Reusing a 6 mm² submain for a replacement 15 kW chilled-water pump
A chilled-water pump serving a 12-storey office is being replaced. The new pump is 15 kW, 380 V three-phase, power factor 0.85, efficiency 0.90, giving a full-load current of about 30 A. It is direct-on-line started. The existing submain is 4-core 6 mm² XLPE/SWA copper armoured cable, 55 m long, clipped to a tray in the plant room and then run for most of its length with four other loaded circuits in a metal trunking riser. The plant room ambient reaches 40 °C in summer. The cable is protected at the landlord's submain board by a 40 A Type C MCB. The maintenance team has already bought a replacement 6 mm² cable and wants you to reuse the existing 40 A MCB. The declared supply is 380 V and the installation is designed to the 4% voltage-drop limit adopted in the Code of Practice. The measured earth fault loop impedance at the pump terminal is 0.38 Ω.
1. Show whether the 6 mm² cable can carry the pump load under the installed conditions, and state what you would specify instead.
Read the worked answer
Step 1 - design current. Ib = 15 000 / (√3 × 380 × 0.85 × 0.90) ≈ 30 A, so the 40 A device is correctly sized above Ib (In ≥ Ib).
Step 2 - apply the correction factors. The tabulated current-carrying capacity It must satisfy It ≥ In / (Ca × Cg).
- Ambient 40 °C: for 90 °C thermosetting insulation, Ca ≈ 0.91.
- Five loaded circuits grouped in trunking: Cg ≈ 0.60 (read the actual value from the grouping table for the number of circuits and the enclosure type).
Required It ≥ 40 / (0.91 × 0.60) ≈ 73 A.
Step 3 - compare. A 6 mm² four-core XLPE armoured cable in this installation method has a tabulated capacity in the region of 50 A, so after derating it can carry roughly 27 A. That is below the 30 A pump current and well below the 40 A device rating. The 6 mm² cable would run hot, age rapidly and could operate the MCB on thermal overload during a long run. It must not be installed.
Step 4 - select. A 16 mm² conductor of the same type, with a tabulated capacity around 80 A, gives about 44 A after the same factors - comfortably above In. Confirm against the actual table for the armoured cable and installation method, not from memory.
Step 5 - protect. A 40 A Type C MCB is acceptable for cable protection but is a poor choice for a motor; consider a motor-rated device with a magnetic setting above the starting current and a thermal setting matched to the pump's full-load current.
2. Check the voltage drop for the proposed 16 mm² cable and confirm whether the 4% limit is met.
Read the worked answer
Use the three-phase volt-drop formula with the tabulated mV/A/m value for the conductor.
Vd = (mV/A/m × Ib × L) / 1000
For 16 mm² four-core copper the tabulated value is approximately 2.4 mV/A/m:
Vd = (2.4 × 30 × 55) / 1000 ≈ 3.96 V
As a percentage of 380 V: 3.96 / 380 × 100 ≈ 1.04%.
That is well inside the 4% allowance, leaving roughly 3% available for the final circuit from the sub-board to the pump motor and its local control panel - which matters, because the 4% figure applies from the origin of the installation to the furthest point of use, not to the submain alone. If the final circuit adds another 1.5% the installation is still compliant.
The same calculation for the rejected 6 mm² cable (tabulated value about 6.4 mV/A/m) gives (6.4 × 30 × 55) / 1000 ≈ 10.6 V, or 2.8%. Note that the 6 mm² cable passes the voltage-drop test but fails the current-carrying capacity test. That is exactly why both checks must be done - a cable can be electrically 'long enough' and thermally inadequate at the same time.
Also confirm the earth fault loop impedance. For a 40 A Type C device the magnetic trip threshold is 10 × In = 400 A, so the maximum permitted Zs is about (0.95 × 230) / 400 ≈ 0.55 Ω. The measured 0.38 Ω passes, but with only modest margin; re-measure after the cable is replaced and before the pump is commissioned, and record the result.
3. The maintenance team argues that the 40 A Type C MCB has 'worked fine for years' and that a direct-on-line start is simple and cheap. Assess the protection and starting arrangement and state what you would recommend.
Read the worked answer
Protection. The 40 A Type C device protects the cable, which is its primary job, and a 40 A rating against a 30 A full-load current gives reasonable overload protection for the conductor. It is not, however, motor protection. The thermal element of an MCB is designed for steady loads, not for repeated starts at several times full-load current, and there is no phase-loss protection. A prolonged single-phasing or a jammed pump would be cleared only by the MCB, after the motor winding has already overheated.
Starting. A direct-on-line start at six times full-load current gives about 180 A at the terminals. The Type C magnetic threshold is 200–400 A, so the device will not trip magnetically on start, but the thermal element sees 180 A for the acceleration period on every start and will eventually nuisance-trip or drift. The starting current also produces a voltage dip at the submain board and at the utility's point of supply; a 15 kW DOL start on an office supply shared with lifts, chillers and IT loads is likely to be visible to other tenants, and the dip must be checked against the CoP and the utility's requirement before it is accepted.
Recommendation:
- Provide a proper motor starter - contactor with a thermal overload relay set to the pump's full-load current, or a motor protective circuit breaker with an adjustable magnetic setting above the starting current.
- Keep the 40 A device as back-up short-circuit protection and for isolation.
- Where the supply cannot tolerate the DOL dip, use a soft starter or a variable-speed drive; a VSD also gives flow control and reduces starting current to near rated.
- Confirm the isolation and lock-off arrangement for maintenance, and record the setting of the overload relay on the commissioning sheet.