Spare capacity calculator

How this is calculated#

The tool finds the maximum demand of an existing supply. It then finds the spare capacity against the main switch, per phase. It also checks whether a new load fits. It reads AS/NZS 3000:2018 Cl 2.2.2.

1. Maximum demand by measurement. You enter the highest demand recorded over 30 minutes on each phase (Cl 2.2.2(c)).
2. Calculated demand. You can also enter a calculated demand per phase. The tool uses the larger of the two, because a measured demand above the calculated one is the maximum demand (Cl 2.2.2).
3. Maximum demand by limitation. The tool takes one fixed or adjustable circuit-breaker setting as the demand on every phase (Cl 2.2.2(d)). For mains and submains, you can enter the breakers they feed. The tool adds their settings per phase. A three-pole breaker counts on each phase.
4. Spare capacity. It subtracts the maximum demand from the main switch rating, per phase.
5. New load. A load in kW becomes amps at its power factor and the phase voltage. A three-phase load splits equally across the phases. A single-phase load goes on the phase you choose.
6. Fit. It adds the new load to the maximum demand on each phase. The load fits when every phase stays at or below the main switch rating.

Worked example#

• 63 A single-phase house, 41 A recorded, 36 A calculated, a 3.5 kW heat pump at 0.9 power factor. The recorded demand is above the calculated demand, so the maximum demand is 41 A. The spare capacity is 22 A. The heat pump draws 16.91 A. That leaves 5.09 A, so it fits.

Limits#

• The tool takes the main switch rating as the capacity. The supply fuse or the network agreement can be lower. Check both.
• A recording is only as good as the day you take it. Record when the demand is at its highest.
• The sum of breaker settings takes no diversity. It is the largest of the methods and is often above the main switch.
• The tool does not size the cable or the protection for the new load. Use the cable size calculator and the RCD selection tool.

See also: Maximum demand by measurement and limitation, Will a heat pump fit a 63 A supply?, EV charger spare capacity on a 63 A main switch, EV charging load and spare capacity and the EV charger calculator.