Circular Saw Breaker Trips: Cable Drop or Breaker Curve?
A 2.2 kW single-phase 230 V motor drives a circular saw in a small workshop. Its nameplate gives cos φ = 0.82 and efficiency η = 0.85. The motor is fed from a 16 A Type C circuit breaker through 35 m of 2.5 mm² copper cable. The measured loop resistance of the cable is 0.49 Ω and the supply source impedance is 0.15 Ω. The saw starts with a measured starting current of 6.5 times rated current for about 1.2 s. The Type C breaker has a magnetic trip range of 5–10 times its rated current. Operators report that the breaker trips occasionally on start-up, especially on cold mornings, but never during continuous sawing. A voltage logger shows 228 V at the distribution board before start and a dip at the motor terminals during start. The workshop owner wants either to fit a larger 20 A breaker or to replace the cable with 4 mm². The motor terminal box has no soft starter and the saw must remain portable on a 230 V single-phase supply.
1. Calculate the motor current, the running voltage drop and the starting voltage drop. Compare the starting current with the breaker's magnetic trip range.
Read the worked answer
Input power = 2200/0.85 = 2588 W. Current I = 2588/(230 × 0.82) = 13.7 A, equivalently 2200/(230 × 0.82 × 0.85).
Total loop resistance = cable 0.49 Ω + source 0.15 Ω = 0.64 Ω. Running drop = 13.7 × 0.64 = 8.8 V, or 3.8% of 230 V; terminal voltage is about 219 V. This is borderline but not the main trip cause.
Starting current = 6.5 × 13.7 = 89 A. Starting drop = 89 × 0.64 = 57 V, so terminal voltage falls to about 171 V, 74% of nominal. The Type C magnetic trip range is 5–10 × 16 A = 80–160 A. The 89 A start is inside the lower part of that band, so a breaker at the sensitive end of its tolerance can trip, especially with a cold motor or a slightly higher inrush. The occasional trips are consistent with magnetic start tripping, not with continuous overload.
2. Evaluate the owner's two options: fit a 20 A breaker or replace the cable with 4 mm². Which is technically sound, and what else should be checked?
Read the worked answer
A 20 A breaker alone is not the right first fix. It raises the magnetic trip band to 100–200 A, which may hold the 89 A start, but it does not reduce the 57 V starting dip. It also reduces overload protection margin for the 2.5 mm² cable and may not be acceptable for the motor's protection coordination. The voltage dip and high starting current remain.
Replacing the cable with 4 mm² reduces the cable loop resistance to about 0.0175 × 70/4 = 0.31 Ω. Total resistance becomes 0.46 Ω; running drop is 6.3 V (2.7%) and starting drop is 41 V, leaving about 187 V at the terminals. That is better but still a deep dip. The robust solution is 4 mm² cable plus a D-curve breaker or a soft starter, after checking the motor capacitor, terminal connections, supply impedance and whether the saw can be started unloaded. A 20 A breaker should not be used to mask a voltage-drop problem.
3. Describe the measurements you would take to confirm the diagnosis and to verify the repair. Include the quantities and the instrument types.
Read the worked answer
Use a true-RMS clamp meter to record running current and a peak-hold or logging meter to capture the starting current and duration. Log voltage at the distribution board and at the motor terminals simultaneously during several starts; the difference gives the cable and source drop under actual starting current. Check the breaker curve and rating against the measured start current, not the nameplate only.
Measure insulation resistance from each motor terminal to earth and between windings, and check the start capacitor capacitance and connections if the motor is capacitor-start. Verify supply impedance and tightness of terminals. After changing the cable or breaker, repeat the start test under the worst condition, such as a cold motor and a full saw load, and confirm terminal voltage stays within the motor manufacturer's limit. Record the results in the machine file.