Commercial LV Cabling Design

Design basis

Three-tenancy commercial complex (a small supermarket, a hairdresser and a butcher, plus communal services), supplied at 400 V three-phase (230 V line-to-neutral) from a 500 kVA transformer with a stated prospective fault current of 15 kA at the transformer. Cable selection to AS/NZS 3008.1.1:2025; installation, earthing, protection and maximum demand to AS/NZS 3000:2018 (+ Amdt 1–3, Ruling 1:2024).

Maximum demand and phase balance

Maximum demand was determined by calculation per AS/NZS 3000:2018 Clause 2.2.2(a), using the Table C2 non-domestic diversity method. Every load was classified once, allocated to a phase, and group-diversity rules applied per phase; three-phase loads add their per-phase current to all three phases. The heaviest phase sets the design current: 123.6 A on phase A, with 7.5 % phase imbalance. Per board: communal 39.9 A, supermarket 39.1 A, hairdresser 24.5 A, butcher 41.8 A. The butcher is simultaneously the heaviest-loaded and longest (15 m) submain.

Consumer mains: worked chain

X-90 single-core copper, separate conduits laid in trefoil, buried 600 mm, soil 20 °C, 15 m route. AS/NZS 3008.1.1:2025 Table 3.8 routes this arrangement to Table 3.13, Column 19 (separately enclosed).

Step 1  Design current    Ib = 123.6 A (heaviest phase)
Step 2  Protective device In = 125 A Type C   (Ib ≤ In)
Step 3  Install method    Table 3.8 → Table 3.13, Col 19
Step 4  Correction        k = 1.04 (soil, T3.45) × 0.99 (depth, T3.46)
                            = 1.03
        Required tabulated CCC ≥ 125 / 1.03 = 121.4 A
Step 5  16 mm² → 101 A (fail);  25 mm² → 132 A (pass)
        Iz = 132 × 1.03 = 136 A → Ib 123.6 ≤ In 125 ≤ Iz 136  OK
Step 6  Voltage drop (limit 1 % = 4.0 V), Rc = 0.927 Ω/km
        ΔV = √3 × 123.6 × 15 × 0.927 / 1000 = 2.98 V = 0.74 %  OK
Step 7  PFC at MSB = 8.0 kA; breaking capacity 10 kA  OK
SELECT  25 mm² X-90 Cu active and neutral; 6 mm² Cu earth

A single-conduit arrangement (Column 17) would instead require 35 mm²; that is not the arrangement specified. The result is current-carrying-capacity driven, not voltage-drop driven.

Submains: worst case worked

V-75 single-insulated copper, one conduit, buried 1 m, soil 20 °C. Table 3.8 routes this to Table 3.12, Column 17. Combined correction k = 1.05 × 0.95 = 1.00. The butcher submain governs.

Step 1  Ib = 41.8 A          Step 2  In = 50 A Type C
Step 4  k = 1.00 → required CCC ≥ 50 A
Step 5  6 mm² → 45 A (fail); 10 mm² → 59 A (pass); Iz = 59 A
        Ib 41.8 ≤ In 50 ≤ Iz 59  OK
Step 6  ΔV = √3 × 41.8 × 15 × 2.23 / 1000 = 2.42 V = 0.61 %  OK
SELECT  10 mm² V-75 Cu; 4 mm² Cu earth
        (16 mm² recommended for practical margin)

Final subcircuits: the thermal-insulation catch

The specification states thermal insulation in all ceiling spaces, and the final subcircuits clip across the ceiling joists, so the cables are not in free air. A cable clipped to a structural member within bulk insulation is a partially-surrounded thermal-insulation installation under AS/NZS 3008.1.1 Clause 3.4.3. Ratings were therefore read from the partially-surrounded column with the 45 °C ambient correction (Table 3.44, factor 0.93). This condition raised most power circuits from 2.5 mm² to 4 mm², the single most consequential installation-condition decision in the design.

All Iz ≥ In and every voltage drop is within budget; the worst-case total path (mains + butcher submain + lighting final) sums to 3.2 % against the 5 % limit.

Earthing and protection

Protective earthing conductors were sized from AS/NZS 3000:2018 Table 5.1 on the MEN system, main earth connected at the MSB neutral bar. Because the 25 mm² consumer-mains active was set by current-carrying capacity and not upsized for voltage drop, the 6 mm² main earth follows directly from Table 5.1. Type C circuit breakers were used throughout (suited to the low inrush of LED lighting, resistive heating and small motors), with 30 mA RCDs on all final subcircuits up to 32 A supplying socket-outlets and lighting. Nominal-current grading (125 A, then 40 to 50 A, then 10 to 20 A) gives current discrimination; full selectivity is to be confirmed against manufacturer time–current curves, a stated limitation of scope.

Fault level and earth-fault loop

Source impedance  Zs = 400 / (√3 × 15 000) = 0.0154 Ω/phase
Consumer mains    R  = 0.884 × 15 / 1000   = 0.0133 Ω
Z at MSB   = 0.0154 + 0.0133 = 0.0287 Ω
PFC at MSB = 230 / 0.0287    = 8 014 A = 8.0 kA

All device breaking capacities (10 kA) exceed the local prospective fault current. Earth-fault-loop impedance was checked for the longest circuit of each conductor size against AS/NZS 3000:2018 Table 8.1 for Type C breakers; the worst-case loop (4 mm² general-power final, 24 m, on the butcher submain) gives Zs ≈ 0.57 Ω against a limit of ≈ 1.15 Ω. The external loop impedance Ze = 0.0345 Ω is a declared assumption; the real figure comes from the network operator's connection-point fault data or an on-site loop-impedance measurement.

Declared assumptions and limits

Assumptions are logged explicitly rather than hidden: the socket-outlet diversity basis, the consumer-mains conduit arrangement (Column 19 basis confirmed against the controlled 2025 edition at transcription), the assumed Ze above, and discrimination pending manufacturer curves. The partially-surrounded ratings used were cross-checked against the 2017 edition and confirmed unchanged in the 2025 edition. Final values are to be verified against the controlling standards and network authority at installation. This write-up is sanitised from graded coursework: the design scenario is paraphrased, and no standards table content is reproduced.