Voltage Drop on a Workshop Radial
A 230 V single-phase radial circuit supplies a small workshop. The cable is 2.5 mm² copper twin-and-earth, 50 m from the consumer unit to the furthest socket, run partly in conduit and partly clipped direct. A 20 A Type B MCB protects the circuit. With a 20 A heater running, the voltage at the origin is 231 V and at the furthest socket 216 V. The owner wants to add a 2 kW (230 V) kettle at that socket and asks whether the existing circuit can take it. The measured earth fault loop impedance at the socket is 0.95 Ω; the supply is TN-S with a 0.8 Ω external loop. The 20 A MCB’s maximum permitted Zs is 1.75 Ω. The installation has no RCD on this circuit. The owner is unwilling to rewire the whole workshop and wants the cheapest compliant option.
1. Calculate the expected voltage drop at 20 A and compare it with the 5% limit. Explain the difference between the calculated and measured values.
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Using ρ = 0.0175 Ω·mm²/m for copper: R = 2ρL/A = 2 × 0.0175 × 50 / 2.5 = 0.70 Ω. Voltage drop at 20 A = 20 × 0.70 = 14.0 V. Percentage = 14.0 / 230 × 100 = 6.1%. The 5% limit is 11.5 V, so the circuit is already non-compliant at its design current. The measured drop is 231 − 216 = 15 V, close to the calculated value; the difference is explained by supply tolerance, connection resistance and measurement uncertainty. The Zs of 0.95 Ω is below the 1.75 Ω maximum for the 20 A Type B MCB, so the measured value does not indicate an immediate disconnection problem, but it does not correct the voltage drop.
2. Evaluate adding the 2 kW kettle: calculate the new current and voltage drop, and decide whether the 20 A MCB and existing cable are suitable.
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Kettle current = 2000/230 = 8.7 A. New total = 20 + 8.7 = 28.7 A. This exceeds the 20 A MCB rating, so the breaker would trip on overload; it also exceeds the safe capacity of the existing 2.5 mm² cable once installation conditions are considered. At 28.7 A, the existing cable drop would be 28.7 × 0.70 = 20.1 V (8.7%), far above 5%. Cheapest compliant options:
- A dedicated 16 A radial for the kettle in 2.5 mm², if the route is short enough and Zs is verified. At 8.7 A the drop on 50 m would be 6.1 V (2.6%), which is acceptable.
- Upgrade the existing circuit to 6 mm² and a 32 A MCB. At 28.7 A the drop becomes 2 × 0.0175 × 50 / 6 = 0.292 Ω × 28.7 = 8.4 V (3.6%), and the 32 A device is more suitable. Verify the cable’s installed current rating and Zs before energising. A 4 mm² upgrade would still give 12.6 V (5.5%) at 28.7 A, so it is not sufficient on voltage drop alone.
3. State the tests and instruments required before and after the alteration, and one documentation requirement.
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Before alteration: safe isolation and prove dead; continuity and polarity of the existing circuit; insulation resistance at 500 V; Zs with a loop impedance tester; RCD test if an RCD is fitted; visual check of cable route and accessories. After alteration: repeat insulation resistance, continuity, polarity and Zs; verify the new MCB rating and cable capacity; test the RCD if required; confirm voltage drop by calculation or measurement at full load. Record the results on a minor works certificate or an electrical installation certificate with a schedule of test results, including the altered circuit, the calculated design current, the cable size and the protective device. Leave the client a copy and note the limitation on adding further load.