There are hundreds of different types and sizes of wire and cable. Some cables are much better at certain jobs
An electrical wire will generally contain a single conductor, usually insulated by a plastic sheath
protecting against short circuit to nearby metal objects.
An overview of cables
Wires and cables break down into four different areas:
Electric wire or cable
Usually carry power from one place to another; either inside
equipment or between power sockets and equipment.
Contain at least one core wire, covered in an insulating sheath
and then by an overall grounded metal shielding layer. The
shielding layer protects the inner wire from acting as an aerial
and picking up interference from nearby power cables or
equipment. The protective shield layer is, in turn, covered by at
least one protective and insulating plastic sheath.
Radio frequency cables
Constructed in a very similar way to single core audio cables,
but allow high frequency signals to connect from one place to
another, as efficiently as possible, without picking up any other
radio or interference signals. Any piece of open wire connected
to RF sensitive equipment will act as an aerial, both receiving
unwanted signals and sometimes transmitting signals that may
cause interference to nearby equipment. Use of the correct RF
cable will prevent this happening.
Made up of a number of insulated wires that allow many data
signals to be connected from one digital circuit to another. If
the data has to travel from one piece of equipment to another,
the wires are sometimes wrapped up in an overall shield or
screen layer which prevents data signals from being radiated
to other sensitive equipment nearby.
|Cables in detail
Cables and wires have a maximum current rating that
should never be exceeded. Choose a wire or cable with a
rating of about 30% higher than the maximum current
expected to flow in the circuit.
Either measure the current using a multimeter or calculate
from the power consumed by the circuit (rated in watts). If
you know the voltage supplied and the power used; simply
divide the power rating of the equipment by the voltage
supplied to it. For example, a 1kW bar fire supplied with 230
volts will draw 4.35 amps (1000/230 = 4.35).
Mains electrical use
Cables between mains sockets and equipment generally need to be flexible and multi-stranded round cables are usually the best choice.
The flat three-core single strand mains wiring (twin and
earth) is used within walls and conduits to distribute mains
around a building. It is prone to fatigue and could break if
used in a position where it is allowed to flex.
The choice of mains distribution cables in a house or other
building is covered by the IEE Wiring Regulations that set
out the different current ratings of cables for the various
types of ring main and spur wiring.
The simplest audio cables are the thin lapped core cables,
suitable for connections between audio or other low level
signal boards inside equipment, or very short external runs
in non hi-fi applications.
Cables with braided screens are good for general purpose
audio use are available in single and twin versions, for mono
or stereo connections. They allow a high-quality signal
transfer over short to medium runs.
There are various special cables for the very best signal
transfer and minimum interference. High-grade cables
include oxygen-free copper, special additional insulating
sheaths, silver-plated and gold-plated conductors for
minimum connection resistance and surface resistance.
If you need to make up long leads, use the best cable you
can afford for the job. If, on the other hand, you just need a
point to point run in a piece of equipment, an inexpensive
thin lapped cable will be sufficient.
Loudspeaker wiring is very different and needs a little
consideration. Little loudspeakers, such as intercoms,
can be wired up using almost any wire. Hi-fi loudspeaker
cabling and in-car entertainment speaker wiring is much
more critical. For any amplifier delivering over a few
watts, a thicker and more substantial cable is needed for
good quality reproduction of sound.
You should select heavy-duty and hi-fi loudspeaker
cables. If the bulk is a problem, there are several special
flat variants that go under the carpet easily. Silver-plated
and oxygen-free copper are also available and provide a
very clean transfer of power to your loudspeakers.
Radio frequency cables
The first thing to consider is impedance; a combination
of DC resistance as well as capacitive and inductive
effects in the cable. It may be considered as the
effective resistance of a cable to an AC signal.
Most RF equipment is designed to work with a cable of a
specific impedance: TVs, DVD players and satellite
equipment are designed to use 75Ω cables, whereas
radio transmitters use 50Ω cable.
Select a cable with the correct impedance for the job,
otherwise the signal could be lost, resulting in poor
reception or, in the case of transmitters, heat up or burn
Attenuation and impedance
Attenuation is a measure of how much signal you will
lose over a given length of cable at a stated frequency,
expressed in Decibels (dB). A cable of the correct
impedance for the application, with the lowest
attenuation figure at the required frequency range will
give the best results.
Networking connections that use the BNC connector are
designed to use 50Ω RF cable. Newer connections use
special multi-way twisted pair cables to reduce both
radiation and interference.
There are various standards for these data cables
(Category 5, for example) and it is always best to check
the instructions supplied with each unit before choosing
What else do I need?
Cable strippers, a craft knife and sharp cutters - makes
the job a lot easier if doing lots of cabling
Soldering iron - when connectors need soldering on to the
Small screwdrivers - will be needed for any screw-fixed
Multimeter or continuity tester – indicates that the
completed cable and connector connect correctly, with no
Conduits or trunking - box up, hide and protect long cable
Cable ties, lacing cord or spiral wrapping – secures wiring
Cable clips - help tidy your wiring and keep it in place
Cable protector - if a cables need to be run across a floor,
consider using a cable protector; a heavy rubber strip that
the cable lies protected inside.
Can I make cables as long as I like?
No. Cable length is restricted by the attenuation
characteristics of the cable. E.g cat5 runs are limited to
100m in length due to signal loss, USB cables are
restricted to 5m due to signal loss. The maximum length of
a cable run is dependant on the cable, characteristics and
the signal being sent down.
How do I join the ends up?
If you have many different terminations, you should use
coloured or numbered sleeving at each end of each
wire to aid fault finding or rewiring. Heat shrink sleeving
joins cables where no connectors are required and shrinks
over the individual joins to provide insulation or an
additional protective layer.
Cable glands and grommets allow neat entry to boxes or
through panels, and prevent cable chafing.
The ends of your cable will need to be securely attached
to the equipment, by screw or clamp terminals or via a
suitable set of connectors.
Check before choosing connectors that their current and
voltage rating is sufficient and safe for the job in hand.
RF cables must be properly connected and joined by
suitable RF connectors of the same impedance.