper wire. There is no blend of aluminium. Also, in South Africa, we have poles, not blades. XD
    – Romans
    Commented Jun 30, 2022 at 15:16
  • @BrydonGibson I've played electric guitar for years and had no idea. But because of your last name...we'll take your word for it! (and an internet search confirmed it). Commented Jun 30, 2022 at 15:51
  • 4
    Maybe it's been standardized, but I know that identifying the polarity of the jack is one of the critical steps in using "someone else's" wall wart for a new purpose. Has NA standardized on negative on the outside of the barrel?
    – FreeMan
    Commented Jun 30, 2022 at 15:54
  • 3
    There is not a required standard for dc there are many positive barrel devices I have an older laptop that was positive barrel negative pin or center conductor
    – Ed Beal
    Commented Jun 30, 2022 at 16:27

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