Phone Extension Wiring 1

Phone Extension Wiring – Which Type of Cable?

I’m often asked about phone extension wiring, usually when folks don’t want move their master socket .

Elsewhere I’ve blogged on how to connect phone extension wiring together. But not on the importance of choice of cable.

You might have “Superfast Broadband” to your premises, but this won’t mean anything if it’s slowed down by sub-standard phone extension wiring.

Some suggestions:-

All Cables Are Not The Same.

Just because it makes a circuit and gives you a dial tone does not mean it’s suitable. Broadband pushes non-fibre technology to its limits – it needs the very best in cable.

Solid Copper Core. Not Stranded.

“Stranded” cable is made up of small copper or steel strands – see our photo above. Broadband signals hate it. It attracts interference. It does not connect easily or reliably in professional-grade jointing devices. Burglar alarm wiring uses it extensively, but that does not have to carry ultra-rapid data signals, just simple voltage. Most professional jointing equipment uses a “punch-down” method of connection known as IDC (Insulation Displacement Connection). This only works well on solid copper cable, not stranded. So don’t use it.

Not Everything that Looks Like Copper, is Copper.

Some cheap cable is aluminium, coated with copper, and called CCA, (copper-coated aluminium). It does not carry signals as well as pure copper cable. It breaks easily, and causes infuriating, time-consuming intermittent faults. The copper rubs off at the joints, allowing corrosion to form. BT used this in their network in the 1970s during a copper shortage, and are still suffering the consequences 40 years later. Look for “CCA” on the outer sheath. It will look like copper on the outside, but the core will be a dull silver colour. Like aluminium, coated in copper, in fact.

The Twist, and Why It Matters.

Electrical signals on a pair of wires can “talk” to each other magnetically and slow each other down, usually when laid side by side over a distance. Professional-grade cable has a twist incorporated into each pair of cables – see our photo above.. This minimises interference “crosstalk”. I have seen broadband speed increase significantly by replacement of a flat under-carpet cable. We replaced it with proper solid core twisted pair cable run along a skirting board. The speed went up right away.

Please Tell Me – What Do The Professionals Use?

A search on eBay for CW1308 will lead to the proper stuff. CW1308 is a BT-specification number for the cable they use for voice and broadband cabling. Some eBay traders are selling it by the length, which is very useful if you don’t need a 50 metre drum. Furthermore, a BT subsidiary called BT cables actually supply it, so you can get the very same stuff that BT/Openreach use, via eBay or trade wholesalers.

If you’re buying anything else, simply check that it is “twisted-pair”, pure copper, and solid core.
What About Cat 5 Data Cable?

Cat 5 network cable is also suitable, providing it’s not CCA. It’s just a bit awkward to terminate in telephone sockets, as it’s 4-pair cable in a large sheath. It’s designed for data sockets.

If you value your broadband speed…

…don’t carry out phone extension wiring with anything less that the proper stuff. Or get me in to replace it if you’ve inherited it.

  1. Frederic Y Zoghbi06-29-2017

    Hi, I’ve read most of your articles and they are very intresting for people with very poor knowledge in electrical wiring and cables who like to do their things themselves… i have a question though that has not been answered in any of your articles: can i connect a telephone cable to an electrical 4mm wire in order to extend my cable? If not a simple connection are there any devices that can be used to link that phone cable to the electrical wire? It is good to mention here that these electric cables are not plugged and do not carry electricity, it just happened that these cable exist at the right place…

    • Rob Govier06-29-2017


      I think I may have addressed this issue by stressing that telephone and broadband cables must have a twist in their pair of wires to function at optimum speed and clarity. Power cables do not have a twist.

      You would probably get some level of service, but certainly not the best.

  2. simon12-04-2017

    Hello, your blog is an excellent source of info on telephone wiring, extremely well presented. I have one question that I have not found the answer for yet – I may not have looked enough.
    I have a very old black BT wire entering my house, the wore colours are not banded like most of your illustrations. The wire was recently chopped during the installation of a new front door. I’d like to repair it and refit the master socket using newer white CW1308 wire. Is there a diagram to show the corresponding colours??
    Many thanks

    • Rob Govier12-04-2017

      Thanks for your kind words. The black cable is called “drop wire”. A quick Google will tell you more about it, but in brief, there are normally two pairs of copper wires in it, and around four white-sheathed spring-steel wires which give it support when suspended from a pole. Caution – there are VERY sharp when cut! They usual wiring colour convention is orange and white for the first (copper) pair, green and black for the second pair. If you have only ever had one line into your house, it’s likely that the orange and black are the active pair. Please note that the cores are 0.9mm, whereas normal white-sheathed internal cable is 0.5mm core, so the two don’t punch down reliably together. You may need jelly crimps. I hope that this helps!

  3. Steven Vine02-18-2018

    Hi Rob

    Thanks for the cable information and general information on your site.
    I have a cable related question (hope this is the right place for it).
    I have to run a 32A 240v ring main, and it will have to run parallel with the incoming BT cable, that connects directly to the Master socket, for about 2m. I can only get the mains cable about 100mm away from the BT cable. Do you think there will be any interference if I do this?

    And in general, does higher power mains cabling have any affect on the BT cable signals when in close proximity? Are there any BT guidelines for these situations?

    Kind regards


    • Rob Govier02-18-2018

      Hello Steve.,

      Three-compartment dado trunking is used extensively within office environments, the main compartment containing ring main cables, and the upper and lower compartments housing data and voice cables. The space between the cables varies around a mean of 100mm, so I think you should be fine.

      Opinions of even highly-experienced ex-BT engineers vary on this topic, and I’m not aware of a BT standard on this. The general guideline is to avoid parallel running with mains cables if a all possible – it removes one extra potential source of fault. However, in the real world, this is usually not possible. Therefore, the observation in my first paragraph applies.

      I discussed this very issue with an electrician on an installation job this week, and he recalled a situation where an alarm cable gave odd faults. It was discovered that it had been buried in the very centre of a large loom of ring main cables, and some strange induction effect was being caused. As with all data cabling, neat is not always best, when it comes to looming cables, especially mains and data.

      In short, I think you should be fine!

      • Steven Vine02-24-2018

        Hi Rob

        Thanks very much for the information. I was not aware of the extensive world of compartmental dado trunking, and a bit of googling has opened my eyes! I,ve seen it all my life in offices, but never gave it any thought.

        Going by what you say I will do the cable runs, keep the separation as great as I can, and deal with any problem afterwards (if any occur).

        Thanks again.


        • Rob Govier02-24-2018


          You’re welcome! If you are able to help boost my on-line searchability by leaving a comment on Facebook, Google+ or other social media channels, please, that would help me to continue to provide free advice.



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