Tenant Electrical Metering and Sub-Distribution

What You Need to Know

Every multi-tenant commercial building needs a clear electrical metering and distribution strategy. This determines how each tenant receives power, how their consumption is measured, how they are billed, and how much capacity is available for their fitout. Getting this wrong creates problems that are expensive to fix after construction: undersized distribution boards that cannot support tenant loads, missing metering that prevents accurate billing, or non-compliant installations that fail inspection.

There are two fundamental approaches. Utility metering (also called direct metering or revenue metering) gives each tenant their own meter registered with the distribution network service provider (DNSP), such as Ausgrid or Endeavour Energy in NSW. The tenant deals directly with their chosen energy retailer. Sub-metering means the building owner installs private meters on each tenant's supply and on-charges electricity based on recorded consumption. Most commercial office buildings in NSW use utility metering for major tenancies and sub-metering for smaller tenancies or shared services.

The electrical sub-distribution system connects the main switchboard (MSB) to each tenant's distribution board (DB) via submains cables routed through risers and ceiling voids. The design must account for current capacity, voltage drop, fault levels, and future flexibility. A well-designed system allocates 80 to 120 VA/m2 for standard office tenancies and provides 20% spare capacity for fitout variations. This article covers the rules, practical considerations, and coordination requirements for getting tenant metering and sub-distribution right.

• AS/NZS 3000:2018 governs all electrical installations. The Wiring Rules require that every final subcircuit is protected by a circuit breaker or fuse at the distribution board, that submains cables are sized for both current-carrying capacity and voltage drop (maximum 5% total from the point of supply to the final outlet), and that distribution boards are accessible for maintenance. Tenant distribution boards must be located within or immediately adjacent to the tenancy they serve. (AS/NZS 3000:2018, Sections 3.6, 3.9, and 4.13)
• NCC 2025 Part J8.3 mandates energy sub-metering. Commercial buildings (Class 5 to 9) with a floor area greater than 500 m2 must install sub-metering to separately measure major energy end uses: HVAC, lighting, domestic hot water, and tenant loads. Each tenancy in a multi-tenant building must have its own energy meter or sub-meter. This is a base building requirement that applies regardless of whether the building pursues a NABERS rating. (NCC 2025, Part J8.3)
• NSW Service and Installation Rules (SIR) govern utility metering. In NSW, the SIR published by Ausgrid and Endeavour Energy specifies the metering panel configuration, meter board dimensions, CT (current transformer) metering thresholds, and the physical location requirements for revenue meters. Metering panels must be in a common area accessible to the DNSP without requiring tenant access. For loads above 80 A per phase, CT metering is required instead of direct-connected meters. (Ausgrid NS220, Endeavour Energy MIR)
• Maximum demand calculations set the capacity allocation. AS/NZS 3000 Section 2.2 provides the method for calculating maximum demand. For commercial offices, the calculation uses a loading of 25 VA/m2 for general lighting and power circuits as a minimum, with diversity factors applied based on the number of circuits and the building use. In practice, tenant capacity allocations of 80 to 120 VA/m2 are typical for office tenancies, with higher allocations for data-intensive or food tenancies. (AS/NZS 3000:2018, Section 2.2, Table C1)
• Voltage drop must be managed across the distribution chain. The total voltage drop from the main switchboard to the furthest outlet in a tenancy must not exceed 5%. In a tall building with long vertical risers, the submains cable from the MSB to a tenant DB on Level 20 can consume 2 to 3% of the voltage drop budget, leaving only 2 to 3% for the tenant's internal circuits. This affects cable sizing: longer runs require larger cables to stay within limits. (AS/NZS 3000:2018, Section 3.6.2)
• Residual current devices (RCDs) are required on socket outlets. AS/NZS 3000:2018 requires RCD protection on all socket outlet circuits in commercial buildings. Each tenant DB must include RCDs rated at 30 mA for personal protection. This applies to all general power outlet (GPO) circuits within the tenancy. Lighting circuits and dedicated equipment circuits may not require RCD protection depending on the installation conditions. (AS/NZS 3000:2018, Section 2.6.3)
• NABERS Energy requires check metering beyond the NCC minimum. Buildings pursuing a NABERS Energy rating must install check metering that complies with the NABERS Energy Metering Guide. This requires Class 1.0 accuracy meters (or better) on all major energy end uses, including separate meters for base building HVAC, tenant HVAC supplementary systems, lighting, lifts, domestic hot water, and any other loads exceeding 10 kW. Data logging at 15-minute intervals is required. (NABERS Energy Metering Guide, 2024)

Start with the tenancy schedule. The architect's tenancy plan defines how many tenancies exist on each floor, their size in square metres of net lettable area (NLA), and their use (office, retail, food, medical). Each tenancy type has a different electrical loading. A 500 m2 office tenancy at 100 VA/m2 needs approximately 50 kVA, which translates to a 80 A three-phase supply at 400 V. A 200 m2 food retail tenancy at 300 VA/m2 needs approximately 60 kVA and a 100 A three-phase supply. These numbers drive every downstream decision: cable sizes, DB ratings, metering type, and riser capacity.

Design the riser and submains. The electrical riser is the vertical pathway carrying submains cables from the main switchboard (typically in the basement or ground floor) to each level. Each tenant's submains cable runs from the MSB to their floor-level DB. In a 10-storey building, the submains to Level 10 might be 50 to 60 metres long. For a 100 A three-phase supply over that distance, you need a minimum 35 mm2 copper cable to stay within the 5% voltage drop limit, though 50 mm2 is often specified to provide spare capacity. The riser must be sized to accommodate all current and future tenant submains cables, including spare conduits for tenancy reconfiguration.

Locate distribution boards strategically. Each tenant DB should be inside the tenancy, typically in a services cupboard near the tenancy entry or adjacent to the core. The DB must be accessible without moving furniture or stored goods. Distribution board sizing should allow for the initial fitout circuits plus 20 to 30% spare ways for future additions. A typical 500 m2 office tenancy needs a 36 to 48-way three-phase DB. Larger tenancies or those with high equipment loads may need a 72-way board or a sub-distribution arrangement with multiple boards.

Separate metering from distribution. In buildings with utility metering, the metering panel is a separate enclosure located in the common area (usually the electrical riser cupboard on each floor or a dedicated metering room). The submains cable runs from the MSB to the metering panel, through the revenue meter, and then to the tenant DB. This means two cable runs per tenant: MSB to meter, and meter to DB. For sub-metered buildings, the sub-meter is installed inside the tenant DB or on the submains cable at the MSB, eliminating the need for a separate metering panel but adding a private meter that the building owner must read and maintain.

Coordinate with the switchboard room design. The MSB must have sufficient space for all tenant circuit breakers, metering CTs (if applicable), and surge protection devices. A 20-tenancy building needs at least 20 outgoing circuits on the MSB, each with an appropriately rated moulded case circuit breaker (MCCB). The switchboard room must provide 1,000 mm minimum clearance in front of the switchboard and 600 mm at the sides for safe access and cable termination.

Plan for tenant fitout changes. Commercial tenancies change hands every 5 to 10 years. Each new tenant may have different power requirements. The base building electrical design should anticipate this by providing spare capacity in the MSB, spare conduits in the riser, and enough space in the metering panel for additional meters. A common approach is to install 25% more riser capacity than the initial tenancy schedule requires. This avoids costly riser upgrades when a new tenant with higher power needs takes over a tenancy.

Handle shared services separately. Common area lighting, lifts, fire systems, and base building HVAC are not tenant loads and must be metered and supplied separately. These loads are typically supplied from dedicated sections of the MSB with their own metering. The essential services supply (fire pumps, emergency lighting, smoke exhaust fans) must have dedicated circuits that are physically segregated from tenant distribution and protected against disconnection.