Cable Selection for a Construction Site Distribution Board
A construction site requires a temporary distribution board 60 m from the site switchboard. The load is 3-phase, 400 V, with a calculated maximum demand of 45 A per phase. The cable will be run on a cable tray in direct sun for part of the route, then buried direct for 15 m where it crosses a vehicle access area. Ambient summer temperature is 40 °C, and the tray section has four other loaded cables grouped with it. The installer proposes 16 mm² copper XLPE/PVC four-core cable throughout. The site has a 30 mA RCD at the switchboard and a 63 A MCCB at the temporary board.
1. Assess whether the proposed 16 mm² cable is adequate, identifying the derating factors that must be applied.
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The cable must be assessed for current-carrying capacity after applying all relevant derating factors, and then checked for voltage drop. The base tabulated rating for 16 mm² copper XLPE is around 80–90 A depending on installation method, but that figure cannot be used directly.
Derating factors to apply include: ambient temperature (a 40 °C correction factor for XLPE, typically around 0.91); grouping (five loaded cables on the tray, which may reduce capacity by roughly 0.8 depending on arrangement and spacing); and the buried section, which has its own rating and soil thermal resistivity considerations, plus a depth and soil condition factor. The vehicle access crossing also raises mechanical protection requirements, which may require the cable to be installed in conduit or with additional protection.
The lowest derated capacity across the route governs. Applying factors of roughly 0.91 and 0.8 to an 80 A base gives about 58 A, which is above the 45 A demand but leaves limited margin and must be checked against the protective device rating. The buried section must be assessed separately. The installer must also check voltage drop over the 60 m run at 45 A; 16 mm² may give a drop approaching or exceeding the allowable limit, particularly if the route is longer than stated.
The installer should not accept the proposal without a documented calculation. A larger conductor, or a different route or installation method, may be required.
2. What protection and installation requirements apply to the buried section and the temporary board, and how should the installer verify the design?
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The buried section must be installed at an adequate depth, with mechanical protection where it crosses a vehicle access area, and with warning tape or marker where required. The cable must be suitable for direct burial if buried without conduit. The installer should record the burial depth and protection method.
The temporary board must be protected against overload and short circuit by the 63 A MCCB, which must be coordinated with the cable's derated capacity so that the cable is protected. If the derated capacity of any section is below 63 A, the MCCB rating or cable size must be adjusted. The 30 mA RCD at the switchboard provides additional protection but does not replace correct cable sizing and protection.
The design should be verified by recording the calculation (base rating, each derating factor, resulting capacity, voltage drop) and by testing after installation: insulation resistance, earth fault loop impedance at the temporary board, polarity and RCD operation. The results should be compared with the design values and retained with the certification records.