Frank Bertie, Chief Technical Officer at NAPIT, looks into understanding the requirements for maximum demand.
When carrying out any new work, whether it is a brand-new building or an addition to an existing installation, BS 7671 requires an assessment of the proposed installation works.
This involves an assessment of the general characteristics of the installation prior to starting design work. Without this knowledge and understanding of the requirements for the installation, the proposed work will more than likely fail to meet the expectations of the client. If an installation has not been designed to take into account any contributing factors regarding the use, loading or operational demands, all manners of problems can arise.
In this article, Iβll cover this process and provide a reminder of the assessment requirements for any new electrical work.
Considerations for new electrical installations
BS 7671 Part One, Chapter 11, lays out the scope for electrical installations. The first step is to determine if the proposed design meets the requirements within Regulations 110.1.1 to 110.1.3. This lists each of the type of electrical installations where BS 7671 applies and includes those details covering a wide range of electrical work that electrical installers will normally be involved in designing.
Regulation 110.2 provides the exclusions from the scope of BS 7671 for which other requirements would have to be considered when designing electrical work in these areas.
Once the type of premises or locations have been established, we can move onto the other considerations that will have to be included in the design.
New electrical installations have a very straightforward assessment process, since the building or premises has been created, the use has been identified, and all of the work will be completed with new products and materials.
Within Chapter 13, Regulation 131.1 provides the requirements that the essential part of the design is to ensure the safety of persons, livestock and property from any dangers which may arise from the reasonable use of the electrical installation.
In the following list, we outline the areas within an electrical installation where the risk of injury may arise:
– Shock currents
– Excessive temperatures
– Ignition of a potentially explosive atmosphere
– Undervoltage
– Overvoltage
– Electromagnetic disturbances
– Mechanical movement of electrically actuated equipment
– Interruptions of power supplies and/or safety services
– Arcing or burning
In the case of existing installations, where additions and alterations are proposed, the designer has extra considerations to make, especially if the installed equipment and services have not been subject to full scrutiny during the initial or subsequent work.
If the original design details are not available to determine the use and loading requirements for which the existing installation was intended to be used, it does place additional responsibility on the new designer to attempt to interpret the original designerβs intent.
Assessment of the installation
Regardless of the installation being new or existing, the designer has to carry out the assessment of the general characteristics of the installation.
BS 7671, Chapter 30, provides the following installation characteristics to be considered:
– Purpose(s) for which it is intended to be used;
– General structure and supplies;
– Any external influences;
– Compatibility of its associated equipment;
– Maintainability;
– All recognised safety services; and
– Assessment for continuity of service.
Based on the aforementioned details, it is vital to assess what the installation will be used for before you commence the design process, to ensure there are no inherent problems resulting from excessive loading or inadequate protection for the circuits.
Maximum demand
Throughout Chapter 31, the term maximum demand is used, and in particular, Regulation 311.1 states how the maximum demand shall be determined. It further expands on this where diversity may be considered for the entire installation or part of the installation.
There is a requirement in Regulation 641.2 that an assessment of the electrical installation is provided to the person carrying out the inspection and testing, which includes the items mentioned previously regarding the installationβs safety and general characteristics.
The Electrical Installation Certificate (EIC) has a box that requires information to be inserted for the maximum demand either in kVA or Amps. It is often listed on the EIC as a figure, but there is very seldom any calculation provided to show how that figure has been determined, or if diversity has been used to calculate.
In some cases, the figure recorded on the EIC may refer to the rating of the service cut-out fuse.
So, how do we assess this maximum demand and take into account diversity for an electrical installation?
A typical example of this can be the increased number of electric vehicle charge points being installed, and this type of electrical installation requires additional notification to the distributor. In reality, all installations that can increase loading that affects the distributorβs or supplierβs equipment may also require notification.
Assessing maximum demand for the installation of EVCPs
For EVCP installations there is a requirement to assess the maximum demand of an existing installation. The maximum demand is the connected load, including any allowance made for diversity, which is referred to as the maximum anticipated load or the actual load. The existing maximum demand must be reviewed as the EVCP electrical load cannot take diversity into account.
To determine the maximum demand of an installation using diversity, Fig 1 shows an example assuming the circuits of a 10-way consumer unit with a main switch rated at 100 A. It has four spare ways and supplies six circuits as follows:
One 40 A cooker circuit
Two 32 A ring final circuits
One 16 A immersion heater
Two 6 A lighting circuits
The calculation in Fig 1 shows there is no allowance for diversity, therefore the EVCP full load shall be used, which is 30.43 A.
The inclusion of the EVCP creates a new maximum demand of 76.8 + 30.4 = 107.2.
Therefore, the new maximum demand exceeds the current rating of the consumer unit. This arrangement is unacceptable and would require an application to the DNO for assessment of the service intake equipment.
In order to reduce the loading, load curtailment would be an option, which can be used to determine the maximum demand.
As you can see in the example in Fig 1, detailed information will be required on the electrical equipment installed within the premises in order to determine the maximum demand figure and insert this figure into the EIC.
Conclusion
By carrying out the required assessment of the electrical installation, the designer will have the necessary information to commence the design, arrange any additional work required to increase the supply to the premises, and then install the proposed work. This will ensure the client has an electrical installation that will meet their operational and safety requirements.
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