Cable Sizing Guide AS/NZS 3008.1 for Commercial Buildings
What You Need to Know
Every cable in a commercial building must pass four checks. Get one wrong and the cable overheats, the voltage sags, the breaker fails to trip on a fault, or the building inspector knocks back the certificate.
The four checks are current capacity, voltage drop, short-circuit withstand, and earth fault loop impedance. AS/NZS 3008.1.1 sets the method for the first three. AS/NZS 3000 covers the fourth and sets the 5% total voltage drop limit.
This memo gives you the principles, the formulas, and a worked example for a 100 A submain over 50 m.
The Rules
- Every cable must pass four checks: current-carrying capacity, voltage drop, short-circuit withstand, and earth fault loop impedance (AS/NZS 3008.1.1 and AS/NZS 3000)
- Total voltage drop from the origin of supply to any point of use must not exceed 5% of nominal voltage (AS/NZS 3000, Cl 3.6.2)
- The 5% limit rises to 7% where the supply is from a dedicated low-voltage substation on the premises (AS/NZS 3000, Cl 3.6.2)
- Reference ambient conditions for tabulated current ratings are 40°C in air and 25°C in soil (AS/NZS 3008.1.1, Tables 4 to 21)
- Apply derating factors for ambient air temperature, soil temperature, soil thermal resistivity, and cable grouping (AS/NZS 3008.1.1, Tables 22 to 29)
- Disconnection time on a fault must be 0.4 s for socket-outlet circuits up to 32 A and 5 s for distribution and fixed-equipment circuits at 230 V (AS/NZS 3000, Cl 1.5.5.3 and Table 8.1)
- Single-phase voltage drop: Vd = 2 × I × L × (mV/A.m) / 1000
- Three-phase voltage drop: Vd = √3 × I × L × (mV/A.m) / 1000
- Short-circuit thermal limit: I²t = K²S² where K depends on insulation type (AS/NZS 3008.1.1, Section 5)
- AS/NZS 3008.1.1 covers cables up to 0.6/1 kV at 50 Hz a.c. only (AS/NZS 3008.1.1, Cl 1.1)
What This Means in Practice
Start every cable calculation by working out the load current. For a three-phase load, I = P / (√3 × V × PF). Add a margin for diversity and future loads. Pick a cable size from the AS/NZS 3008.1.1 tables that meets the load current after derating.
Derating is where most designers go wrong. A cable rated 100 A in the table might only deliver 70 A when it sits in a hot ceiling space with five other cables next to it. Check Table 27 for the ambient temperature factor. A 50°C plant room derates a 90°C cable by about 12%. Then check Tables 22 to 26 for grouping. Six circuits bunched in a riser can derate by up to 35%. Multiply the factors together and apply to the table value.
The voltage drop check often forces an upsize. A cable that passes the current check on a 50 m run might fail on a 200 m run. The fix is the same: pick the next cable size up and recheck.
Indicative current ratings, single-core copper in conduit at 40°C
| Conductor (mm²) | Indicative rating (A) |
|---|---|
| 1.5 | 17 |
| 2.5 | 24 |
| 4 | 32 |
| 6 | 41 |
| 10 | 57 |
| 16 | 76 |
| 25 | 101 |
| 35 | 125 |
| 50 | 151 |
| 70 | 192 |
| 95 | 232 |
| 120 | 269 |
Guide values only. Verify each design against the current AS/NZS 3008.1.1 tables. The exact value shifts with insulation type, installation method, and arrangement.
The short-circuit check confirms the cable can survive the fault current for the time it takes the upstream breaker to clear. Use I²t = K²S². K is approximately 111 for PVC copper (V75) and 143 for XLPE copper (X-90). If the calculated minimum cross-section exceeds the chosen cable, upsize.
The fault loop impedance check is the last gate. The measured Zs at the furthest point of the circuit must let enough fault current flow to trip the protective device within the disconnection time. Long runs of small cable raise Zs and can fail this check even when the other three pass.
Key Design Decisions
Size for Voltage Drop, Not Just Current
A 100 A submain over 50 m might pass the current check at 25 mm² but fail the voltage drop budget. Size the cable for the longest run in the circuit, not the average. Allocate the 5% budget across the consumer mains, submains, and final circuits before you start sizing.
Plan for Derating Up Front
Plant rooms run hotter than 40°C in summer. Cable risers carry more circuits than the design day allows for. If the design assumes table values without derating, the cable runs hot and the insulation degrades.
Allocate the Voltage Drop Budget Early
A typical commercial split is 0.5% on consumer mains, 1.5 to 2% on submains, and 2.5% on final circuits. Lock this in before sizing the first cable. Without a budget, every designer assumes their share is the full 5% and the total exceeds the limit.
Verify Fault Loop Impedance on Long Runs
Long runs of small cable raise the earth fault loop impedance. The breaker may fail to trip within the required disconnection time even though the cable passes the other three checks. Calculate Zs at the design stage. Re-check on commissioning with a loop tester.
100 A Submain, 50 m Run
A 400 V three-phase submain feeds a tenant switchboard. Continuous load is 100 A. Distance is 50 m.
Step 1: Current check. Try 25 mm² copper XLPE four-core in conduit. Indicative rating is about 101 A. Just passes.
Step 2: Voltage drop. Indicative mV/A.m for 25 mm² XLPE three-phase is about 1.50.
Vd = √3 × 100 × 50 × 1.50 / 1000 = 13.0 V % = 13.0 / 400 = 3.25%If the upstream consumer mains drop 1% and the downstream final circuits need 2%, the total reaches 6.25%. The 25 mm² cable fails the budget.
Step 3: Upsize to 35 mm². Indicative mV/A.m drops to about 1.10.
Vd = √3 × 100 × 50 × 1.10 / 1000 = 9.5 V = 2.38%Total becomes 5.38%. Still over budget. Upsize to 50 mm² or tighten the downstream budget.
The lesson: the current check is the floor, not the answer. Voltage drop usually drives the final size on commercial submains over 30 m.
Who Needs to Know What
References
- AS/NZS 3008.1.1:2017, Electrical installations, Selection of cables, Cables for alternating voltages up to and including 0.6/1 kV, Typical Australian installation conditions
- AS/NZS 3000:2018 (incorporating Amendment 1:2020), Electrical installations (known as the Australian/New Zealand Wiring Rules)
- AS/NZS 3000 Clause 1.5.5.3 and Table 8.1, automatic disconnection of supply
- AS/NZS 3000 Clause 3.6.2, voltage drop
- National Construction Code 2022, Volume One, references to AS/NZS 3000
- Service and Installation Rules of NSW (Ausgrid, Endeavour, Essential Energy)
Note: AS/NZS 3008.1.1 tables are copyrighted by Standards Australia. Indicative current ratings shown in this memo are for guidance only. Verify against the current published AS/NZS 3008.1.1 tables for any design.