Connection using spare capacity in the existing supply

How does Connection using spare capacity in the existing supply work, step by step?

1. 1

   Step 1: Establish what the existing connection actually is

   The first task is documentary. The designer establishes the agreed capacity of the existing connection with the network operator, the form the supply takes, who owns which part of the equipment on site, and where the boundary between network and customer assets sits. On older sites the paperwork is often thin, contradictory or describes an arrangement that has since been altered. Where records and reality disagree, reality wins and the records are corrected. This step is desk and enquiry work, and it is worth doing thoroughly because every later decision rests on the number it produces.

2. 2

   Step 2: Measure real demand over a full year

   Nameplate ratings describe what equipment could draw, not what the site does draw, and the gap between the two is usually large. On most projects the assessment uses metered half-hourly consumption data across at least twelve months, so that seasonal peaks, weekday and weekend patterns and holiday shutdowns are all visible. Where that data is not available - an unmetered supply, a recently altered site, a new tenant - temporary monitoring is installed for a representative period. The output is a demand profile, not a single figure, because the shape of the day is what decides whether charging load can be fitted around the existing operation.

3. 3

   Step 3: Model the charging load honestly

   The designer then adds the proposed charging demand to the measured profile. Charging load is modelled from the number of connection points, their rated output and a realistic view of how many are drawing at once. Diversity is genuine on a large car park of slower units where cars sit for hours; it is thin on a small bank of high-powered units where every arriving vehicle wants full output immediately. Assumptions are stated explicitly and agreed with the client, because the difference between an optimistic and a conservative diversity assumption can be the difference between this route and an expensive network application.

4. 4

   Step 4: Test the combined worst case

   The combined profile is tested against the agreed capacity at the worst coincidence rather than the average. That means the day of highest building demand overlaid with full occupancy of the charging bays. A margin is added for the building's own known future growth, since a scheme that consumes every last unit of headroom leaves the occupier unable to change anything afterwards. Where the combined case sits comfortably inside the agreed capacity, the route is confirmed. Where it does not, the project moves to load management, to a reduced scheme or to an application for more capacity.

5. 5

   Step 5: Confirm the existing equipment can carry the new load

   Having enough capacity at the meter is not the same as having equipment fit to distribute it. The designer surveys the existing switchgear and distribution equipment for condition, age, remaining spare ways, available space and the practicality of adding to it. Assessment of the existing installation, and any decision about what may be reused, is work for competent qualified people, carried out under permit with the equipment treated as live until proven otherwise. It is common for this survey to be the point at which the cheap route stops being cheap, because the capacity exists but the board it would come from does not have room for it.

6. 6

   Step 6: Agree the position with the network operator

   Even where no increase in capacity is sought, the network operator is normally notified of what is being connected, and on many projects that notification is a formal requirement rather than a courtesy. The operator decides whether the proposal is acceptable within the existing agreement, whether any conditions attach to it, and whether the load type triggers any additional assessment. Getting that position in writing early protects the programme. Discovering after installation that the operator takes a different view of the same connection is an expensive way to learn the same fact.

7. 7

   Step 7: Design and build the distribution within the site

   With the route confirmed, the project becomes a straightforward one of taking the available capacity from the existing intake to the charging bays. That means the distribution equipment, the routes for cable across the site, the bases the chargers stand on and the metering and communications the operator of the chargers needs. Because the supply already exists and is in use, this work happens on a live site and around an occupier who has to keep trading. Sequencing, temporary arrangements and clear agreement on any planned outages matter as much as the electrical design.

8. 8

   Step 8: Commission, then verify against the model

   Commissioning is carried out by competent qualified people to the designer's specification, and the results are recorded. The step that is most often skipped is the one after: monitoring actual combined demand once the chargers are in service and comparing it with the model that justified the scheme. Real utilisation almost never matches the forecast exactly. Where it exceeds it, the site finds out from data rather than from a nuisance outage, and the operator has time to restrict output or apply load management before there is a problem.