Energy sub-metering: do you measure up?

Sub-metering is a small but important part of the Building Regulations Part L2: Conservation of fuel and power in buildings other than dwellings, and there is still widespread ignorance of its requirements. As a result, in some installations there is inadequate provision of metering while in others unnecessarily costly or complex systems are specified.

Building Regulations L2

Building Regulations Part L2 is published as two documents: L2A covers new non-domestic buildings and L2B covers existing non-domestic buildings.

The regulations call for sub-metering so that at least 90% of the estimated annual consumption of each fuel (electricity, gas, LPG, oil, etc) is assigned to different end uses such as lighting, heating, ventilation, pumps and fans. The purpose is to develop a sub-metering strategy so that users can identify areas where improvement can be introduced in order to achieve energy savings of 5-10% or better.

Other key sub-metering stipulations are:

• Reasonable provision of energy meters in existing buildings can be achieved by following the recommendations of CIBSE Technical Memorandum TM39:Building energy metering (A guide to energy sub-metering in non-domestic buildings.)
• Reasonable provision of energy meters would be to install sub-meters in any building with 500 m2 of floorspace or more.
• Meters should be used to monitor the performance of any low and zero-carbon (LZC) system.
• In buildings with a total useful floor area greater than 1000 m2, facilities should be provided for automatic meter reading and data collection.

TM39 is an updated version of CIBSE General Information Leaflet GIL65: Metering energy use in new non-domestic buildings. This can be downloaded free from www.cibse.org/pdfs/GIL065.pdf. GIL 65 gives guidance for designers on how to develop a metering strategy to optimise cost, improve operators’ understanding of their buildings and achieve the required reduction in energy use.

L2 requirements apply to premises with a floor area greater than 500 m2, including separate buildings on multi-building sites. Specific recommendations are made for plant and equipment for which separate metering should be provided:

• Motor control centres feeding pumps and fan loads greater than 10 kW
• Boiler installations greater than 50 kW
• Chiller installations greater than 20 kW
• Electric humidifiers greater than 10 kW
• Final electricity distribution boards greater than 50 kW.

This last recommendation is especially relevant because the majority of distribution boards are rated 100 A or more, and therefore three-phase boards will generally be rated above 50 kW. Furthermore, the traditional practice of feeding lighting and small power loads from a common distribution board means that separate metering may be required for grouped power and lighting loads.

Metering solutions

Approaches to sub-metering vary. In some cases all metering is provided at the main switchboard. This has the advantage that meters are all in the same location so manual collection of data is easy.

On some sites, MCCB panelboards provide sub-distribution to final distribution boards and to loads such as lifts, ventilation or air-conditioning plant. This will require sub-metering at the panelboard. Bolt-on metering chambers and associated meter packs that can be used to monitor specific loads are available for panelboards.

If final distribution boards supply more than one type of load (typically lighting and small-power) and metering at the main switchboard or panelboard, separate feeders and two distribution boards instead of one will be needed. But if the distribution board can meter grouped loads it is possible to use a single feeder.

Different solutions are available for metering at the final distribution board.

• Custom-built boards incorporating metering – generally an expensive option.
• Separate meter packs. These are used with standard distribution boards and provide a more cost-effective solution.
• Distribution boards with integral metering – suitable products are now available.

In each case there are options for a single meter to monitor the entire board, or for split metering to monitor separately grouped lighting and small-power loads. These options are available with both Type A (single-phase) and Type B (three-phase) distribution boards.

It should be borne in mind that in some split-metering applications, one meter measures one group of MCB ways while the second measures the entire board. This calls for external calculation for the second group of MCB ways.

Ideally, meters should include remote reading capabilities, even if these are not put to immediate use. As a minimum, this should be a pulsed output offering remote measurement of kWh.

A better solution is a Modbus design that provides information via an RS485 connection. With one of these connections, information can be read directly from the meter. Some data registers, such as peak demand, can be reset remotely. If the meter is connected to an effective energy or building management system, it can provide a more informative energy monitoring capability.

Summary

Specifiers and installers do not need to go to the expense of custom-built distribution boards and panelboards to ensure compliance with Building Regulations Part L2. A range of metering solutions is now available for Type A and Type B boards including add-on meter packs for use with standard distribution boards and distribution boards with integral metering capabilities (see box).

The design of these units minimises the amount of on-site work for the contractor and standardised design allows boards to be sourced through the normal electrical wholesaler network.

Where greater sophistication is required, ethernet connectivity can be used to integrate the sub-metering into a comprehensive energy management architecture for effective monitoring, control and management of the complete energy infrastructure in large sites.