Transformer Sizing Calculator

Parameters

Every field this calculator exposes, what it means, the range it accepts, and the traps worth knowing before you rely on the answer.

Total loadkW

The connected load the transformer has to serve, in kilowatts of real power. Enter kilowatts, not kilovolt amperes: the power factor field does the conversion. The form accepts 1 to 10,000 kW in 1 kW steps. This is the load field with no default, and primary voltage, secondary voltage, supply phase and cooling type are also required and every output scales directly from it.

Demand factorratio, 0.1 to 1.0

Scales connected load down to the simultaneous peak. Default 0.8. Gotcha: this is one flat factor applied to the whole load. If you have already worked out maximum demand per AS/NZS 3000 Table C1 or Table C2, enter that figure as the total load and set the demand factor to 1.0, otherwise you apply diversity twice and undersize the transformer.

Power factorratio, 0.7 to 1.0

Displacement power factor of the load seen at the transformer secondary, used to convert kilowatts to kilovolt amperes. Default 0.85. If the site has power factor correction fitted, use the corrected value. A lower power factor means more kilovolt amperes for the same kilowatts, so this field moves the selected size directly.

Supply phasesingle phase or three phase

This does not change the required kilovolt amperes at all. It only sets the divisor for the two current figures: single phase divides by the voltage, three phase divides by the voltage multiplied by the square root of 3, which is 1.732.

Primary voltagevolts, line to line

High voltage side. Typical Australian distribution values are 6,600, 11,000, 22,000 and 33,000 volts. The form accepts 1,000 to 66,000 volts. Gotcha: a compliance check flags any primary below 1,000 volts as a fail, so a low voltage to low voltage isolation transformer shows a failed check even though the kilovolt ampere answer is still correct.

Secondary voltagevolts

Low voltage side, line to line for three phase or line to neutral for single phase. Typical values are 400 or 415 volts three phase and 230 or 240 volts single phase. Accepts 100 to 1,000 volts, and a check flags anything at or below 100 volts.

Future growthpercent, 0 to 100

Added to the maximum demand before a standard size is picked. Default 20. Between 20 and 30 percent over a 20 year horizon is normal practice. This figure also feeds the loading check, so raising it increases the selected size and the reported loading percentage at the same time.

Cooling typeONAN, ONAF, AN or AF

Oil natural or air, natural or forced circulation. Recorded for the report only. It does not change the required kilovolt amperes, the selected size, either current, or the loading. No forced cooling uprating is applied.

Ambient temperaturedegrees Celsius

Design ambient at the transformer location, default 40. Also recorded only. No thermal derating is applied for hot sites and no uprating is applied for cool ones.

Assumptions and limits

This is a demand and standard size selection tool. It is deliberately simple, and being clear about where it stops is more useful than pretending otherwise.

What the tool assumes

• The whole load sits behind one demand factor and one power factor. There is no per circuit or per category diversity, and no separation of continuous from intermittent load.
• The standard size ladder is a generic list of 10, 16, 25, 50, 63, 100, 160, 200, 250, 315, 400, 500, 630, 750, 800, 1,000, 1,250, 1,500, 1,600, 2,000, 2,500 and 3,150 kilovolt amperes. It is indicative of the AS 2374 and AS 60076 families rather than transcribed from any one manufacturer catalogue. Your supplier range may differ.
• Both currents are the steady state full load currents at the selected standard rating, not at the actual site demand. They ignore magnetising current, no load losses and tap position.
• The loading percentage compares the required kilovolt amperes, which already include the growth allowance, against the selected size. It therefore describes loading at the end of the growth horizon, not loading on day one.
• The 80 percent loading limit used by the compliance check is a common design convention, not a numbered clause requirement.

What it does not do

Treat the output as a first pass sizing figure. Check it against the current edition of the relevant standard and have the responsible person for the installation review and sign off the final selection. Nothing here is validated or certified.

Worked examples

Three examples covering three phase, single phase, and a case where the loading check fails. Every figure below comes from the same arithmetic the calculator runs.

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.