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How this is calculated#

A load schedule lists the boards of a site and what each one draws. The tool works the maximum demand of each board by the switchboard diversity method of AS/NZS 3000 Appendix C, and carries it up the tree to the main board.

Para C2.4.4 sets the method for switchboards that conform to the AS/NZS 3439 or AS/NZS 61439 series. The maximum demand of a board is the sum of the overload ratings of its circuits, times a factor from Table C4. The factor falls as the number of circuit protective devices downstream rises.

On each board the tool counts its final subcircuit devices and one device for each submain it feeds. It adds the ratings of the final subcircuits and the rating of each submain device. The submain is a circuit of the board that feeds it, so a sub‑board's demand is never added twice. Table C4 prints no factor below two devices, and the tool then takes the sum as the demand.

The ratings are per phase. On a three‑phase board, enter the sum on the most loaded phase, and count a three‑pole device once. The tool gives each board's demand in amps and in kVA, and the site's demand at the main board.

Each sub‑board's demand is checked against the device that feeds it. A board whose demand is more than its submain device gets a warning.

The schedule prints as a sheet with one row a board.

Worked examples#

• A workshop site with four boards has a site maximum demand of 283.5 A a phase, 196.4 kVA. The main board's ratings add to 405 A.
• A house with a garage board has a maximum demand of 149.1 A, 34.3 kVA. The main board's ratings add to 213 A.
• A main board feeding one busy board has a site maximum demand of 200 A a phase, 138.6 kVA. The main board's ratings add to 250 A. The busy board's demand of 192 A is more than its 160 A submain device.
• A main board and two distribution boards has a site maximum demand of 325.5 A a phase, 225.5 kVA. The main board's ratings add to 465 A.

Limits#

The method is only for boards that conform to the AS/NZS 3439 or AS/NZS 61439 series. For other boards, use another Appendix C method, such as the maximum demand tool. With the toggle off, the tool takes no Table C4 factor and gives the sum of the ratings.

It works from device ratings, not from the loads. A board full of large devices with small loads gives a high demand. A load-by-load assessment can give a lower figure.

See also: Maximum demand, Circuit loading and Max demand by phase.