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2 6
E L E CT R I C AL CONNE CT I ON
S P R I NG 2 01 5
METHOD IN THE MADNESS
Careful cabling
O
nce a cable is properly selected
and installed an electrician
generally won’t think of it again,
and the installation owner or system users
will certainly never consider it.
But if you select the wrong cable, the end
users will have problems and safety may be
compromised.
Retrofitting can be an expensive and
annoying business if you do get it wrong.
Take those that installed the Infinity or
Olsent brand cables – what a nightmare,
particularly if cables were installed
horizontally through walls.
So how do we get it right?
STANDARDS
We need to comply with all the
requirements of the Wiring Rules and
AS/NZS 3008.1.1
Electrical installations –
Selection of cables
.
First there is the general and overarching
guidance provided by Part 1 of the Wiring
Rules. This includes protection against
danger and damage, protection against
electric shock, basic protection, fault
protection, residual current protection, etc.
Then we get into the specific guidance of
Part 2, Section 3, for selecting wiring systems.
You will also need to understand the detailed
requirements of AS/NZS 3008.1.1.
CONDUCTORS
Generally we have a choice of copper or
aluminium.
Copper is a ‘noble’ metal, so it corrodes
slowly and has good conductivity.
Copper is the preferred choice for cables
in an installation, even though it is more
expensive than aluminium. Most fittings and
accessories are fitted with copper or brass
connections, so there is no galvanic reaction
(that is, corrosion – any two dissimilar metals
touching each other are likely to corrode
unless adequate measures are taken).
However, aluminiummay be selected for
longer lengths of consumer mains or sub-
mains, as overall it may be cheaper and is
quite suitable. But you will need to use some
form of bi-metallic connectors, generally
somewhat expensive, to connect at one or
both ends of the cable. Also, a larger conduit
may be required to fit the larger sized
aluminium cable.
The economics may not be there for
shorter lengths.
Certainly you should consider aluminium
when quoting for longer lengths of
consumer mains and sub-mains, but don’t
forget to include the cost of bi-metallic
connectors, heatshrink or links, and larger
conduit, if applicable. You will also need
crimpers, which can be quite expensive.
My son was asked to quote on a
proposed installation where the conduit
had been installed for the consumer mains.
The conduit, if compliant, is a little small
and he may not be able to use aluminium
cable. Copper mains are a possibility, but we
have to check voltage drop and maximum
demand before making a decision.
ENVIRONMENTS
If high temperatures are a factor, then a
mineral insulated metal sheathed (MIMS)
cable is likely to be needed.
If moisture is a particular problem, then
water-blocking (insulation between the
strands) or water-jacketed (includes a metal foil
layer in the insulation) cables may be required.
Ultraviolet stabilisation or protection will
be required if the cable is exposed to the
sun during the day.
Mechanical protection may be required if
there is a possibility of damage from some
mechanical force.
See Clause 3.3.2 of the Wiring Rules for
other types of environmental conditions
that might be encountered, and the
management options.
CONDUCTOR STRANDS
The number and shape of conductor
strands will depend on the degree of
flexibility required.
A single-strand cable is fairly rigid, and a
cable with many strands is quite flexible.
A single strand might be suitable if it is
not moved again after installation, but a
cable that is regularly moved will need to
be flexible.
Seven-stranded cables are mostly used in
installation wiring.
CURRENT CARRYING
The current-carrying capacity of a cable is
affected by the method of installation and
external influences that might restrict its
operating temperature.
Some of the first steps in selecting a cable
are to determine the:
>
maximum demand of the circuit (Clauses
1.6.3 and 2.2.2);
>
route of the cable (this will determine the
installation method);
>
number of conductors (two, three or four
cores); and,
>
normal operating temperature of the
cable (Table 1 of AS/NZS 3008.1.1).
The current-carrying capacity of cables is
detailed in Tables 4 to 21 of AS/NZS 3008.1.1,
but derating or rating factors may also apply.
Derating factors are detailed in Tables 22
to 26 for such things as bunched circuits,
circuits on trays, racks, cleats in air, or groups
Selecting the right cable
is a tricky business.
Chris
Halliday
offers guidance on the
conductive material, insulation
type and cable size for a range
of requirements and conditions.