Residential Telecommunications

Using 8 Position Jacks for Telephone Lines

This is an important note about using 8 position (8P) jacks for telephone circuits. According to Savvy Home Networks:

"In newer homes, the jack size you'll often find is an 8 pin-position connector (commonly referred to as "RJ-45"). These are a little wider than older phone jacks, which were only 6 pin-positions wide. Standards now require 8 position jacks for both network and telephone (TIA-570-B section 5.2.3). Standard phone cords plug into either jack size and work correctly."

Even though the latest residential standard calls for 8P jacks I personally think using them for voice circuits can lead to trouble for a few reasons.

  1. Most residential phone plugs use 6P connectors, not 8P. If you insert a 6P telephone plug into an 8P telephone jack you may not get a snug fit since there will be a gap on either side of the plug. This may lead to short circuits, stuck plugs, and damaged jack pins.
  2. The wiring table at Jacks and Plugs - Voice suggests that phone equipment supporting the 6P, three-line RJ25 standard (though I believe such equipment is rare) will not work with 8P jacks wired to the T568A standard (I haven't tested this).
  3. Most importantly, 8P computer local area network (LAN) connectors are the same size and shape as 8P telephone connectors. IF YOU PLUG LAN EQUIPMENT (E.G A COMPUTER OR A ROUTER) INTO AN 8 POSITION TELEPHONE JACK WITH AN 8 POSITION CABLE THE LAN EQUIPMENT CAN BE PERMANENTLY DAMAGED. This could happen, for example, when LAN equipment is plugged into a phone jack and an incoming call produces a 90V AC ring signal on one of the cable pairs. Most LAN equipment is only designed to handle voltages in the 1V to 5V range. Similarly, connecting an 8P LAN equipment socket to a 6P telephone jack with a 6P telephone cable can damage the LAN eqipment too.

    Electrical equipment manufacturers would never make 110V plugs that fit into 240V sockets. I find it bewildering that telecom equipment manufacturers made it possible for 5V equipment plugs to connect to jacks that can supply 90V.

In my mind the 6P RJ standards listed at Jacks and Plugs - Voice are a safer choice for phone jacks than 8P jack standards like T568A and RJ61. 6P phone plugs will fit 6P RJ jacks properly and 8P computer plugs will not fit them at all.

I'm not alone with this view. Here's another person's view on mixing 6p connectors with 8P connectors.

"The consequences of putting a 6P plug into an 8P ... can cause problems. Some phone plugs get mechanically latched in the RJ-45 housing and won't release. That ruins the jack.

Often when you plug a 6P into an 8P jack, the plug gets inserted crosswise. The Hawaiian word for that is kapakahi, in case you were wondering. A kapakahi plug can cause a short circuit between the pairs inside the RJ-45. In modern homes, you generally have more than one phone line. Your family phone may run on the blue/white pair one but the home office may be on another pair. The short circuit can knock it out of action.

Some RJ-45s serving LANS use POE or Power Over Ethernet. Shorting out one of those pairs is not hazardous but can result in expensive grey smoke coming out of the data boxes."

-- Tom Moore, Home Toys Article - April 2007 - About RJ Jacks

Here also are some excerpts from a discussion forum that looks like it's home to some real pros (Phone System Tech Support: Survey: NEC Dterm line cords). Note that when they refer to RJ11/12 and RJ45 connectors they are really refering to 6P and 8P connectors respectively.

  • "The problem is putting a RJ11/12 plugs into a RJ45 jack is that you damage the outside contact springs which can mean problems if you use the outlet as a data jack in the future." -- paulw
  • "I personaly don't like the 8pin modular jacks and using a 6pin plug in them. Not only can it damage the outer two pins of the 8 pin jack, but the plugs can actually wiggle a little bit and cut off the current flow causing all sorts of intermittant problems." -- Tip&Ring
  • "T&R - I totally agree with you about the problems of pluging rj11 in rj45 jacks. I always terminate my voice cable in a 6p4c rj11 jack. Recently, I was called my a national chain to go correct a problem with an extension that didn't work after their regular installer completed the job and left town. The problem - the extension was wired with a rj45 jack and the rj11 plug would not make a consistent connection. Replaced jack with a standard rj11 and all was well." -- BillyBob
  • "We never install any 8 Pin 8 conductor jacks for any telephone system that uses the 6 pin 4/6 conductor plugs! I have gone behind one too many IT nerds that want to use the 8 Pin 8 Conductor Cat-5,E,or whatever the latest trend is this month jack and show them by wiggling the cord and watching the phone just go nuts that we will not do it and if they still won't go along, I have a disclaimer letter saying that we are not responsible." -- Tip&Ring

To be fair I should note that a few people on the same discussion forum reported no problems at all using 8P jacks.


To mitigate the problems described above I suspect some manufacturers may be manufacturing 8P jacks that accept 6P plugs snugly without getting stuck or doing any damage. The issue of mistakenly plugging LAN equipment into telephone jacks remains however.

Tom Moore mentions a different strategy in the article referenced above. He says there is an insert for 8P jacks that provides a straight path for 6P plugs. I've never seen one of these and I'm not sure whether they prevent 8P plugs from being inserted or not.

Finally, external breakout boxes are available that convert 8P jacks into 6P jacks. However, buying one box for every phone jack or telephone in the home seems like more expense and trouble than simply installing 6P jacks in the first place would be.

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