I've run Cat5e network cable throughout an office and have a patch panel where ADSL & phones come in.

The wall sockets are all RJ45 so I can run either phone or ethernet over them.

If someone accidentally were to plug their computer into the wrong outlet (one which has the phone signal on it), will it do any damage to the ethernet card in the computer? I think in NZ the lines are up around 45V.

What about if the phone is ringing?

I was thinking of putting a reducer into those ports anyway so it'd be clear that the phone goes on those. i.e. http://www.jaycar.com.au/productView.asp?ID=YT6055

2 Answers 2


In theory this is not an issue with UK/US compatible phone wiring and low speed networks (10/100BaseT) as the standard for running telephony over data cable has the 'hot' phone signal on either pin 4 or 5 of the connector and these are not used for 10/100 networks BUT there's no guarantee that the phone pair have not been swapped over, putting the hot (not grounded) connection on a used pin, or that someone hasn't decided to use their own wiring standard. I'm not sure if Kiwi phone standards follow the UK/US model and didn't have much luck searching out this info.

As soon as other network standards are considered, such as gigabit Ethernet (1000BaseT), then there is an issue because all or more of the line wiring is in use and so your network card will possibly be fried by the ringing voltage on the line.

The reducers seem like a good idea, but that's a lot of money for bits of plastic - maybe look at other suppliers or online auction sites and mark up all sockets so that there's less chance of a mix-up.

Possibly useful wiring tables here: http://www.evonet.com/evonet/index.asp?Page=102&Format=Print

Executive summary:

Something's likely to get damaged sometime!

  • 1
    To be honest that Executive Summary could apply to anything.
    – Rich
    Commented May 13, 2011 at 13:20
  • I'm quite sure you're right. What I didn't realise is that I had assumed 100BaseT uses 2 pairs. I thought it used all 4 pairs of wires. So it makes sense that the voltage of the phone wouldn't hit the used pins. However, for 1000BaseT it might be a problem. I know the reducers are really expensive for bits of plastic. But so is the computer, so for a few dollars I'll use them for the few phones that we have.
    – hookenz
    Commented May 16, 2011 at 10:10

I couldn't tell you for sure, but 10baseT uses +2.5 V or −2.5 V, 100baseTX uses +1 V, 0 V, or −1 V, while in the US on-hook is -48 V, and ringing is 80 V AC, so I would think that there is a good chance that the much higher phone voltages could likely damage ethernet equipment

  • Yes having said that I have momentarily done it without damage. But not when the phone was ringing. Do ethernet cards have protection against this kind of thing?
    – hookenz
    Commented May 13, 2011 at 1:07
  • I did a little searching... One company sells some surge protection devices that they say brings surges down to a "safe" 45V. Also, search for "GR-1089" which is standards for network equipment, though it appears it is mostly concerned with surges and not a steady current
    – mvario
    Commented May 13, 2011 at 13:07

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