There are hundreds of different types and
sizes of wire and cable. Although any piece of wire may be used to
join point A to point B, some cables are much better at certain
jobs than others.
Generally, a wire will contain a single
conductor, usually insulated by a plastic sheath so that it will
not short out to nearby metal objects, other wires, or allow
anyone to be electrocuted. Cables usually contain more than one
conducting wire, insulated from each other and then covered by an
overall protective and insulating sheath. Broadly speaking, wires
and cables break down into four different areas:
This usually carries power from one place
to another, either inside equipment or between power sockets and
These usually contain at least one core
wire, covered in an insulating sheath, which is then further
covered by an over-all grounded metal shielding layer that
protects the inner wire from acting as an aerial and picking up
interference from nearby power cables or other equipment. The
protective shield layer is, in turn, covered by at least one
protective and insulating plastic sheath.
RF cables are constructed in a very similar
way to single core audio cables, but are specially constructed to
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 from the world around it, and sometime transmitting
signals that may cause interference to nearby equipment. Use of
the correct RF cable will prevent this happening.
These are 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 the data
signals from being radiated to other sensitive equipment nearby.
The first thing you need to do is to work
out which of these basic types will be most suitable for the job
you want your wiring to do. From the descriptions above, this
should be fairly easy, but then the more difficult part begins.
The answer to this is yes, you probably can
make the cable as long as you need to, provided you think
carefully about the job the cable will be doing, and where it is
going to be used.
Once you know what sort of cable you need,
you will probably find that there are many different wires or
cables that would seem to do the job. However, not all cables will
do the job as well as others. Letís go through our main four types
again but look at the reasons you might choose one cable or wire
The main considerations here are the amount
of current to be carried by the cable, and whether it needs to be
very flexible, or is fixed in position once wired up.
Cables and wires have a maximum current
rating and this should never be exceeded. In fact you should
choose a wire or cable with a rating of about 30% higher than the
maximum current you expect to flow in the circuit.
If you donít know the current that will be
drawn by your circuit, you can either measure it using a
multimeter or calculate it if the power consumed by the circuit is
known (rated in Watts). It is simple to work out the current that
will flow in a supply cable 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 =
For mains electrical use, cables are
supplied in just a few standard current ratings so, once you know
the current drawn by the device to be wired, you simply need to
look at the appropriate range of cables and pick a cable with a
rating comfortably higher than the figure you have calculated.
Cables between mains sockets and equipment
generally need to be flexible, and multi stranded round cables are
usually the best choice here. The flat three core single strand
mains wiring (twin and earth) used within walls and conduits to
distribute mains around a building are inflexible and prone to
fatigue and break if used in a position where they are allowed to
The choice of mains distribution cables in
a house or other building is, in any case, covered by the IEE
Wiring Regulations, which set out the different current ratings of
cables to be used for the various types of ring main and spur
Within equipment, the same considerations
exist. You need to work out how much current will flow through
internal wiring, then decide whether the wiring will be fixed in
place, or whether a multi-stranded, more flexible wire would be
more suitable, often the case if occasional movement of circuit
boards may be necessary for service or modification.
Power supply and switching contact wiring,
amplifier outputs and so on will be carrying the highest currents,
and you need to choose thicker wires to carry these without
heating. Thinner wires offer more resistance than thick ones,
meaning that power will be lost in the wiring if too thin a wire
is used. The longer you need a lead to be, the more resistance it
will present to the circuitry it is connected to. This means that
signal or power will be lost in the lead, but you can minimise
this by choosing a cable with a lower resistance, usually a
This is particularly important where the
voltages involved are fairly small, and proportionally more
voltage drop occurs across a long lead. For example, the use of
thin wiring to supply an amplifier in the back of a car from a
fuse under the bonnet may result in a drop of several volts along
the length of the lead when the amplifier is being used. This
reduces the power supply available at the amplifier to perhaps 9
Volts, resulting in a poor quality distorted sound at anything
above medium volume.
So, use a thicker wire for less loss and
better performance and efficiency, especially over long lengths.
Having decided that you need a screened
audio cable (except for loudspeaker outputs) you simply need make
a suitable choice from the many cables on offer.
The simplest audio cables a
re 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. They are not suitable for longer runs or for high
quality audio connections.
Cables with braided screens are good for
general purpose audio use. They allow a high quality signal
transfer over short to medium runs and are available in single and
twin versions, for mono or stereo connections.
At the top of the range come various
special cables for the very best signal transfer and minimum
interference and hum pick up. High grade cables include oxygen
free copper, special additional insulating sheaths, silver plated
conductors for minimum connection resistance and surface
Where capacitance per metre is quoted,
lower capacitance leads will have less of a loading effect on high
frequency signals and are suitable for longer runs.
If you need to make up long leads for line
level connection of, for example, a video recorder and a Dolby
surround amplifier, then 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 fine.