[MEP Center Engineering]

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An outdoor lighting final circuit at 230 V single-phase carries 10 A at unity power factor to fittings about 60 m from the distribution board. The designer used 2.5 mm² copper PVC cable with a C16 MCB, because the current is low.

• What voltage-drop limit applies to a final circuit, and is it measured from the board or from the origin of the installation?
• Is 2.5 mm² acceptable here?
• Are there other checks that long final circuits commonly fail?

[MEP Center Engineering]

The usual limits are counted from the origin of the installation, not from the final board: IEC 60364-5-52 (Annex G) recommends 3 % for lighting and 5 % for other uses when supplied from a public LV network, and 6 % and 8 % from a private transformer. At 60 m, 2.5 mm² fails the voltage-drop check and also the fault-loop check with a C16.

Step 1: Set the budget

Subtract the drop already used upstream. If the sub-main to this board uses 1 %, the lighting final circuit has 2 % left of a 3 % total. NEC treats this as an informational note (3 % for a branch circuit, 5 % for feeder plus branch), and many specifications make it mandatory.

Step 2: Calculate

Single-phase: ΔU = 2 × L × I × (R cos φ + X sin φ). For small cables at cos φ = 1, X can be ignored. Use R at the operating temperature (70 °C for PVC): R₇₀ = R₂₀ × (1 + 0.00393 × 50).

Cable	R at 70 °C (Ω/km)	ΔU at 10 A, 60 m (V)	ΔU (%)	Total with 1 % upstream
2.5 mm²	8.87	10.6	4.63	5.63 % (fails)
4 mm²	5.52	6.6	2.88	3.88 % (fails)
6 mm²	3.69	4.4	1.92	2.92 % (passes)

The circuit needs 6 mm², even though 2.5 mm² carries 10 A easily.

Step 3: Check fault-loop impedance (often forgotten)

Automatic disconnection in a TN system needs Z_s × I_a ≤ U₀. A C16 must trip magnetically at 10 × 16 = 160 A, so Z_s ≤ 230 / 160 = 1.44 Ω. Use conductor resistance at operating temperature, or apply a 0.8 factor to values at 20 °C, not both.

• 2.5 mm² with a 2.5 mm² protective conductor: loop R = 2 × 0.060 × 8.87 = 1.06 Ω. With Z_e = 0.40 Ω, Z_s = 1.46 Ω, which fails.
• 6 mm²: 0.44 + 0.40 = 0.84 Ω, which passes.
• Alternatives are a B16 (Z_s ≤ 2.88 Ω) or RCD protection. Outdoor circuits usually need a 30 mA RCD anyway; see RCD Types AC, A, F and B.

Practical points

• Use the real cable route length, including vertical drops and the loop through fittings, not the straight-line distance.
• For distributed loads (lamps spaced along the route) the drop is lower than with the whole load at the end. Calculate it segment by segment rather than halving it blindly.
• For motors, check the starting voltage drop as well (often limited to 10 to 15 % at the motor terminals during starting). See Motor Starting: DOL, Star-Delta, Soft Starter or VFD.
• LED drivers tolerate a wide voltage range, but emergency luminaires and control gear may not. Do not relax the limit on that basis.

Common mistakes

• Measuring 3 % from the final board instead of from the origin.
• Using resistance at 20 °C for voltage drop and then calling the result conservative.
• Upsizing phase conductors but leaving a small protective conductor, so Z_s still fails.

For deeper study: Electrical Power Systems Design course.

Check the edition of the code that applies to your project.

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