Cable Selection for a Long Run to a Detached Shed
A client wants a 32 A single-phase submain run 48 m from the main switchboard to a detached workshop containing a 5 kW welder, a 2.2 kW compressor and lighting. The cable will be run in buried conduit under a paved driveway for 30 m, then clipped to a shed wall in free air for the remainder. Ambient temperature in the ceiling space at the switchboard end reaches 45 °C in summer. The client has already bought 6 mm² two-core plus earth copper cable and wants you to install it because it was cheap. You check the manufacturer's current-carrying capacity tables and note that 6 mm² in buried conduit is rated around 47 A under reference conditions, but derating for 45 °C ambient and grouping with two other circuits in the same conduit trench applies.
1. Assess whether the client's 6 mm² cable is adequate, showing your reasoning.
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Start with the design current. The welder and compressor are intermittent loads, but the submain must carry the maximum demand, which is the sum of the connected loads unless diversity is justified. A 5 kW welder at 230 V draws about 21.7 A, the compressor about 9.6 A, and lighting perhaps 2 A, giving a gross figure near 33 A. That exceeds the 32 A breaker, so the protective device rating must be checked against the cable's derated capacity, not against the nominal 47 A figure.
Apply derating. A 45 °C ambient correction factor for thermoplastic-insulated cable is roughly 0.79, and grouping with two other loaded circuits in the same buried conduit attracts a further factor of about 0.7. Multiplying 47 A by 0.79 and 0.7 gives approximately 26 A, which is below the 32 A design current. The cable is therefore not adequate even before voltage drop is considered.
Check voltage drop as a cross-check: a 48 m run at 33 A on 6 mm² copper gives a drop of roughly 9 V, or about 4 percent, which is at or beyond the usual 5 percent limit for a submain plus final subcircuits combined. The client's cheap cable fails on current capacity and is marginal on voltage drop. Recommend a larger conductor, typically 10 mm² or 16 mm² depending on the final calculation, and explain the reasoning to the client before proceeding.
2. How would you communicate this to the client and what alternatives would you offer?
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Lead with the safety and compliance reason, not the cost. Explain that the cable's capacity is reduced by the hot ceiling space and by sharing a buried conduit with other circuits, so it cannot safely carry the welder and compressor at full load. If it were installed, the breaker could fail to protect the cable, leading to overheating and possibly fire, and the installation would not comply with the Wiring Rules, so it could not be certified.
Offer practical alternatives: upgrade to a larger conductor sized by calculation, split the load so the welder runs on its own circuit, or reduce the maximum demand by interlocking the welder and compressor so they cannot run together. Provide a written quote for each option with the calculated figures so the client can see why the larger cable is required rather than merely asserted.
Acknowledge the client's purchase without being dismissive: the 6 mm² cable may be usable elsewhere on the property, such as a shorter lighting or GPO subcircuit, so the money is not necessarily wasted. Document the advice given and the client's decision, and do not install the undersized cable even if pressed, because your licence and the client's safety are at stake.