Cable Sizing for a Workshop Submain
A workshop is fed from a distribution board 38 m away by a three-core armoured cable clipped direct to a wall for 22 m, then in thermal insulation for 8 m, then in free air for 8 m. The load is a 400 V three-phase motor circuit drawing 45 A at full load with a starting current of six times full load for 8 seconds. The supply is TN-S with a prospective fault current of 4.5 kA at the origin. Ambient temperature is 35 °C. The client has specified a 63 A BS 88-2 fuse and a 10 mm² copper conductor because 'it fits the gland'.
1. Assess whether the client's 10 mm² proposal is defensible. Show the reasoning steps you would use, stating any assumptions.
Read the worked answer
Work through the standard design sequence rather than accepting the gland argument.
- Design current Ib = 45 A.
- Protective device In = 63 A, so Ib ≤ In is satisfied.
- Correction factors: ambient 35 °C gives roughly 0.94 for XLPE or 0.87 for PVC; grouping not stated, assume 1.0. The 8 m in thermal insulation is the problem: for a cable totally surrounded, derating can be around 0.5, and for one side in contact with insulation roughly 0.75. A partly enclosed run of 8 m is significant relative to the 38 m total.
- Required tabulated capacity It ≥ In / (Ca × Ci). With Ca 0.87 and Ci 0.75, It ≥ 63 / 0.65 ≈ 97 A.
- A 10 mm² three-core armoured cable clipped direct is typically around 60–70 A, well below 97 A. Even ignoring the insulation factor it is marginal at 63 A.
Conclusion: 10 mm² is not defensible; a larger conductor (likely 25 mm²) or rerouting the insulated section is needed. Verify against the actual cable manufacturer's tables before committing.
2. The motor starts at six times full load for 8 seconds. Explain how this affects fuse selection and whether the 63 A BS 88-2 fuse is suitable.
Read the worked answer
A BS 88-2 fuse must ride through the start without nuisance operation but still protect the cable. The starting current is 6 × 45 = 270 A for 8 s. A 63 A BS 88-2 fuse will typically carry several times its rating for a short period; at 270 A (about 4.3 × In) the pre-arcing time is normally well over 8 s, so the fuse should hold on start. That part is acceptable.
However, fuse suitability is only one of two checks. The cable must still be protected: the fuse's disconnection time at the relevant overcurrent must not damage the conductor, and the tabulated current-carrying capacity after correction must be at least In. Since the corrected capacity of 10 mm² fails that test, the fuse choice does not rescue the cable. Also check the adiabatic withstand k²S² ≥ I²t for the fault current, and verify earth-fault loop impedance gives disconnection within the required time (0.4 s for TN final circuits, 5 s for distribution circuits). Recommend a motor-rated device (e.g. an aM fuse or MPCB) coordinated with the starter rather than a general-purpose fuse if starting is frequent.