Load Assessment

Since the supply of electricity is a service the electrical load cannot be fully assessed until the requirements of that service have been notified to the project electrical designer. Some loads will be determined by the project electrical designer, for example the lighting loads and for office space the load per work station, many other loads will have to be notified by the other disciplines for example lifts, air handling and refrigeration, these will be notified via the project electrical engineer, unless the project electrical designer is also carrying out that function. It is not only the size of the loads that are of interest it is also necessary to know the level of supply security and the geographic locations of the loads.

The level of supply security required will determine whether one or two supplies from the PES (Public Electricity Supplier) are required, and whether generators and uninterruptible power supply (UPS) units are also required.

The size of the site and the geographic locations of the loads will determine the locations of the switchrooms/sub-stations.

To this end it is necessary to have a form which can be filled in by the members of the project design team, these are the notification of electrical supply requirements forms. The forms must be carefully laid out as when it all goes wrong; no project is ever completed without problems at some point in the design and construction phase; they will form part of the paper trail to demonstrate how the job was done and when load information was received and when supplies were notified to the other project disciplines by the project electrical designer. So what information is required, the item or plant ident, both its alphanumeric code and title, its location, its load in kW and Amps, whether three or single phase, if a motor its direction of rotation when looking back towards the motor from the drive end, the date the connection is required, the date the supply is required to be made live, a signature box for the person requesting the supply together with a box for the name in block capitals, date requested, company and date received.
On the reverse of the form the supply parameters, load definitions, conditions of connection, and requirements prior to supply should be detailed.

The pdf version of this section includes an example of an Electricity Supply Requirements Notification form. It is a separate file entitled electricity supply requirements notification form.pdf with an original size of A4 (210mm x 297mm) with two pages which should ideally be printed on one sheet of paper.

Prior to receiving the notification of electrical supply requirements an outline load assessment may be prepared, in modern offices the work station density can be as much as one unit per seven square metres, each work station is approximately 750VA which equates to 107VA per square metre. If the work station density is known then a more accurate figure can be calculated. Lighting levels are now approximately 500lux this equates to a load of approximately 14 VA per square metre, however that load predates the introduction of LED lighting, areas lit by LEDs will have an approximate load of 3VA per square metre. Server farms, data centres, wan bridges etc can be assessed at 10kVA per square metre. Then there are the plant areas, refrigeration can be assessed as the electrical load per square metre plus the lighting electrical load per square metre multiplied by the area plus the occupancy multiplied by seventyfive, plus the south and west glazed area in metres multiplied by 400 the total is then multiplied by the refrigeration factor of 40% to give the refrigeration electrical load. Air handling unit loads can be assessed as being the total floor area in square metres multiplied by 16VA to give the electrical load. The domestic water pumps, hose reel pumps, sprinkler pumps, canteens, coffee machines, print rooms, lifts, escalators can all be allocated an assessed total load of 10% of the sum of the electrical loads above.

For factories and warehouses the client usually has more information regarding the loads, shops and hotels are a very individual things and will require a greater input from the client and other members of the project team; although the large chains usually have standard layouts therefore load data will be available from previous projects.

For institutional buildings such as schools and hospitals there is again much data to be used from previous projects.

For stadia an approximation can be made from the capacity, for each person a load of 125VA may be assumed for stadia that are not enclosed and 155VA for stadia that are enclosed.

Theme parks present a unique problem in that no two are the same and that newer, bigger and better attractions are always being installed. To prosper a theme park needs a new major attraction every three years with minor changes to attractions every year. The loads in a theme park can be extreme, for example, linear drive powered roller coasters can have a load of 2.5MW or more applied for 3.5 seconds every minute, the application in one step of this type of load can cause severe problems with other rides and attractions due to the voltage fluctuations that occur when a load of this size is applied. It may also breach the maximum permitted load application that may be applied to the PES network. As a general rule a load of 15kVA per hectare can be used for outline design. The maximum voltage drop that may be inflicted on the PES, in the England, at the point of common coupling due the application of a load is 1% if the application is more than once in any three hour period or 3% if less frequent.

It is vital to remember that the above are approximations to enable outline design to be undertaken, the actual loads will only be properly known when all the notification of electrical supply requirement forms have been received and the data on them incorporated into the design.

Once the initial load for the project is determined it should have a notional increase per year added for ‘creeping load growth’ to determine the load at the end of life of the electrical installation. Historical data for the type of establishment or site being designed will give a reasonable annual load increase, where this is not available a factor of 1-1.5% should be used for general purposes, however for theme parks that factor should be 2% per year plus an additional 5% every third year. This will then give the load at the end of the twenty-five life span of the electrical installation. The end of life load should be used to determine the size of the equipment to be used in the installation. It is also the load that should be used in negotiation with the PES to determine the required reserve capacity.

The reserve capacity is the term used for the contracted level of electricity supply. When planning a distribution network a PES needs to allocate loads for each consumer, with domestic supplies they use standard loads with all other loads they sell the capacity as well as the load. Therefore if you negotiate a 10MVA reserve capacity then that level of supply is allocated to you even though you may have a maximum load of only 5MVA, if during the lifetime of the system the reserve capacity is exceeded the PES will require a new application for an increase to the reserve capacity. There will be costs involved which the PES are permitted to recover from you; and that can also include any costs incurred by the PES to reinforce their network. That is why it is important to have a reserve capacity that reflects the anticipated load at the end of life of the installation.

The supply security level definitions shown on the reverse of the electricity supply requirements notification form are reproduced below:-