Air-conditioner cable size: how many mm² do you need?

“How many mm² does this AC need?” is a question we hear every day from contractors and technicians. The right answer starts with a small label on the outdoor unit: the nameplate. This guide walks you from the unit’s current to the breaker, then to the cable size after correcting for our ambient temperature, and finally to the run length.

The figures come from the tables of the Saudi Electrical Code SBC 401, which is built on IEC 60364; the sources are listed at the end. The final decision still belongs to the project electrical engineer or consultant, because installation method and room temperature differ between projects.

One ton of refrigeration is 12,000 BTU/h, about 3.517 kW of cooling capacity. That is not the electricity the unit draws; the electrical input is lower and depends on efficiency, which the Saudi standard SASO 2663 for small units now rates by seasonal efficiency (SEER). Two units of the same tonnage can draw noticeably different currents.

Read three things from the nameplate:

• Phases, voltage and frequency, e.g. 220–240 V, 1 Ph, 60 Hz or 380–415 V, 3 Ph, 60 Hz.
• Rated or running current in amperes.
• Central and package units usually add two more: minimum circuit ampacity (MCA) and maximum breaker or fuse (MOCP / Max fuse). When they are given, they are the manufacturer’s binding figures: the cable must not be below the first, and the breaker must not exceed the second.

Council of Ministers Resolution 425 (20/9/1431 H) changes residential and commercial supply from 127/220 V to 230/400 V. SASO notes that 127 V is still found in parts of the Kingdom and is being phased out under the Saudi Electricity Company’s supervision. SEC’s Distribution Planning Standard (2004 edition) sets the nominal frequency at 60 Hz and lists low-voltage service voltages of 220/127 V and 380/220 V at ±5 %. In practice: confirm the voltage at your meter before you calculate.

If you are given power rather than current:

• Single-phase: I = P ÷ (V × cos φ)
• Three-phase: I = P ÷ (√3 × V × cos φ), with V the line-to-line voltage.

A three-phase unit spreads its load over three conductors, so the phase current is lower and the cable can be smaller for the same power.

The core rule in the code (SBC 401 clause 43-3.2) is: design current ≤ breaker rating ≤ corrected cable capacity (Ib ≤ In ≤ Iz). The table applies it to single-core PVC copper wires in conduit (method B1), with the code values multiplied by the 45 °C factor of 0.79:

For these sizes the protective (earth) conductor equals the phase size up to 16 mm² (code Table 54-3), and each AC unit gets its own circuit and breaker in the panel.

The right question is: what current can 16 mm² carry in your site conditions, and what is the total AC current on each phase? From the table above, 16 mm² single-core wires in conduit on a three-phase circuit carry about 53.7 A at 45 °C, so the sub-main is protected at 50 A. A multi-core 16 mm² cable in conduit (B2) is rated 62 A at 30 °C, about 49 A at 45 °C, which means a 40 A breaker.

Illustrative example (the current is assumed; use the figure on your units’ nameplates): if each single-phase AC draws 10 A and the three-phase sub-main has a 50 A breaker, four units per phase make 40 A and leave a margin, so 12 units spread evenly over three phases, each with its own breaker in a sub-panel. Do not load a phase right up to the breaker rating, and check voltage drop if the sub-panel is far away.

A long cable drops voltage at the unit, and a compressor struggles on low voltage. SBC 401 (2007 edition, clause 52-5) recommends no more than 4 % drop between the origin of the installation and the equipment, and the IEC guidance value for non-lighting loads on a public supply is 5 %. The calculation is simple:

ΔV = (V/A/km) × I × L(km)

Example: a single-phase unit at 230 V on a 16 A breaker, 2.5 mm² single-core wires, 40 m: 17.82 × 16 × 0.040 = 11.4 V, i.e. 4.96 %, above 4 %. With 4 mm²: 11.14 × 16 × 0.040 = 7.1 V, only 3.1 %. So 2.5 mm² is fine at 25 m, but at 40 m choose 4 mm². The specific values are in the voltage-drop table of our cable sizing guide.

Aluminium is lighter and cheaper but carries less current for the same size: in method C with two loaded conductors, 16 mm² aluminium is rated about 66 A against 85 A for copper (Table A.52-2). The code also requires terminations used with aluminium to be tested and approved for that use. For AC final circuits we recommend copper; aluminium is an option the designer may study for large mains.