Choosing a workshop submain cable when the base table rating looks generous
A licensed contractor sends you, a final-year vocational trainee, to complete the submain for a small fabrication workshop. The single-phase lighting circuits are already in place; the new three-phase submain must feed a welding socket, a 5 HP (3.7 kW) air compressor and a bench grinder, giving a connected load of 9 kW at an assumed power factor of 0.85. The submain is 55 m of 3-core copper PVC cable drawn in conduit on a wall, and the run shares its conduit tray with two other loaded circuits. Site conditions: maximum ambient inside the workshop 40 °C; the manufacturer's base current rating in conduit at 30 °C for 2.5 mm² is 22 A, for 4 mm² is 30 A, for 6 mm² is 38 A. The derating factor for 40 °C is 0.87 and the grouping factor for three circuits is 0.70. The utility permits a maximum voltage drop of 4 % from the meter to the load. The senior electrician wants to use 2.5 mm² cable to cut cost and fit a 32 A MCB. You have the cable tables and a megger.
1. Calculate the full-load line current and decide whether the senior electrician's 2.5 mm² cable and 32 A MCB proposal can be accepted.
Read the worked answer
Full-load current. For a balanced three-phase load, I = P / (√3 × V × cos φ) = 9000 / (1.732 × 415 × 0.85) = 9000 / 611 ≈ 14.7 A.
What 2.5 mm² can really carry on this route. The 22 A figure assumes 30 °C, one circuit, free air. Here two derating factors apply: 22 × 0.87 × 0.70 = 13.4 A.
Comparison. 13.4 A is below the 14.7 A load, so the cable would be overloaded from the first day, and the overload would be invisible because the cable is buried in conduit where nothing can be seen or smelled until the insulation fails.
The 32 A MCB makes it worse, not safer. A protective device must satisfy Ib ≤ In ≤ Iz. Here Ib = 14.7 A, In = 32 A, Iz = 13.4 A, so the breaker would sit roughly 2.4 times above the cable's capacity. The cable would overheat and fail long before the breaker ever thought about tripping.
Voltage drop check. About 18 mV/A/m for 2.5 mm² three-phase gives 18 × 14.7 × 55 / 1000 ≈ 14.5 V, i.e. 3.5 % of 415 V. It just scrapes inside the 4 % limit, leaving nothing for the lighting circuits downstream.
Verdict: reject both the 2.5 mm² cable and the 32 A MCB, and record the reason in the site register.
2. Select the correct cable size and protective device, and show the three checks that justify your choice.
Read the worked answer
Step 1 - current capacity. Try 4 mm²: base 30 A, derated 30 × 0.87 × 0.70 = 18.3 A. That is greater than the 14.7 A load, so it passes.
Step 2 - device coordination (Ib ≤ In ≤ Iz). Choose a 16 A three-pole MCB: 14.7 ≤ 16 ≤ 18.3. All three terms are satisfied. A 20 A device would also be legal on paper (20 > 18.3 fails), so 16 A is the largest permissible - do not round upwards for convenience.
Step 3 - voltage drop. About 11 mV/A/m for 4 mm² three-phase: 11 × 14.7 × 55 / 1000 ≈ 8.9 V, i.e. 2.1 % of 415 V, comfortably inside the 4 % limit and leaving margin for the final circuits.
Step 4 - device curve. The compressor starts direct-on-line and draws roughly six times full load for a fraction of a second. A type C MCB tolerates 5–10 times its rating magnetically, so a 16 A type C will ride through the start without nuisance tripping. Motor overload protection still belongs in the compressor's own starter, not in the MCB.
Step 5 - protective conductor. Because the phase conductor is 16 mm² or less, the circuit protective conductor must be the same cross-section, so a 4 mm² earth core is required. If a separate earth wire is pulled, it must not be smaller than 4 mm².
Answer: 4 mm² three-core copper PVC submain with a 16 A three-pole type C MCB and a 4 mm² CPC.
3. State what you would test and verify before the submain is energised, and identify the one thing that must not be left in the panel.
Read the worked answer
Insulation resistance. With the main switch off, the MCB off, and all loads, lamps and electronic devices disconnected, test each phase to phase, each phase to neutral and each phase to earth with a 500 V megger. New wiring should read in the tens of megohms; anything below 1 MΩ must be investigated before energising. Record the readings.
Continuity and polarity. Confirm CPC continuity from the DB earth bar to every point, and check that the phase, neutral and earth go to the correct terminals of the welding socket. A reversed neutral and earth in a socket is a fatal fault, not a cosmetic one.
Earth electrode resistance. Measure the electrode with the three-pole method against the utility's stated limit and record it; it must be rechecked before the certificate is issued.
Phase sequence for the compressor. A three-phase motor's direction depends on phase rotation. Bump the compressor for two seconds and confirm rotation matches the arrow before coupling it to the load; a scroll or reciprocating compressor run backwards can be damaged in seconds.
Must not be left in the panel: the 32 A MCB. If it is found installed, either the cable must be uprated to match it or, as here, the breaker must be replaced with the 16 A device. A panel that is 'working' with an oversized breaker is not a compliant installation.