After moving into our house (built in mid-80s), we noticed a blank wall plate right about where you would expect a wall phone jack to be in the kitchen. I opened it up and found a loop of 3 pair telephone wire. I cut into it and hooked up the green and red wires from both sides to keep the loop intact, but had no dial tone. Opening up our other phone jacks, I discovered that the jacks seem to be wired in series, with the incoming pair and outgoing pair altering between red/green and blue/white (see image below). After using the blue/white pair instead of the red/green pair, the jack worked. Is there any reason why this would have been done originally? In particular, is there a reason for me to go back and restore the pattern by switching out the appropriate wires?
Telephone line color code changes
telephone
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Probably you have two or more phone jacks daisy-chained together. One cable goes to wherever the phone line enters the house and the other runs to another phone jack somewhere else in the house.
This is a common practice. I did some pricing online, and it seems that Cat 5e is comparable in price to 4-conductor phone cable, so the builders may simply have used it because they had it to hand.
L1, L2 cross-references to TIP and RING respectively per standard US phone nomenclature. TX/TS can be wired as the second line TIP2/TX, RING2/TS or a sort of daisy chain, but in a single line system, can be ignored.
Wiring on this will be
- Green => Tip => L1
- Red => Ring => L2
This pair will connect to the center pair of contacts in the modular socket/jack. Most phone wall cords sold are RJ-11 with a single pair to save money.
What the phone company has given you is a standard T-568A (Cat 4 or 5) color coded single pair. If you examine it closely, one wire's insulation will be white with blue stripe and the other will be solid blue.
It cross-references to the old RING/L1, TIP/L2 wiring convention as follows:
- White/Blue => Green => Tip => L1
- Blue => Red => Ring => L2
RJ Modular wiring:
- RJ-11 Single Pair
- RJ-14 Two Pair
- RJ-25 Three Pair
POTS Local Loop voltages (positive ground to reduce electrolytic corrosion):
- Tip is at ground potential
- Ring is -48V DC (nominal)
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Best Answer
As you have found, traditional US household telephone wire contains two pairs of conductors. This was normally used to allow two telephone lines to be brought into the house, each using one pair, with both lines available at all places to ease switching back and forth or to let phones with additional switches (or, later, circuitry) switch between those two lines.
Red/green was usually the primary line, buy it was common for some wall jacks to swap the pairs so a single-blind phone would connect to the other line.
In older buildings, where the phone wiring has been changed repeatedly, the two lines may have gotten cross-connected in ... creative ... ways. Ma Bell was somewhat notorious for sloppy, least-effort solutions.
(Alternatively. rare now, you may also occasionally see the second pair just connected to a transformer. These were used to power the lights in "Princess" phones.)