Bakery Subpanel: Conduit Fill Is Not the Only Limit
A bakery expansion adds a 400 V subpanel 25 m from the main board. The contractor proposes one 40 mm rigid PVC conduit carrying six single-phase 16 A circuits (each 2.5 mm² Cu PVC, L+N) and one 3-phase 20 A circuit (3×2.5 mm² + N), with a single 2.5 mm² PE. The conduit runs through a ceiling void at 45 °C and is grouped with four other loaded conduits for 8 m. The contractor says the 40 mm conduit is only 34% full and the 16 A breakers protect the 2.5 mm² conductors, so no further check is needed. The bakery also has a 12 kW deck oven and a 6 kW proofing cabinet on the subpanel. You are asked to review the conduit and conductor design before the ceiling is closed.
1. Determine the number of current-carrying conductors and check whether 2.5 mm² Cu on 16 A and 20 A breakers is acceptable after derating.
Read the worked answer
The six single-phase circuits contribute six line conductors; the three-phase circuit contributes three line conductors, so there are nine current-carrying conductors. The shared neutral may also count if harmonic-rich loads are present; the PE is not counted. A 2.5 mm² PVC Cu conductor in conduit has a base ampacity of about 21–24 A. Apply an ambient factor of about 0.79 for 45 °C and a grouping factor of about 0.60 for five loaded conduits: 21 × 0.79 × 0.60 ≈ 10 A; even using 24 A gives only about 11.4 A. A 16 A breaker therefore does not protect the conductor, and the 20 A three-phase circuit is worse. The 34% conduit fill is not the governing issue; heat dissipation is. Upsize to at least 6 mm² for the 16 A circuits, but the margin is thin; 10 mm² gives roughly 21 A corrected. Recalculate with the exact IEC table and use a separate PE sized for the phase conductors.
2. Evaluate the 12 kW oven circuit and the contractor's claim that breaker size alone proves conductor protection.
Read the worked answer
The 12 kW oven at 400 V, 3-phase, 0.95 power factor draws about 18.2 A. If it is fed by 4 mm² Cu over 25 m, the voltage drop is approximately 4.4 V, or 1.1%, which is acceptable. But the conductor must still be derated for the same 45 °C ambient and grouping if it shares the conduit. The statement that a 16 A or 20 A breaker protects 2.5 mm² is incomplete: protection requires the corrected ampacity to be at least the breaker rating under the actual installation method. The 20 A three-phase circuit on 2.5 mm² clearly fails after correction. Separate the power circuits from the lighting/socket conduits or increase conductor size and conduit size; do not accept the installation on fill percentage alone.
3. Propose a compliant minimum arrangement and the checks needed before the ceiling is closed.
Read the worked answer
Propose separate conduits for the socket circuits and the three-phase power circuits. For the six 16 A single-phase circuits, use 10 mm² Cu if they remain grouped at 45 °C; for the 20 A three-phase circuit, use at least 6 mm² after calculation, preferably 10 mm² for margin. Use separate PE conductors of at least the same size as the associated line conductors where required, and verify the conduit fill with the new sizes. Provide a revised derating calculation, voltage-drop calculation for the 25 m run, and a coordination check for the 20 A breaker. Before closure, inspect conduit routes, confirm firestopping at penetrations, and record insulation resistance and continuity tests. The contractor's 34% fill statement does not address thermal derating.