How to steal spare pairs from data wiring

The 8-position, 8-pin modular jacks commonly used for 10Base-T and 100Base-T data networks (and commonly mis-identified as "RJ-45" jacks) have eight wires connected to them.

HOWEVER, at the far end, usually a network hub, switch, or router, only four of the wires (two pairs) are connected.

That means that half of the copper running around inside your walls is probably not carrying any data.

If you need to connect another PC, or a phone, where you already have a data jack, and it would be difficult, time-consuming, or costly to run new wire to that location, it could be very tempting to steal a pair or two of the un-connected wires from the existing jack, and connect them to a new jack.

Here's the bad news: If you do this, there are a couple of guys up high on a mountain in Switzerland, or maybe in a Burger King in Silicon Valley, who will be extremely pissed-off at you for violating the hallowed ethernet standards.

Here's the good news: We won't tell them about what you did, and if no one else rats you out, they won't find out; and even if they did find out, they can't fine you or put you in jail or confiscate your tool kit.

The minor warning: Theoretically, there is some loss of data speed when you steal a pair or two, but daring and resourceful and lazy geeks have been doing this for years, and no one noticed any difference in the speed of sending email, playing games or downloading porn.

The major warning: The spare pair scam works with 10Base-T and 100Base-T wiring, not with 1000Base-T, and maybe not with power-over-ethernet wiring. So, know how the network is being used, and how it is likely to be used in the future, before you begin your surgery.

The chart below shows the "available" wires in both 568A and 568B wiring plans:

Jack Pin #	Wire Color (T568A)	Wire Color (T568B)	Signal
1	White/Green	White/Orange	Transmit+
2	Green	Orange	Transmit-
3	White/Orange	White/Green	Receive+
4	Blue	Blue	Unused!
5	White/Blue	White/Blue	Unused!
6	Orange	Green	Receive-
7	White/Brown	White/Brown	Unused!
8	Brown	Brown	Unused!

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Stealing data wire to use as phone wire is pretty straightforward. If you need just one pair, grab the white/blue pair, and put it on pins 4 and 5 in the new jack. If you need a second pair, grab the white/brown pair, and put it on pins 3 and 6 in the new jack. Blue goes where blue belongs, and brown looks kind of like orange, so it's an easy job.

If you need to connect a PC, rather than a phone, it can be a little bit trickier. You're going to need the white/blue and white/brown pairs again, but they'll need to go on pins 1, 2, 3, and 6 in the new jack.

Make sure you do the same things on both ends, and leave notes and tags to explain what you did to help anyone who has to work on the network or phone system after you.