Motor loads and Isolators: A practical guide
May 4, 2026 | 3 min read
AS/NZS 3000 outlines that selection of switching devices requires the correct selection of the device to suit the load and the installer to select the correct device to comply with the wiring rules.
Disclaimer: this content is guidance only and does not replace the requirement of reading the latest version of AS/NZS 3000 in full, and other relating standards and local legislation.
Isolator, switches, switch-disconnector...same-same, but different?
The term isolator, switch, switch disconnector a.k.a “load-break switch” are used often in industry, but they have different purposes, as seen below.
Table 1 - Device functions
Determine what function and application you are trying to achieve - check AS/NZS 3000:2018 Clauses 2.3.2.2 to 2.3.7.2.
Motor ratings, do they matter?
The standards have two ways to interpret motor duty/switching for switches or isolators: “M-rating” and “AC-23” utilisation category.
- M ratings (AS 3133) – indicates that the device has been tested for motor duty, it can make and break up to a given locked-rotor current. (LRC or LRA). Check for the “Mxxx” value.
- AC 23 utilisation category (AS 60947.3) – An AC 23 rating, like “AC-23A: 20 A at 415 V”, the switch is suited for motor duty (inrush and locked rotor current). Check the AC-23 rating with the FLC (full load current) of the motor.
So, which one do I follow?
The standard your device is compliant to.
If you are installing an AS 3133 isolator, follow the M-rating.
If you are installing an AS (or IEC) 60947.3 isolator, follow the AC-23 ratings.
Always ensure the device you choose matching the requirement of your installation and is accepted within the latest standard AS/NZS 3000.
Let’s use a domestic single phase air conditioner installation as an example. Where do we start? Easy – latest version of AS/NZS 3000.
- Do I need a switching or isolation device? Yes – Then use an AS 3133 compliant isolating switch.
AS/NZS 3000:2018, Clause 4.19 “Airconditioning and heat pump systems incorporating a compressor shall be provided with an isolating switch (lockable) in accordance with Clause 2.3.2.2, installed adjacent to but not on the unit…” What Sizing? Check the air conditioner rating plate and manual - identify the total:
- FLA/RLA (full-load current/rated load amps) or the rated current if higher (include any control circuit currents)
- LRA/LRC (locked-rotor amps/locked-rotor current), “maximum running current” or similar.
Ensure the total FLA/RLA and the total LRA/LRC are lower than the rated current and M-rating of the isolating switch.
AS/NZS 3000: 2018, Clause 4.13.1.2, “The isolating switch shall have a rating of not less than—
(a) the full-load current of the motor when installed directly in the motor supply circuit; or
(b) the control-circuit current when installed in the motor-starter circuit.
Any switch operating directly in the motor-supply circuit shall be capable of safely interrupting the locked-rotor or stall current of the motor.”
Figure 1 -Example rating plate
Based off these steps and the example rating plate above, the isolating switch NL120L with an M-rating of M120 and a rated current of 20A, will satisfy AS/NZS 3000:2018
In summary, correct installation and device selection hinges on adherence to AS/NZS 3000 requirements. Ensuring an isolating switch is appropriately rated for both the full-load and locked-rotor currents protects motor applications and maintains compliance. Always refer to the latest version of AS/NZS 3000 to keep up to date with changes and manufacturer information for guidance.
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