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The National Calculation Method (NCM) describes the procedure, for buildings other than dwellings, for demonstrating compliance with the carbon emission requirements of regulation 17C of the Building Regulations and calculating ‘operational ratings’ and ‘asset ratings’ in the production of Energy Performance Certificates (EPC’s).

The NCM provides the underlying method and the standard data sets necessary to calculate the annual energy use of a proposed building and comparing it with the energy use of a ‘notional’ building of a similar type, under similar circumstances. This is done by calculating the Target Emission Rate (TER – the CO2 emission rate for the notional building) and comparing this with the Building Emission Rate (BER) for the proposed building. The BER must not be higher than the TER.

The calculations can be performed using approved simulation software (Approved Dynamic Simulation Models (DSMs)) or by using the Simplified Building Energy Model (SBEM), a computer programme developed by BRE and available to download from the NCM website. Unlike some of the DSM’s, SBEM is a ‘simplified’ compliance tool, rather than a design tool.

A design stage SBEM assessment should be completed before the construction starts, and then an as-built SBEM when construction is completed, which includes the results of an air permeability tests and demonstrates the building has been constructed in accordance with the design. This is referred to as a Building Regulation UK Part L report (BRUKL).

2013 revisions to Part L of Building Regulations, which took effect on 6 April 2014 have been implemented in a new version of SBEM, cSBEM. SBEM may still be used on projects for which transitional arrangements apply, see 2013 changes to the approved documents for part L of the building regulations for more information.