Challenge: Calculate the Maximum Demand for This Industrial Workshop

You have been handed a load schedule for a new metalworking workshop. Supply is 415 V three-phase from a dedicated transformer. The loads are:

Calculate the maximum demand using AS/NZS 3000:2018, Appendix C methodology. Then select a standard transformer rating.

Step 1: Convert each load to VA (apparent power at the supply terminals).

For motors, the supply VA includes both efficiency and power factor losses:

VA = kW / (PF × η) × 1000

Total connected load = 238,964 VA = 239.0 kVA

Step 2: Apply diversity factors per AS/NZS 3000:2018, Appendix C.

Diversity factors account for the reality that not all loads operate simultaneously at full rated power. The methodology varies by load type:

Motors: Per Appendix C, the largest motor is taken at 100%, and remaining motors are diversified. For industrial workshops with CNC and production equipment:

• Largest motor (hydraulic press, 69,652 VA): 100% = 69,652 VA
• Second largest motor (milling machine #1, 37,520 VA): 75% = 28,140 VA
• Third largest motor (milling machine #2, 37,520 VA): 60% = 22,512 VA
• Fourth to sixth motors (CNC lathes, 19,175 VA each): 50% = 28,762 VA total

Diversified motor demand = 69,652 + 28,140 + 22,512 + 28,762 = 149,066 VA

Socket outlets: First socket outlet at 100%, remainder at 50%:

Diversified socket demand = 2,400 + (3 × 2,400 × 0.50) = 2,400 + 3,600 = 6,000 VA

Lighting: Generally taken at 100% for commercial/industrial (all lights on during working hours):

Diversified lighting demand = 8,421 VA

HVAC: Continuous load, taken at 100%:

Diversified HVAC demand = 18,728 VA

Total maximum demand:

MD = 149,066 + 6,000 + 8,421 + 18,728 = 182,215 VA = 182.2 kVA

Compare this with the total connected load of 239.0 kVA. The diversity factor reduces the demand to 76% of connected load — a realistic figure for a workshop where the hydraulic press, milling machines, and CNC lathes do not all operate at full power simultaneously.

Supply current at maximum demand:

IMD = 182,215 / (√3 × 415) = 182,215 / 718.8 = 253.5 A

Standard dry-type transformer ratings (kVA): 100, 150, 200, 250, 315, 400, 500, 630, 750, 1000.

The calculated maximum demand is 182.2 kVA. Selection considerations:

• 250 kVA provides 37% headroom above calculated MD. This allows for motor starting surges (which can momentarily double the demand), future load growth, and calculation uncertainty.
• 200 kVA provides only 10% headroom — insufficient for motor starting transients. The hydraulic press alone draws 5–7 times its running current during DOL start (approximately 350–490 kVA momentarily), which would cause unacceptable voltage dip on a 200 kVA transformer.
• 315 kVA is conservative but justified if the workshop plans to add equipment within 5 years.

Recommended: 250 kVA transformer for current loads, or 315 kVA if expansion is planned.

The upstream cable from the transformer to the main switchboard must be sized for the transformer’s full rated current, not just the calculated MD — because the transformer can deliver its rated current and the cable must not be the bottleneck if future loads approach the transformer rating.

This calculation exposes several errors that appear repeatedly in practice:

1. Forgetting motor efficiency. Using 15 kW directly instead of 15,000 / (0.85 × 0.92) = 19.2 kVA understates each CNC lathe demand by 28%. Across all motors, this error would understate motor demand by approximately 35 kVA — enough to select an undersized transformer.
2. Double-diversifying. Applying diversity to individual motors AND then applying a second diversity to the total motor group. Appendix C applies diversity once: rank motors by size, apply the descending percentage, sum. Do not then apply an additional “motor group” diversity on top.
3. Ignoring motor starting. The maximum demand calculation gives the steady-state demand. Transformer sizing must also consider starting transients. A 55 kW motor starting DOL draws approximately 330–440 kW momentarily (6–8 × FLC × PF at start). If two large motors can start within seconds of each other, the transient demand can briefly exceed the transformer’s continuous rating. This is acceptable if the voltage dip remains within limits (<15% at the motor terminals per AS/NZS 3000, Clause 3.6.2), but it must be checked.
4. Treating socket outlets like motors. Socket outlets use a different diversity methodology (first at 100%, remainder at reduced rates) because their usage is unpredictable and intermittent. Motor diversity is based on operational scheduling. Mixing the two methodologies in the wrong direction overstates or understates the result.

Standards referenced: AS/NZS 3000:2018, Appendix C (maximum demand), AS/NZS 3008.1.1:2017 (cable sizing), AS 2735.1 (dry-type transformer ratings).