Choosing the right cable size: current, voltage drop and installation method

Many sites pick cables “by habit”: 4 mm² for the AC, 16 for the board… and it works until a run gets long or a conduit fills up with circuits. This guide lays out the method a design engineer uses, with figures and sources, so you can check your choice before ordering.

The Saudi code SBC 401 is built on IEC 60364: ampacity tables sit in Annex B.52, voltage drop in clause 52-5 and Annex G.52. Follow the steps in order, then take the largest size any check produces.

Work out the load current Ib: single-phase I = P ÷ (V × cos φ), three-phase I = P ÷ (√3 × V × cos φ). SEC’s planning standard requires customers to keep a power factor of at least 0.85 lagging at the interface. Then choose the breaker In ≥ Ib from the standard series (10, 16, 20, 25, 32, 40, 50, 63 A…).

If the method changes along the route (say conduit, then tray, then an insulated wall), size on the worst section. Note that buried cables are referenced to 20 °C soil, while SEC’s standard rates its own buried cables at 35 °C ground temperature; do not use method D values without the ground-temperature correction.

Iz = I0 × k1 × k2, where I0 is the code table value at 30 °C, k1 the temperature factor (PVC: 0.87 at 40 °C, 0.79 at 45 °C, 0.71 at 50 °C) and k2 the grouping factor (two circuits 0.80, three 0.70, four 0.65). The condition is Iz ≥ In. Full tables are in our guide “How many amps can 2.5 mm² wire carry?”.

A note on catalogues: some building wires on the Saudi market use 90 °C-rated PVC rather than 70 °C, as in the Saudi Cable Company catalogue. In that case use the manufacturer’s tables for the actual insulation, and check that equipment terminals are rated for the same temperature.

The working formula:

ΔV = (V/A/km) × Ib × L(km)  →  ΔV% = ΔV ÷ U × 100

with the specific value from the table below and U = 230 V single-phase or 400 V three-phase. SBC 401 (2007 edition, clause 52-5) recommends no more than 4 % from the origin of the installation to the equipment, and IEC Table G.52.1 gives 3 % for lighting and 5 % for other uses on installations supplied from the public network. Follow your project specification.

Worked example: a three-phase sub-board at 400 V, design current 30 A on a 32 A breaker, 60 m away, four-core PVC copper cable in conduit (B2) at 45 °C.

1. Current: 6 mm² = 34 × 0.79 = 26.9 A, below 32, fails. 10 mm² = 46 × 0.79 = 36.3 A, passes.
2. Voltage drop on 10 mm²: 3.93 × 30 × 0.060 = 7.1 V, i.e. 1.77 %, acceptable with margin for final circuits.
3. For comparison, if 6 mm² had passed on current: 6.55 × 30 × 0.060 = 11.8 V, i.e. 2.95 %, using up most of the 4 % before the final circuits.

Result: 10 mm². At 120 m the 10 mm² drop would be about 3.5 %, and the size goes up to 16 mm² (2.48 × 30 × 0.120 = 8.9 V, i.e. 2.2 %).

On a short circuit the breaker must clear before the conductor exceeds its insulation limit. The adiabatic check is k² × S² ≥ I² × t, with k = 115 for PVC-insulated copper up to 300 mm² (115 A/mm² for one second from 70 to 160 °C). A 2.5 mm² wire withstands 115² × 2.5² ≈ 82,700 A²s; compare that with the let-through I²t the breaker manufacturer publishes at your prospective fault current. On large feeders and time-delayed breakers this check becomes decisive.

Once you have the size, order with a complete specification: cross-section and cores (e.g. 4×10 mm²), conductor material, insulation and sheath, armour if any, voltage grade (450/750 V for building wires, 0.6/1 kV for LV power cables), reference standard and required length. Konoz is an authorised distributor for Riyadh Cables, MESC and Bahra Cables, supplying every region of the Kingdom from Makkah; send the specification and we match it against the manufacturers’ catalogues rather than guessing a part number.