Demand over diversity – A case study


The energy balance within modern properties is constantly changing. This has been further accelerated by the current ‘cost of living’ crisis, with the trend towards more home working also having had a notable affect.

The old idea was that a ‘normal’ house would have two Ring Final Circuits, two Lighting Circuits, one Immersion Heater, one Cooker and one Shower. However, this stereotype has been blown apart with the efficiency improvements of heating appliances and availability of low energy electrical equipment, and that’s before we factor in PV, EV, heat pumps and other green tech.

So, how do we go about accurately determining the maximum demand for a property such as a small flat, and what if we have five flats all fed from the same supply?

The complexity of the diversity calculations required, the fact that the figures in the IET On-site guide are way out of date now, and the large margins for error built into these calculations, make this a daunting task for any electrician. But this was exactly the dilemma faced by West London based Mark Crowdy of Electraworks Limited during a recent customer visit.

Mark’s customer owns a large town house in Richmond, London that had been converted over 50 years ago into 5 Flats. The existing 100A single phase service to the property had been split by the DNO into a 6-way single-phase supply, 5 flats and one landlord’s supply for communal areas. This installation had worked very well for the last 50+ years with no known overloading issues.

However, recent works had highlighted that the old service head and tails were looking their age, and conversations with the DNO were underway about updating the supply. This then raised another dilemma.

Would the existing 100 Amp single phase supply be sufficient, or would a larger three-phase supply be needed?

Faced with the responsibility of advising his customer, and the huge cost implications of having a new, larger, DNO supply connected to the property, Mark knew he needed to obtain accurate and reliable maximum demand data. After some online research Mark reached out to Chauvin Arnoux UK, we listened to his application and specified a PEL, power and energy logger, along with a 1-metre-long flexible current sensor which was capable of looping around all 6 tails at once, and therefore capturing the total current in them all.

At a site visit a few days later, the PEL was setup and installed. This was a quick and easy process and involved no interruption of the supply, or disruption for the tenants. The PEL was set to log current for a period of 12 days to capture the full range of loading on the property and locked safely away in the utility cupboard. By performing this activity in the colder weather, the results would include any heating loads that might not be present in the summer months.

The initial results seen on-site for the few hours we were there, were interesting, with the highest recorded current being just 39 amps.

Mark returned to Richmond 12 days later to retrieve the PEL and was soon on his way back to his office to review the over 280 hours of logged data.

Would the log reveal that the 100A main fuse/cut-out was overloaded and an expensive and disruptive upgrade to the supply would be required, or would the results show the supply as being adequate and only therefore requiring updating at the expense of the DNO?

Analysis of the logging data was a simple process using the PEL Transfer software, which displayed Mark’s current log sampled every second both as a table of values, and as a current vs. time graph.

A graph of the total log (Figure 1) showed the maximum current drawn from the supply during the logging period was 48.83A which occurred at 19:12:47 on 9th February 2023. This event spanned a period of approximately 2 minutes. The average current drawn from the supply during the logging period was 8.85A.

The data showed that the maximum current drawn from the supply, during the 12-day logging period was less than half the 100A main fuse/cut-out rating, and far less than the textbook diversity calculation would suggest.

Mark was able to create a report for his client detailing his findings and confirming that the installation was indeed operating well within its supply capacity, and a larger supply wouldn’t be required. Great news and a significant cost saving for them!

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