16 A Socket Circuit Tripping With a Heater and Compressor
In a small fabrication workshop, a 230 V single-phase subcircuit from the main distribution board feeds three 16 A industrial sockets. The cable is 2.5 mm² copper PVC-insulated, run in conduit for 28 m. On a Monday morning, a 2 kW water heater and a 1.5 kW air compressor are plugged into two sockets. The 16 A type C MCB at the DB trips after 10–15 minutes, not immediately. A multimeter shows 232 V at the socket with no load and 208 V with both appliances running. The earth continuity from socket to DB is 0.4 Ω. The workshop owner says, 'Just put a 32 A MCB in; the cable is new.' The compressor nameplate shows a starting current of 6 times full-load current. You are asked to advise before any change is made.
1. Calculate the running current of the two appliances at 230 V and compare it with the 16 A MCB rating. What does the 10–15 minute trip time suggest?
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At 230 V: heater 2,000/230 = 8.7 A; compressor 1,500/230 = 6.5 A; total 15.2 A. That is below 16 A, so a healthy 16 A MCB should carry it thermally. But at the measured 208 V, the same power needs about 3,500/208 = 16.8 A, and motor current is higher because a motor is not a constant-power load. The 10–15 minute trip is the thermal overload characteristic, not the instantaneous magnetic trip. So the circuit is running just above its 16 A rating, and the voltage drop makes it worse. The fault is not a short circuit; it is overload plus a bad connection or excessive run.
2. The measured voltage drop is 24 V (about 10%). Is that explained by the 2.5 mm² cable alone? What else would you check?
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Using about 18 mV/A/m for 2.5 mm² single-phase, drop ≈ 18 × 16.8 × 28/1,000 ≈ 8.5 V, or 3.7%. The measured 24 V, about 10.3%, is much higher, so the cable alone does not explain it. Check for loose line or neutral terminals, a hot or discoloured socket, damaged cable, high-resistance earth path, and the actual current with a clamp meter. Measure voltage at the DB and at the socket under load to separate supply dip from local drop. The 0.4 Ω earth continuity is not the main cause of the voltage drop, but it still needs comparison with the limit. Do not accept the owner's instruction to fit a 32 A MCB: it would not protect 2.5 mm² cable.
3. Give a safe remedy that does not simply upsize the MCB. Include circuit separation, cable/protection coordination and the order of work.
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Isolate and lock off; inspect and remake terminations; separate the compressor from the heater so the fixed heater is not on the same socket circuit; calculate Ib, In and Iz with derating. A 2.5 mm² PVC conduit cable may be around 20–24 A depending on installation, so a 20 A MCB may be acceptable only if Iz and voltage drop pass. For a 28 m run, 4 mm² may be needed to keep drop at or below 5%. Use a motor-rated protective device or curve for the compressor, or a dedicated circuit, and provide 30 mA RCCB protection for sockets. Test insulation resistance, earth loop, polarity and RCD trip. Document the new schedule and energise only after tests pass.