A load off my mind

On first reading, the latest British Council for Offices guide to office specification seemed, at last, to define how to apply electrical diversity to office power and air-conditioning system design – making the debate between landlord and tenant services engineers about base and build load allowances a thing of the past.

However, I don’t think it does so well on close inspection. To me, the guide still lacks clarity on the subject of electrical load allowances because, while it acknowledges the significance of diversity in rating an electrical infrastructure, it then ignores it in reckoning spare capacity. And when it comes to air-conditioning loads, I think the guide still pushes us to put too much casual cooling capacity into plant but not enough at the floor plates.

Let me explain. The guide sets out electrical and cooling allowances for office small power loads. It sets on-floor loads at 20-25 W/m^2, although it notes that, if this load is spread across enough general office area, it will rarely exceed 15 W/m² at the riser. It applies similar thinking to the related air-conditioning load.

However, the guide hedges about applying diversity to electrical systems, except in calculating the overall building demand. It seems to promote rating the whole network for the on-floor load and then adding 10 W/m² of further capacity – apparently backing away from its assertion that the diversified small power load already provides 10 W/m² of spare capacity.

My empirical data suggests that the BCO has it right with 15 W/m² for loading – providing small power loads are measured far enough upstream for diversity to come into play. In my tests, office small power loads only once much exceeded 10 W/m2 at the switchboard.

My data also showed that total lighting and small power loads remained pretty constant throughout the day, irrespective of how users moved between the range of office work spaces. With occupancy sensors controlling lights and power saving business equipment, it seems users “carry” their casual electrical demand with them – about 250 W each.

So it appears that there is a first-order relationship between the intensity of electrical spot loads across floor plates and occupant densities. Thus, when you subdivide an office floor plate into its different kinds of usage, some traditional correlations between electrical load allowances and accompanying air-conditioning loads begin to unravel.

The guide pushes us to put too much casual cooling capacity into plant but not enough at the floor plates

Terminal cooling devices have to deal with local hot spots, including heat from people. My analyses showed that the distribution of casual cooling loads across a floor plate followed an S-curve – starting at nothing in areas like store rooms and rising progressively through work station areas into more intensive kinds of collaborative spaces and ending with big numbers in the equipment rooms.

The BCO sets uniform allowances for peak on-floor small power, lighting and metabolic casual heat gains that total not more than 45 W/m^2, yet my S-curve indicates that this would only meet the local spot cooling needs of 70% of the areas in my sample floor plates.

When it comes to chillers and distribution systems, my data suggested that the total maximum casual load was less than 25 W/m^2. Depending on interpretation, the BCO guidance works out at about 32 W/m2 – or 42 W/m2 with its recommended spare capacity added.

So I question some of this guidance. Why does it promote one small power load and then add extra spare capacity? And why does it push us to put casual cooling capacity at the wrong end of the system? Although this guide has moved things on, there is still a way to go.

Paddy Conaghan is a senior partner at Hoare Lea Consulting Engineers