CHRIS HALLIDAY: Careful cabling


power cablesSelecting 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.

Once a cable is properly selected and installed the 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?

We need to comply with all the requirements of the Wiring Rules and AS/NZS3008.1.1 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/NZS3008.1.1.

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, aluminium may 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.

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/NZS3008.1.1).

The current-carrying capacity of cables is detailed in Tables 4 to 21 of AS/NZS3008.1.1, 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 of circuits. Rating factors are shown in Tables 27 to 29 for such factors as cables in heated concrete, soil temperature, burial depth and soil thermal resistivity.

In using Tables 4 to 21, circuit designers or electricians should consider the following cable situations:

  • Vertically or horizontally spaced;
  • touching the surface of, say, wall lining material;
  • exposure to the sun;
  • enclosed or not;
  • partly or completely surrounded by thermal insulation; and
  • buried directly or in an enclosure such as conduit.

They should then go to the correct column of the table.

Voltage drop
Too long a cable run with too much load will cause excessive voltage drop, which may result in unsafe or non-operation of the equipment on that circuit.

This will breach the Wiring Rules, so we must consider voltage drop when selecting a cable (see Electrical Connection autumn 2015 article on voltage drop).

Fault loop impedance
The loop impedance of circuits must be considered for all circuits and should comply with Tables 8.1 and/or 8.2 of the Wiring Rules.

Table B1 provides guidance on circuit lengths. Voltage drop requirements will generally, but not always, be more stringent than loop impedance requirements. Further detail is provided in the aforementioned article.

Short-circuit temperature
Cables may have to endure the thermal effect and mechanical forces caused by fault currents for up to five seconds.
Reputable supply
If the Infinity and Olsent debacle has taught us anything, it is that we should buy only reputable brand cables from reputable suppliers.

This goes for all other electrical equipment and components. The message from government fair trade departments is “if it sounds too good to be true – then it probably is”.

Selecting a particular type and size of cable is not easy, and there will be other factors not included here.

Failure to consider all requirements may mean that people are put at risk, or the electrician must bear the cost of returning to remedy the installation.

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