Distribution transformers step down High Voltage (11kV / 22kV) grid power to standard Australian Low Voltage (230V single-phase / 400V three-phase). Sizing transformers correctly under AS 2374 and AS/NZS 3000 requires accounting for total apparent power (kVA), power factor, and a 20% safety margin to handle starting inrush currents without thermal degradation.
The Transformer Sizing Formula — Single-Phase and Three-Phase
Transformers are rated in apparent power (kVA) rather than real power (kW) because thermal heating limits depend on total vector current and voltage running through the primary and secondary copper windings:
Single-Phase: Apparent Power (kVA) = (Voltage × Current) / 1,000
Three-Phase: Apparent Power (kVA) = (Voltage × Current × 1.732) / 1,000
Required Transformer Capacity (kVA) = Calculated kVA × (1 + Safety Margin / 100)
The multiplier 1.732 accounts for the square root of 3 (sqrt(3)), handling 120-degree phase displacement between active lines. Standard Australian secondary voltages are 230V phase-to-neutral and 400V phase-to-phase.
Step-Up vs Step-Down Transformers: Operating Principles & Sizing
Transformers operate on Faraday's law of mutual induction. Depending on the turns ratio between the primary winding ($N_1$) and secondary winding ($N_2$), the transformer steps voltage up, down, or isolates the circuits 1:1:
- Step-Down Transformers (Primary Voltage > Secondary Voltage): The most common commercial application in Australia. Incoming high-voltage distribution (11,000V or 22,000V) is stepped down to 400V 3-phase and 230V single-phase to power building consumer switchboards, pumps, and plant machinery.
- Step-Up Transformers (Primary Voltage < Secondary Voltage): Used where local generation must be matched to high-voltage transmission lines or long-distance feeds. Common examples include commercial rooftop solar arrays stepping 400V inverter output up to 11kV/22kV for utility grid export, or remote mining bore pumps running long sub-mains where higher voltage prevents excessive I²R line losses.
- 1:1 Isolation Transformers (Primary Voltage = Secondary Voltage): Primary and secondary windings have equal voltage (e.g. 230V to 230V, or 400V to 400V) with complete galvanic separation. Used in hospital operating theatres, marine marina berths, and sensitive telecommunications facilities to eliminate ground loops and protect delicate equipment.
Full Load Current (FLC) Calculation Equations
When sizing primary switchgear, drop-out expulsion fuses, and secondary circuit breakers, use the standard Australian full load current formulas:
Three-Phase Secondary FLC (A) = (kVA × 1,000) ÷ (Secondary Voltage × 1.732)
Three-Phase Primary FLC (A) = (kVA × 1,000) ÷ (Primary Voltage × 1.732)
Single-Phase FLC (A) = (kVA × 1,000) ÷ Voltage
Transformation Turns Ratio = Primary Voltage ÷ Secondary Voltage = Secondary Current ÷ Primary Current
Worked Example 1 — Sizing a Single-Phase Step-Down Transformer for a Rural Shed
Consider a remote agricultural shed in regional NSW running a 230V single-phase sub-main. The peak continuous load on the sub-board is 54 Amps:
Step 1 — Identify Load Parameters
- Load Current (I) = 54 Amps
- Secondary Voltage (V) = 230 Volts
- Safety Headroom Margin = 20%
Step 2 — Apply Single-Phase kVA Formula
Calculated kVA = (230 V × 54 A) / 1,000 = 12.42 kVA
Required kVA = 12.42 kVA × 1.20 = 14.9 kVA
Step 3 — Hardware Selection
The requirement is 14.9 kVA. Select a standard 15 kVA pole-mount transformer. Winding output should be wired with heavy sub-main conductors sized using our AS3008 Cable Sizing Calculator.
Worked Example 2 — Sizing a Three-Phase Kiosk Transformer for a Manufacturing Plant
Calculate required capacity for a ground-mount kiosk transformer stepping down 11,000V (11kV) grid lines to 400V 3-phase for a factory drawing 120A continuous load:
Step 1 — Identify Load Parameters
- Load Current (I) = 120 Amps
- Secondary Voltage (V) = 400 Volts (3-Phase)
- Primary Voltage (V) = 11,000 Volts
- Safety Headroom Margin = 20%
Step 2 — Apply Three-Phase kVA Formula
Calculated kVA = (400 V × 120 A × 1.732) / 1,000 = 83.1 kVA
Required kVA = 83.1 kVA × 1.20 = 99.7 kVA
Step 3 — Hardware Selection & FLC Rating
The site requires 99.7 kVA. Install a standard 100 kVA kiosk transformer. At rated full load, primary current drawn from the 11kV grid is only 5.25A per phase, allowing small drop-out fuses. Secondary rated current is 144.3A. Total site load should be cross-verified with our Maximum Demand Calculator.
Australian Standard Transformer kVA Reference Chart
Typical full load current (FLC) ratings for standard single-phase and three-phase transformers in Australia:
| kVA Rating | Single-Phase FLC @ 230V (A) | Three-Phase FLC @ 400V (A) | Typical Grid Application |
|---|---|---|---|
| 5 kVA | 21.7 A | 7.2 A | Remote telecom mast / small rural shed |
| 10 kVA | 43.5 A | 14.4 A | Acreage bore pump station / site office block |
| 15 kVA | 65.2 A | 21.7 A | Dairy shed / rural homestead connection |
| 25 kVA | 108.7 A | 36.1 A | Commercial machinery shop / welding bay |
| 50 kVA | 217.4 A | 72.2 A | Small industrial complex / commercial showroom |
| 75 kVA | 326.1 A | 108.3 A | Concrete batching plant / mechanical workshop |
| 100 kVA | 434.8 A | 144.3 A | Shopping plaza / medium commercial office complex |
| 150 kVA | 652.2 A | 216.5 A | Cold storage warehouse / light manufacturing line |
| 200 kVA | 869.6 A | 288.7 A | Supermarket / logistics transport hub |
| 300 kVA | 1304.3 A | 433.0 A | Heavy industrial fabrication park / sawmill |
| 500 kVA | 2173.9 A | 721.7 A | Data centre stage / food processing plant |
| 1000 kVA | 4347.8 A | 1443.4 A | Large public hospital / heavy chemical plant |
Common Mistakes When Sizing Transformers
- Confusing kW with kVA: Base transformer capacity on apparent kVA load rather than real kW power. A 100 kW load at 0.75 PF equals 133 kVA. Installing a 100 kVA transformer will cause severe thermal overloading.
- Omitting Inrush Headroom: Sizing transformers to run at 98% capacity. Continuous high-temperature operation accelerates oil and winding degradation. Always maintain at least 20% headroom.
- Ignoring Motor Starting Surge: Direct-On-Line (DOL) induction motor starting draws 6× to 8× full load current, causing severe primary voltage dips if the transformer is undersized.
When is Transformer Sizing Required?
- When establishing temporary site switchboards for major commercial construction.
- When upgrading main switchboard capacity to install commercial HVAC chillers or heavy industrial machinery.
- When installing isolation transformers for sensitive medical scanning suites or IT server rooms.
- When designing customer-owned HV kiosk substations connected to utility networks (Ausgrid, Essential Energy, Energex, Western Power).
Australian Standards & Compliance
Winding configurations and transformer installations must comply with:
- AS 2374: Power Transformers (general requirements, temperature rise limits, and insulation testing).
- AS/NZS 3000: Wiring Rules — switchboard clearances, earthing arrangements, and protection coordination.
- AS 60038: Standard voltage ratings and grid supply tolerances.
Related Electrical Design Tools
Complete your substation and main distribution calculations with our free tools:
- Power Factor Correction Calculator — Reduce reactive current to lower kVA load on transformers.
- kW to Amps Calculator — Convert appliance kilowatt ratings to full load current.
- AS/NZS 3008 Cable Sizing Calculator — Size sub-main conductors for current carrying capacity.
- Maximum Demand Calculator AS3000 — Evaluate switchboard total demand under Table C1 rules.
- Circuit Breaker Sizing Calculator — Size main circuit breakers and protection devices.