Transformer Sizing Calculator

Transformer Sizing Guide for AS 2374 and AS 60076

Transformer sizing is the process of selecting a distribution or power transformer with sufficient capacity to serve the site load today, with enough margin to accommodate future growth over the asset's 25 to 30 year design life. In Australia, distribution transformers must comply with AS 2374 (power transformers) and AS 60076 (IEC harmonised standard), which specify performance, testing, and rating requirements. The sizing process starts with the maximum demand calculation per AS/NZS 3000, applies demand factors and diversity to determine the actual design load in kVA, adds a growth allowance, and selects the next standard transformer size from the manufacturer's range. This calculator handles each of those steps and includes basic protection sizing for both the HV primary and LV secondary sides.

Key concepts

• Maximum demand vs connected load. The connected load is the sum of all equipment nameplate ratings, but not everything runs at the same time. Maximum demand applies demand factors from AS/NZS 3000 Tables C1 and C2 to calculate the actual simultaneous peak, which is typically 50 to 70 percent of connected load for commercial buildings and 30 to 50 percent for residential developments.
• Standard transformer sizes. Australian distribution transformers are manufactured in standard kVA ratings: 100, 200, 315, 500, 750, 1000, 1500, 2000, and 2500 kVA. Smaller single-phase units (10, 16, 25, 50 kVA) serve rural and light commercial applications. Always select the next standard size above the calculated demand. Non-standard sizes require custom manufacturing with longer lead times and higher cost.
• Growth allowance. A growth allowance of 20 to 30 percent is standard practice for a 20 year design horizon. Undersizing means an expensive transformer replacement within a few years. Oversizing beyond 30 percent increases capital cost and no-load losses (iron losses), which run continuously regardless of load.
• Protection coordination. The HV primary is typically protected by fuses (drop-out or full-range HRC) or a circuit breaker with overcurrent relay. The LV secondary uses a main circuit breaker or switch-fuse. Protection devices must be coordinated so that an LV fault is cleared by the LV protection before the HV fuses operate, preserving supply to other transformers on the same feeder.

Common scenarios

1. New commercial building supply. A 6-storey office building with a connected load of 800 kW and diversity factor of 0.65 gives a maximum demand of 520 kW. At 0.90 power factor, the apparent power demand is 578 kVA. Adding 25 percent growth brings the target to 722 kVA. The next standard size is 750 kVA, which would be the selected transformer. The supply authority confirms their network can support this rating at the proposed point of connection.
2. Residential subdivision pad mount. A 50-lot residential subdivision with a maximum demand of 4 kVA per lot (after diversity per AS/NZS 3000 Table C1) gives a total of 200 kVA. Adding 30 percent growth for future air conditioning and EV charging brings the target to 260 kVA. A 315 kVA pad-mount transformer is selected. The supply authority may require a larger unit depending on their network planning guidelines.
3. Industrial site with large motor loads. A manufacturing facility with 600 kW of process motors and 150 kW of ancillary loads. The largest motor is 110 kW with a starting current of 6 times FLC. The transformer must handle both the 750 kW steady state demand and the transient demand during the largest motor start without excessive voltage drop (typically limited to 5 percent on the transformer secondary). A 1000 kVA unit provides adequate margin for both steady-state demand and motor starting.