Process to determine wire size

The size (cross-sectional area) of a conductor must allow it to carry the load current safely, without overheating and without an excessive voltage drop. The steps are:

1. Find the total load. Add the power ratings of all appliances on the circuit:

$$P_{total}=\sum P$$

2. Calculate the load current.

Single phase:

$$I=\frac{P}{V\cos\phi}$$

Three phase:

$$I=\frac{P}{\sqrt3\,V_L\cos\phi}$$

3. Apply a safety margin and diversity factor. Increase the current by 20-25% for future expansion (or use a diversity factor if not all loads operate at once).

4. Select the wire size for current-carrying capacity. From the manufacturer's or a standard table (such as IS/IEC), choose a conductor whose rated ampacity is at least the design current, considering the ambient temperature, the method of installation (conduit, open) and the number of cables grouped together. Typical copper sizes: 1.5 mm² (lighting, up to 10 A), 2.5 mm² (power sockets, 16 A), 4 mm² (geysers, ACs, 25 A), 6 mm² (32 A).

5. Check the voltage drop. The drop in the cable should not exceed about 3-5% of the supply voltage:

$$\Delta V=I\cdot R=I\cdot\frac{\rho L}{A}$$

Here $L$ is the total conductor length (go and return for a single-phase circuit). If the drop exceeds the limit, choose a larger size.

6. Check the short-circuit withstand and coordinate with the protective device (MCB rating at or above the load current and not above the cable rating).

Example: a 2 kW heater at 230 V, pf 1, draws $I=8.7$ A; with a 25% margin this is 10.9 A, so a 2.5 mm² copper wire with a 16 A MCB is selected.