Warm Plug Top on a 3 kW Water Heater Circuit
A household has a 230 V, 3 kW storage water heater supplied by 30 m of 2.5 mm² two-core-and-earth PVC cable. The cable is clipped in a hot loft, where its tabulated current rating is derated from 24 A clipped direct to 17.5 A. The circuit has a 16 A MCB, a 13 A fused spur, and no RCD. The heater current is 13.0 A. With the heater on, the consumer-unit voltage is 232 V and the voltage at the heater is 223 V. The plug top at the fused spur is discoloured and warm after 20 minutes, but the cable itself is not warm. The owner asks whether the cable is overloaded, whether the voltage drop is acceptable, and whether the installation is safe. The electrician must also identify the most likely cause of the warm plug top and state the immediate action.
1. Is the 2.5 mm² cable overloaded? Compare the load current with the derated current rating and the protective device rating.
Read the worked answer
The heater current is 13.0 A. The derated cable rating in the hot loft is 17.5 A, so the cable is not overloaded: 13.0 A is about 74% of its derated capacity. The 16 A MCB also satisfies the usual In ≤ Iz relationship because 16 A < 17.5 A, although the margin is small. The 13 A fuse in the fused spur is close to the heater current but is intended to protect the flexible cord and the heater. The important qualification is ambient temperature: if the loft is significantly hotter than the assumed 30 °C, the 17.5 A rating may fall further. The cable itself is not warm, which supports the conclusion that it is not carrying an overload. The immediate problem is therefore not cable overload.
2. Calculate the cable voltage drop and compare it with the measured drop. What does the difference indicate?
Read the worked answer
The loop resistance of 30 m of 2.5 mm² copper cable is R = 2ρL/A = 2 × 0.0172 × 30/2.5 = 0.413 Ω. At 13 A the cable drop is 13 × 0.413 = 5.4 V. The expected heater voltage is 232 − 5.4 = 226.6 V, but the measured value is 223 V. The extra drop is 3.6 V, which corresponds to an additional resistance of 3.6/13 = 0.28 Ω. At 13 A that loose joint dissipates 3.6 × 13 = 47 W, enough to discolour and carbonise the plug top. The cable drop alone is 5.4/232 = 2.3%, which is acceptable. The total drop is 9/232 = 3.9%, above the usual 3% target, but the cable is not the main contributor. The likely fault is a high-resistance connection at the plug top, fused spur, or socket.
3. State the safety deficiencies and the immediate action. What tests should be made after repair?
Read the worked answer
The absence of a 30 mA RCD on a water-heater circuit is a serious shock-risk deficiency, especially if the heater is in a bathroom or wet area. The discoloured warm plug top is a fire risk and must not be ignored. The immediate action is to switch off the circuit at the consumer unit and isolate it, then inspect and replace the plug top, fuse carrier, and socket as necessary. Remake the terminations and check that the correct fuse is fitted. After repair, test earth continuity, insulation resistance, polarity, and the RCD trip time. Also confirm the cable’s derated current rating for the actual loft temperature and check that the heater is correctly earthed and bonded. Do not restore the circuit until the high-resistance joint has been eliminated and the RCD protection is provided.