A Complete Guide to Telephone Cable

Telephone cables connect circuits in a system. For example, they join landline telephones or PCs to telecommunications networks. They play a vital role in our interconnected world, transmitting both sound and data from location to location.

Telephone cable makes use of electrically conductive materials such as copper wiring to transmit data electromagnetically. The outer insulation is typically PVC (polyvinyl chloride) or polyethylene (PE). They are low-frequency devices, meaning that they operate below 300 kilohertz.

This guide offers a comprehensive introduction to telephone cables. Learn how they work, what they are used for and the different types available.

There are three broad types - internal, external, and internal/external. Telephone cabling designed for external use is thicker and designed to be weatherproof, while internal cabling is more flexible and can be used in a variety of locations. Internal/external cabling is the most versatile option and can be used both indoors and outdoors.

Two further distinctions can be made:

• Flat cords vs coiled cords. The former connects telephones to wall sockets while the latter is the familiar coiled cabling used to connect receivers to a landline handset. Coiled cable uses a separate connector to the main phone line, typically a smaller RJ9 or RJ22. It can be longer than it appears - for example, a one-foot length of coiled cabling might be six feet in length when uncoiled
• Straight cords vs reverse cords. The first type is principally used for data, while reverse cords are for voice services. Straight cords feature an exactly duplicated arrangement of wires to pin connections on either side of the cable, while in reverse cables, the order of connections is mirrored i.e. in reverse order on the second side

Other noteworthy types include:

• The RJ11 telephone cable is an international standard and the best option for connecting single telephone lines to single devices. It is used in domestic and commercial settings in the UK and around the world
• Overhead telephone cabling is used outdoors for voice, data and even TV signals. These heavy-duty cables are mounted on utility poles and are especially popular in North and South America

Here in the UK, BT has established a number of commonly used telephone cable specifications. These are known as the CW1308 standard.

Cables made to the BT specification typically feature paired 0.5mm insulated copper conductors, but the number can vary hugely, from as little as 2 to as many as 200. The insulating sheath is normally PVC, but you may also see:

• LSF (low smoke and fume). This refers to the insulation’s resistance to fire. Conventional PVC emits thick black smoke and hazardous (Halogens) fumes when it burns or is exposed to high temperatures and this can interfere with attempts to escape in the early stages of a fire. LSF has been formulated to do neither
• LSOH or LSZH (low smoke zero halogen). Hydrogen chloride - a halogen, or salt-producing chemical element - is another dangerous gas emitted by some types of PVC when it burns or melts. It can cause breathing difficulties and lung damage if inhaled or skin burns if touched. If mixed with water it will form hydrochloric acid. LSOH insulation will emit neither fumes nor halogen in the event of a fire

A variation of the standard, the CW1308B, has reinforced insulation for use both outdoors and indoors. This resists not only weather but also UV (ultraviolet) light.

The CW1128 is an additional BT telephone cable specification. This is designed solely for outdoor use, with reinforced armoured models suitable for use below ground.

The term core refers to the individual conductors within a cable, each of which transmits different forms of data. In telephone cables, they are typically made from copper wire, with a diameter from 0.3 to 0.9mm, and an insulating PVC or PE jacket about half as wide again. However, aluminium and aluminium alloys are now gaining popularity. These are generally twice as large. Telephone cables may contain multiple cores, each twisted into pairs.

Common telephone sizes include:

• Two cores, one pair
• Four cores, two pairs
• Six cores, three pairs
• Eight cores, four pairs
• Ten cores, five pairs

In addition to the specifications discussed above, UK telecoms giant BT devised a now-standard colour code for use with six-connection telephone sockets:

Connection	Cable Colour	Function
2	Blue	Dial tone, speech
3	Brown	Activates extension phones on the same circuit
5	Orange	Dial tone, speech

Connections 1, 4 and 6 are not normally connected.

An older version of this code is sometimes still encountered by electricians. These can be swapped to the new colours when rewiring is needed. The older colour code is:

Connection	Cable Colour	Function
2	Blue with white bands	Dial tone, speech
3	Orange with white bands	Activates extension phones on the same circuit
5	White with blue bands	Dial tone, speech

RJ11 cables combine two pairs of colour coded wire - green and red, plus black and yellow. These are attached to a modular telephone jack. They use the RJ telecoms connection standard.