Current limits for different types of motors

4 October 2023

Recent enquiries indicate some confusion around current limits for different types of motors.

This article provides clarification on determining the maximum allowable inrush current for various motor applications, including single-phase, three-phase and rural single/split-phase loads as per Section 10 of the WASIR.

Single-phase starting currents

Table 5 in Section 10 of the WASIR covers single-phase starting currents. The starting current for metro and major regional areas is limited to 45A. Other locations in the SWIS network are limited to 18A.

Three-phase starting currents

Table 6 in Section 10 of the WASIR covers the requirements for three-phase starting currents. There are three sections, and loads greater than 6kW require a small calculation.

For example, a 10kW motor requires the current to be reduced to 45A as follows: 10kW –  6kW = 4kW,   4kW x 3.2A = 12.8A,   12.8A + 33A = 45A.   

Note: Table 3 in Section 7 states the kW values for starting currents on the Horizon Power network. 

Inrush currents for motors in rural supplies with single/split-phase loads

Table 7 of Section 10 of the WASIR covers these motor types.

240V single-phase motors may be started direct online (DOL).

Provided the inrush current does not exceed the maximum levels in the table below, 480V split-phase motors up to 7.5kW may be started DOL, where the installation is served by a sole use transformer 25kVA or larger.