Workshop Motor Cable Drop and Breaker Upsizing
A workshop adds a 2.2 kW, 240 V single-phase induction motor driving a bench grinder. The motor is 35 m of cable run from the distribution board, wired in 2.5 mm² copper conductors in PVC conduit. The motor's full-load current is 10 A and its starting current is six times full load for 0.8 s. The existing circuit breaker is a 16 A Type B MCB. With the motor running, a voltmeter at the motor terminals reads 231 V while the distribution board voltage is 240 V. The motor starts with a noticeable dip in workshop lighting and occasionally trips the MCB. The workshop owner wants to replace the MCB with a 32 A Type B to stop tripping. The cable is protected by the MCB and runs alone in the conduit.
1. Calculate the expected voltage drop at full load and compare it with the measured 9 V. What does the difference suggest?
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Resistance per conductor: R = ρL/A = 0.0175 × 35/2.5 = 0.245 Ω. The loop has two conductors, so R_loop ≈ 0.49 Ω. Expected drop at 10 A: V_drop = I R_loop = 10 × 0.49 = 4.9 V. That is about 2.0% of 240 V, within a typical 5% limit.
Measured drop: 240 − 231 = 9 V. That implies either a current of about 9/0.49 ≈ 18.4 A, or an effective loop resistance of 9/10 = 0.9 Ω. Since the motor full-load current is 10 A, the extra 0.41 Ω is suspicious. Possible causes are a poor termination at the MCB or motor, a damaged or partly broken conductor, a joint in the run, or a measurement taken during starting or with a different load. Check the current simultaneously with the voltage, inspect and remake terminations, and measure continuity/loop resistance. A 9 V drop at 10 A is not explained by the cable length alone.
2. Evaluate the owner's proposal to fit a 32 A Type B MCB. Include cable current-carrying capacity, overload protection and motor starting.
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2.5 mm² copper in PVC conduit has a current-carrying capacity of roughly 18–24 A depending on the installation table and derating. A 32 A Type B MCB is larger than the cable's capacity, so the cable could overheat and degrade before the breaker trips on overload. The breaker would no longer be protecting the cable; it would only protect against a very high fault current.
Type B magnetic tripping is 3–5 times the rated current: 3–5 × 32 = 96–160 A. The motor starting current is 6 × 10 = 60 A for 0.8 s, so the 32 A Type B would probably not trip magnetically - but that is not the right reason to fit it. The correct approach is to keep a breaker that protects the cable and choose a trip curve that tolerates the start. A 16 A Type C has a magnetic range of 5–10 × 16 = 80–160 A; 60 A is below that, so nuisance magnetic tripping is less likely, while the 16 A rating still protects a cable with adequate capacity. Verify the cable's derated capacity, the thermal trip curve at 60 A for 0.8 s, and the fault-loop impedance. If 16 A Type C still trips, investigate the motor or the starting method; do not simply increase the breaker.
3. Explain the lighting dip and recommend three checks or design changes to reduce it.
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Starting current is about 60 A. With the existing loop resistance, the starting drop is 60 × 0.49 ≈ 29 V, so the motor terminal voltage momentarily falls to about 211 V, roughly 12%. The same dip appears on the distribution board and affects the lighting. The run is also long enough that the steady-state drop is noticeable.
Checks and changes:
- Terminations and cable condition - remake the MCB and motor terminations, check for damaged insulation or partial conductor breaks, and measure loop impedance. The measured 9 V at 10 A suggests extra resistance that should be found first.
- Increase conductor size - 4 mm² copper has R = 0.0175 × 35/4 = 0.153 Ω per conductor, loop ≈ 0.306 Ω. Steady drop at 10 A ≈ 3.1 V; starting drop at 60 A ≈ 18.4 V, about 7.7%, a clear improvement.
- Protective device and starting - use a 16 A Type C MCB if the cable capacity and fault loop impedance allow, rather than a 32 A device. For a grinder, check that it starts unloaded; a soft starter or a larger motor is a bigger change. Keep lighting on a separate circuit if flicker is unacceptable.