Multiple mode inverter (MMI) and supply cable rating

Requirements

This guidance relates to installation and safety requirements that must be carried out by Registered Electrical Contractors (RECs) and Licensed Electrical Workers (LEWs) to ensure a safe and compliant battery system installation. LEWs and RECs have various legal obligations to install, be responsible for installation and inspect electrical work and equipment safely under the Electricity Safety Act 1998 (the Act) and the Electricity Safety (General) Regulations 2019.

This guidance is also relevant to Licensed Electrical Inspectors (LEIs) to ensure the completed installation is tested and verified in accordance with the Act and regulations.

As part of the legislative framework, the following Australian Standards are applicable to a Battery Energy Storage System (BESS):

• AS/NZS 5139: 2019 Electrical installations – Safety of battery systems for use with power conversion equipment
• AS/NZS 4777.1: 2024 Grid connection of energy systems via inverters, Part 1: Installation requirements
• AS/NZS 4509.1:2009 Stand-alone power systems, Part 1: Safety and installation
• AS/NZS 3000: 2018 Electrical installations (the Wiring Rules)

The full text of the Australian Standards can be purchased from the Standards Australia store.

Technical guidance

This page provides clarity on clauses from the Standards that relate to the safe and compliant installation and inspection of a BESS.

When a BESS is installed, existing installations must be assessed to check that safety has not been affected.

Licensed electricians and inspectors are encouraged to review the guidance below before contacting us. If you have a question that is not addressed in the Battery Energy Storage Systems section please contact the Renewable Energy Safety team.

Questions and answers

If I am installing a battery system with a single a.c. port that provides both supplementary and alternative supplies with a lower rated output current than the input current, can I consider using the lower rated current value for the selection of a.c. cables and circuit breakers (CB) when no backup is installed?

Clause 5.3.4 of AS/NZS 4777.1:2024 sets out the requirements for the inverter supply cable rating when installing a multiple mode inverter (MMI). When you install an MMI with a single port, the larger of the two values must be used to select the MMI supply cable.

The second paragraph of clause 5.3.4 states:

Example

A battery system with a single a.c port providing both supplementary and alternative supplies has the following specifications:

• Inverter Output max. a.c. Current = 21.7 A
• Inverter Input max. a.c. Current = 43.4 A

The a.c. max input current is the greater, therefore the value of 43.4 A must be used when calculating and selecting the a.c cables and protective devices in this example.

The cable and protective device shall be selected in accordance with AS/NZS 3000 and AS/NZS 3008.1 Electrical installations - Selection of cables series and comply with the requirements of AS/NZS 4777.1, regardless of the connected arrangement or any programmed limitations. That is, whether back up loads are connected or not.

Programmed limitation cannot be used due to the requirements in AS/NZS 4777.1 clause 2.3.1 which states:

If I am installing a battery system with dedicated alternative supply port and separate supplementary supply port, how do I determine the appropriate MMI supply cable rating?

Clause 5.3.4 of AS/NZS 4777.1:2024 sets out the requirements for the inverter supply cable rating when installing an MMI with a dedicated alternative supply port and a separate supplementary supply port.

Where the multiple mode inverter has a dedicated alternative supply port and a separate supplementary supply port, the supplementary supply cable current-carrying capacity shall be not less than the total of the rated charging current of the energy storage for the IES plus the maximum demand of the alternative supply or the overcurrent protection device rating provided for the supplementary supply cable.

The clause provides 2 options to meet compliance, the MMI supplementary supply cable current-carrying capacity shall be either:

1. Not less than the total of the rated charging current of the energy storage for the IES plus the maximum demand of the alternative supply, or
2. The overcurrent protection device rating provided for the supplementary supply cable.

This configuration is becoming the most common for MMI installations.

Example

A 10kW single phase MMI data sheet lists the following specifications:

• Inverter Output max. a.c. Current = 45.7 A
• Inverter Input max. a.c. Current = 78.3 A
• EPS (alternative supply) Output (with Battery) max. EPS Current = 45.5 A.
• Max. Charge Current = 50 A
• Max. EPS Current 45.5 A. + Max. Charge Current 50 A = 95.5A

This MMI has 2 separate ports: a dedicated alternative supply port and a separate supplementary supply port.

Alternative supply cable selection

The alternative supply cable shall be selected to carry greater than 45.5 A or provided with an overcurrent protection device that is appropriately rated to protect the cable (clause 5.3.5.1).

Clause 5.3.5.1 states:

Supplementary supply cable selection

The remaining current listed on the inverter in this example is due to the backup circuits being supplied power via the inverter supply cable during grid operation. The combination of the inverter being able to convert a.c. to d.c. (grid supply to force charge the batteries), while simultaneously supplying power to the backup circuits, means that up to 95.5 A could potentially flow in the cable (although unlikely).

There is no benefit to installing 25 mm2 conductors (rated to handle greater than 95.5 A), to supply the inverter when the entire site is fed by 16 mm2 consumer mains on a 63 A main switch. This is why clause 5.3.4 provides 2 means of compliance, described above.

Therefore, in this example the MMI supplementary supply cable shall be rated either:

1. Not less than 95.5 A (45.5 A + 50 A = 95.5 A), or
2. If a 50 A CB is installed providing protection to a set of 16 mm2 conductors, this would be considered compliant to the clause as the 16 mm2 conductor is provided adequate overcurrent protection by the 50 A CB.

A current limiting CB may comply with the above clause, being selected and designed to provide adequate overcurrent protection to the supplementary supply cable. In rare cases, it may also cause nuisance tripping. This is not a safety or non-compliance issue as identified within the standard, but potentially annoying for the system owner if it occurs.

You must also consider clause 3.4.1, which states:

To determine the total IES rated current, refer to its definition in clause 1.3.26 rated current which states:

Therefore, Inverter output max. a.c. current = Total IES rated current.

Using the example figures from the product data sheet, the total IES rated current would be the Inverter Output max. a.c. Current = 45.7 A. This means a 16 mm2 cable protected by a 50 A main switch would comply with the standard.

Further reading

Disclaimer

The above guidance provides practical and technical guidance. It does not constitute legal advice. Licensed Electrical Workers, Registered Electrical Contractors and Licensed Electrical Inspectors should seek independent advice about their obligations under the Electricity Safety Act 1998 and the Electricity Safety (General) Regulations 2019.

References to the Standards have been reproduced without modifications by Victorian Energy Safety Commission (ABN 27 462 247 657) with the permission of Standards Australia Limited.

Users must not copy or reuse this work without the permission of Standards Australia or the copyright owner.