Electrical Installation Design and Calculations
Installation design and calculations test whether you can size a circuit from the load through to the protective device and prove it is safe: design current, device rating, cable capacity, voltage drop and short-circuit withstand.
The design sequence
- Assess the load and apply demand/diversity factors to obtain the design current Ib. For a motor, Ib is the nameplate full-load current, not the mechanical output divided by efficiency.
- Choose the device rating In so that Ib ≤ In ≤ Iz.
- Correct the tabulated cable capacity: Iz = Itab × Ca × Cg × Ci (ambient temperature, grouping, thermal insulation), then apply any device-location derating.
- Check voltage drop: 3% for lighting and 5% for other circuits, measured from the origin of the installation.
- Check thermal withstand of the conductors to the prospective fault current with k²S² ≥ I²t, and size the protective conductor accordingly.
Worked example
A 16 A single-phase socket circuit runs 25 m in 2.5 mm² copper, with mV/A/m ≈ 18 (phase and neutral at operating temperature). ΔU = 18 × 16 × 25 / 1000 = 7.2 V, i.e. 7.2/230 = 3.1%. That passes the 5% limit for a socket circuit, but it would fail a 3% lighting limit and it consumes most of the allowance for the whole run.
Traps
- Skipping the derating factors and quoting the catalogue Iz directly.
- Adding every connected load without diversity, then blaming the breaker for tripping.
- Using the three-phase mV/A/m value on a single-phase circuit.
- Neglecting motor starting current, which needs a type C or D device or a starter.
- Forgetting that the neutral and protective conductors are part of the calculation.