Workshop submain: the cable on the van is not the cable for the job
A registered electrician quotes to run a 230 V single-phase submain from the main switchboard of a house to a detached workshop. The route length is 65 m, including rises. The client wants a 32 A supply for a welder and a bench saw. There is 100 m of 6 mm² two-core TPS on the van, so the quote is built around using it, clipped to battens in a ceiling space where the summer ambient reaches 35 °C. Ze at the main switchboard is 0.35 Ω and the main switch is a 63 A MCB. The installation volt drop allowance is 5 % of 230 V. The client has already accepted the quote and booked the day off work.
1. Determine whether the 6 mm² TPS satisfies the volt drop requirement for a 32 A load. Show the calculation and state how far short it falls.
Read the worked answer
Calculation (single-phase)
Vd = (mV/A/m value × I × L) / 1000
Using AS/NZS 3008 Table 42 values for copper, single-phase (go and return), at operating temperature:
- 6 mm² ≈ 7.5 mV/A/m
Vd = (7.5 × 32 × 65) / 1000 = 15.6 V
Allowance = 5 % × 230 = 11.5 V
15.6 V is 6.8 %, so the cable fails by about 4.1 V. It is not compliant and cannot be installed for this load.
Judgement point: the 5 % is measured from the point of supply to the furthest point of any final subcircuit, not just to the workshop board. Using 6.8 % on the submain alone leaves nothing for the socket circuit inside the workshop, so even a 'marginal pass' at the submain would be bad design.
2. Select the smallest standard conductor that satisfies volt drop for the submain, then list the other design checks that must still be passed before that cable can be signed off.
Read the worked answer
Selection
10 mm² copper, single-phase ≈ 4.4 mV/A/m
Vd = (4.4 × 32 × 65) / 1000 = 9.15 V = 4.0 % - compliant, but it consumes most of the allowance, so the final subcircuits in the workshop must be short and lightly loaded.
Other checks that must still be passed
- Current-carrying capacity with derating. Apply the ambient correction for 35 °C and any grouping or thermal insulation factor. The derated capacity must be at least the rating of the protective device, and the protective device must protect the cable (Ib ≤ In ≤ Iz).
- Earth fault loop impedance and disconnection time for the submain's protective device, using the actual R1+R2 of the chosen conductors plus Ze of 0.35 Ω.
- Conductor and neutral sizing - whether a separate earth conductor is needed (two-core TPS has no integral earth), and whether the neutral can carry the load including harmonics from the welder.
- Maximum demand - confirm the 32 A is real, not a wish list; the 63 A main switch and existing load must accommodate it.
- Mechanical and environmental suitability - 10 mm² clipped in a ceiling space, bends, and support.
3. The client refuses to pay for the larger cable. Set out the options you would put in writing, and explain what you would need to confirm before any of them can be adopted.
Read the worked answer
Options to put in writing
- Reduce the supply. If the real maximum demand is lower (say 22 A, with the welder and saw not used together), 6 mm² gives (7.5 × 22 × 65)/1000 = 10.7 V = 4.7 %. That technically passes, but leaves 0.3 % for the workshop's own final subcircuits, so it only works if the workshop sockets are effectively at the board.
- Provide a new point of supply at the workshop. This makes the workshop a separate installation with its own MEN link and electrode and removes the long submain - but it moves cost to the network connection and needs distributor approval.
- Install a load-management or interlock arrangement so the welder and saw cannot run together, reducing maximum demand.
- Keep 6 mm² and accept a reduced supply - only if the reduced demand and the downstream volt drop both check out.
What must be confirmed first
- Measured or documented maximum demand, not the client's guess.
- The actual route length after installation (the 65 m is an estimate).
- Distributor requirements and cost if a new point of supply is considered.
Position: volt drop is a compliance requirement, not a preference. The electrician cannot install the 6 mm² as quoted, and should record the change and the reason before work starts, so the variation is not disputed later.