Electrical wiring: standard colours, common sizes, and how cable size is decided

Knowing wire colours isn't only for electricians; it helps you understand what you see when removing a socket or light fitting, and spot non-compliant wiring.

1) Colours under IEC 60445

The international standard IEC 60445 (7th edition, 2021) sets the conductor identification colours: neutral blue, AC line conductors black, brown or grey with no preferred order, and the protective conductor green-and-yellow1. The table below is the allocation used in the UK according to the IET table2:

The allocation in the table is British, not a Saudi requirement; we could not open the Saudi Electrical Code SBC 401 to check which colours it adopts, so ask the design engineer. In older homes you may find other colours (such as red or black for phase). Don't assume a wire's function from its colour in old wiring; it must be tested with instruments by a technician.

2) Circuit division and cable sizes

Sizes are measured in square millimetres (mm²) of copper. Schneider Electric's electrical installation guide summarises what national standards typically recommend for dividing circuits in a dwelling3:

The guide adds that phase, neutral and earth conductors in a circuit should be the same size up to 32 A circuits3.

How many sockets and lighting points per room?

An example of recommended minimums in dwellings, from the same guide3:

This is an indicative minimum from an international reference. Actual design and socket positions in bathrooms are governed by zone requirements and the Saudi code SBC 401.

3) Why is there no size that "fits everything"?

Choosing the correct cable size is an engineering process under IEC 60364-5-52 and the local code, depending on:

1. The load current and the breaker protecting the circuit (the breaker must protect the cable).
2. Circuit length: the longer the cable, the greater the voltage drop, and a larger size may be needed.
3. Installation method: in conduit in the wall, surface, buried, or grouped with other cables in the same conduit.
4. Ambient temperature: in sun-exposed roofs temperature rises, reducing the cable's current capacity.

The installation guide confirms this: conductor size and protective device rating depend on the loads, location temperature, the construction material the cables pass through, installation method, and the influence of neighbouring circuits3.

4) Voltage, phases and the main cable

• Voltage: a Council of Ministers resolution changes the voltage system in residential and commercial units from 127/220 V to 230/400 V4. Details in Voltage and plugs.
• Single or three phase: a three-phase supply brings three phases and a neutral (colours in Section 1). The meter type and capacity are set by the utility based on the approved loads in the design, not by the owner alone.
• Spreading circuits across phases: the designer balances the load; adding ACs or heaters later on one phase can trip the breaker or cause heating. Any large addition should be reviewed with a qualified electrician.
• Wire vs cable: a wire is one insulated conductor run inside conduit; a cable is several conductors in one sheath (such as the main cable from the meter to the consumer unit). The main cable size is part of the design, not a site guess.
• Where do concealed cables run? The well-known British rule: vertically or horizontally in line with an accessory, or within set zones near wall corners; otherwise cables need other protection5. Ask for the drawing before drilling into a wall.

This section is for understanding and requesting the right documents, not for working on live electricity. See Breakers and the electrical path and Earthing.

5) When buying cable

1. Check the real cross-section; some counterfeit products are smaller than stated.
2. Buy cable complying with approved standards, from reliable sources.
3. Stick to the standard colours to ease maintenance and protect whoever works after you.
4. Ask for the circuit schedule (which breaker feeds which points) and keep it.