Submain to a detached workshop: volt drop, cable selection and earthing
A rural property near Gatton has a single-phase 240 V supply. The main switchboard is on the house; a detached workshop 120 m away is fed by a direct-buried submain of 6 mm² copper TPS (active, neutral and a 6 mm² earth), protected at the house by a 32 A MCB. The workshop has its own earth electrode driven at the slab, bonded to the workshop earth bar, and its distribution board contains a main switch and two MCBs. The owner adds a 20 A welder and lighting and complains that the welder 'buzzes and the lights dim'. With a 15 A resistive load switched on, you measure 231 V at the house main switch and 219 V at the workshop distribution board. Supply is 240 V nominal, 50 Hz. The workshop has metal water pipework and a steel frame.
1. Determine whether the submain complies with the voltage-drop requirement at the intended 20 A load. Show your working and state the limit you are applying.
Read the worked answer
Limit applied: AS/NZS 3000 limits voltage drop from the point of supply to any point of the installation to 5% of nominal voltage - 12 V on a 240 V system.
From the measurement: 12 V drop at 15 A over 120 m (single phase, so out and back):
Vd = 2 × I × L × (mV/A/m) / 1000
12 = 2 × 15 × 120 × X / 1000 → X ≈ 3.3 mV/A/m
That is consistent with 6 mm² copper at operating temperature (roughly 3.7 mV/A/m at 75 °C), so the reading is plausible and there is no measurement error to explain it away.
At the design load: Vd = 2 × 20 × 120 × 3.7 / 1000 ≈ 17.8 V ≈ 7.4% of 240 V.
Conclusion: non-compliant. Even at 15 A the drop is already at the 5% limit with no margin. The cable is thermally fine (6 mm² is rated well above 20 A), so volt drop - not current-carrying capacity - is the governing constraint. The welder's high inrush current and poor terminal voltage explain the buzzing and dimming.
2. Select a minimum cable size for the 20 A design load and list the other checks you must complete before signing the work off.
Read the worked answer
Required mV/A/m: X ≤ 12 × 1000 / (2 × 20 × 120) = 2.5 mV/A/m.
10 mm² copper at 75 °C is about 2.2 mV/A/m, giving Vd ≈ 2 × 20 × 120 × 2.2 / 1000 ≈ 10.6 V (4.4%) - complies with a small margin.
So 10 mm² is the minimum. Consider going up a size if the owner may add load or if the welder is assessed at its full rated input current rather than a diversified value.
Other checks before sign-off:
- Tabulated current-carrying capacity from AS/NZS 3008.1.1 with the correct installation method and derating factors (burial depth, soil thermal resistivity, grouping, ambient).
- Protective device rating not exceeding the derated cable capacity.
- Earth-fault loop impedance at the far end of the submain, verifying the 32 A MCB (and the workshop MCBs) will disconnect within the required time.
- Protective earthing conductor size: for a 10 mm² phase conductor the PE must be 10 mm² (AS/NZS 3000 Table 5.1).
- Maximum demand for the workshop, and verification that the supply and main switchboard can carry it.
3. Explain the earthing arrangement at the workshop: is the local electrode correct, and what bonding do you require?
Read the worked answer
The property is one MEN installation. The workshop earth bar is (and must remain) connected back to the main earth bar at the house by the submain protective earthing conductor - that is the fault-current return path.
- The workshop must not have a MEN link. Re-bonding neutral to earth at the workshop creates a parallel neutral path, causes diverted neutral current on the PE and will nuisance-trip RCDs (and can be dangerous if the submain neutral is lost).
- A local electrode at the workshop is permitted - it can help with potential grading - but it must be bonded to the workshop earth bar and does not replace the submain PE or act as the sole earth return.
- Bond the metal water pipework and the steel frame to the workshop earth bar with conductors sized to AS/NZS 3000 Table 5.1 (minimum 6 mm² for this installation).
- Verify by earth continuity and equipotential bonding resistance testing between the earth bar and each bonded part, and confirm the electrode connection is accessible and labelled.