After two years, I will graduate as an electrical engineer. My job will require installing bulbs, Washing machines, heaters, refrigerators and air conditioners ... etc. I know that each device requires a specific current. My question is: How to determine the diameter or cross section area of a cable that should be used ? I've googled and found some complicated calculations and complex tables. I would like to know what the easiest way to know the right cable is. I see some electricians can determine the cable immediately when they are asked, and I don't want to take long time in calculations while I'm at work. So, Is there a simple equation or a method to pick the right cable ? Thanks in advance,

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    \$\begingroup\$ The criteria listed are not dependent on voltage. \$\endgroup\$ Jul 24 '15 at 21:05
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    \$\begingroup\$ For the kind of equipment you're talking about, it's more a matter of building code than engineering calculations. Get a copy of your local code or regulations and after you've done a couple of jobs you'll remember the limits for the types of wire you use often. \$\endgroup\$
    – The Photon
    Jul 24 '15 at 21:47
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    \$\begingroup\$ Definitelly there is an easy way, but it comes after a deep and complete knowleges and some experience in the electrical science. Electricity it is not a field to play, since humal lifes at least it stakes here. \$\endgroup\$
    – GR Tech
    Jul 24 '15 at 21:51
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    \$\begingroup\$ Side-note: light-bulbs and washing machines are not 'medium voltage' or 'high voltage' to an electrical power engineer. To a power engineer, "low voltage" is 1,000V, medium voltage is 33,000V, and high voltage is 132,000kV. \$\endgroup\$ Jul 26 '15 at 18:51

I have previously answered some similar questions: How do I tell what gauge wire I need? and High voltage cable cross section .

I will expand a bit more, below.

In Australia, we have two relevant standards:

  • AS 3000 - Wiring Rules. Covers minimum requirements for a safe installation, including that:

    • cables should not melt due to overheating,
    • cables should not catch fire in fault conditions,
    • cables should be large enough to meet a 'fault loop impedance' requirement for safety in case of earth fault, and so on.
  • AS 3008 - Selection of Cables. Includes numerous tables and calculations for calculating the current-carrying capacity of various sizes and types of cables, under various installation conditions.

In some applications there are short-cuts. The final sub-circuits of commercial and domestic wiring, i.e. power points, washing machines, etc. will usually be 2.5mm² copper. Lighting circuits will usually be 1.5mm². And so on. You will learn these shortcuts as you work.

2.5mm² is good for about 20 amps under most installation conditions. But not all installation conditions. Part of your work as an engineer is to determine when the 'short-cut' will do, and when the 'short-cut' is inappropriate and detailed calculations are required.

Things to consider in cable selection:

  • Current requirement of the load (amps)
  • Voltage drop - for long cable runs this is just as much a concern as the current-carrying capacity.
  • Ambient conditions - hot surroundings or poor ventilation decrease the capacity of the cable, so you need a bigger cable for the same current.
  • Cable fault withstand capacity - the ability of the cable to withstand the temperature rise in case of a short-circuit. If the cable is too small, it will catch fire before the fuse/circuit breaker operates.
  • Cable insulation material - EPR cables have a higher rating than XLPE, which in turn has a higher rating than PVC. So if you use EPR, your cable can be smaller than the equivalent in PVC.
  • Physical installation considerations - sometimes better to use two smaller cables than one big cable, if you have to make a sharp bend.
  • Spacing between cables - if installing multiple cables in the same run, they heat each other up, which decreases their capacity, so the cables have to be bigger.
  • Special requirements - metallic screening to prevent radiated electromagnetic interference? De-rating due to non-sinusoidal load current i.e. switching power supply? Full size neutral or half size neutral? And so on.

When you become a professional engineer, and you are responsible for cable selection, you will be introduced to the relevant standards in your country, such as AS 3000 and AS 3008.

Follow them. In my country they are the minimum requirement. No short-cuts. No short-cuts even if the electrician says it's OK.

Usually the electrician knows what they are talking about (more than you, the engineer), but if you are signing off on an electrical design then it is your name on the dotted line, not the electrician's. If it blows up and burns something down, the electrician will be nowhere to be found - the lawyers will come after you.


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