In the 22 weeks set aside for the project, the electrical contractor Drake and Scull Technical Services had to completely replace the lighting, fit 445 variable air volume (vav) air conditioning units, and renovate the underfloor busbar power supply to socket outlets.
Flexible prefabricated wiring was chosen for the lighting and vav circuitry for two main reasons. First, it is very quick to install and it allows flexibility to the end tenants. At the time the work was started, the whole building had not been let, so there were no end clients to specify their particular requirements.
Second, as the timelines were so tight, the electrical work began while the demolition and stripping out work was still going on. This potentially difficult situation was improved by only needing to employ four electricians as opposed to 12 for a conventional project. It is unlikely that this number of electricians would have been available without having to pay premium rates.
Drake and Scull project manager John Moran estimated that only 3333 h would be required for the project using prefabricated wiring as opposed to 8706 h by conventional methods. The saving in labour costs of 62% means that, despite an increased capital cost for materials, there was an estimated overall cost saving of 35% (£56 000) on this part of the project.
Flexible prefabricated wiring does not need metal conduit, which is time-consuming to install. In the Icon building, wire baskets have been used to secure the flexible, but heavy, multicore cables that link the main distribution board on each floor to up to five zonal distribution boxes (or hubs). Even the wire baskets are not strictly necessary if other suitable attachment points are available.
From each zonal distribution box, lighter flexible cables with pre-fitted connectors distribute the power to the luminaire circuits. In this project, eight circuits (typically) emanate from each zonal distribution box, but up to 20 circuits could be accommodated.
The first cable simply plugs into the zonal distribution box at one end and into the first luminaire in the circuit at the other. The luminaires all have factory-fitted connectors. In this project the flexible cables link one luminaire to the next in a daisy chain.
The cables and circuits are rated to allow about 20% more luminaires to be added to any particular circuit without any risk of overloading.
Once the heavy multicore cables are in place, two skilled electricians can wire up the luminaires on a whole floor of the building (more than 1500 m2) in just over two days.
Points to watch
This type of system works most effectively when the luminaire array is a regular shape and where the zonal distribution boxes can be optimally positioned – an open plan office area such as the Icon building is ideal.
The maximum time-saving benefit will be achieved when all the flexible cables connecting the luminaires are the same length. If different lengths are supplied for each fitting then installation becomes like a jigsaw puzzle.
However, if all cables are supplied at the maximum required length and the area is very irregular there may be some very large excesses of cable left in the ceiling void.
This was just one of the problems encountered by the John Lewis Partnership (JLP) when they tried flexible prefabricated wiring in the offices attached to a Waitrose supermarket in Billericay, Essex. The area was quite small and required only one zonal distribution box. However, the area consisted of two corridors at right angles, with offices on both sides of the corridors.
Furthermore, the corridors were fire escape routes and solid firewalls lined both sides of the corridors. Therefore all the lighting circuits had to enter the offices through firewalls, which meant that longer lengths of cable were needed to minimise the number of breaks in the walls, in line with the Fire Regulations. The irregular shape of the area led to quite large excesses of cable being lodged in the ceiling void.
Despite the installation problems caused by the firewalls, and some time lost due to late delivery of parts, the project still saved £1000 over a conventional installation. Bill Wright, head of electrical engineering at JLP, estimates that a further £1000 would have been saved had the problems not occurred.
Factors for success
Good design to minimise waste, and timely supply of the right parts, are essential to a prefabricated wiring project. For the Icon project, an extra 5% of cables were supplied to each area (eg: an extra two cables in an area of 40 luminaires) to give flexibility for last minute changes in design and spares for future alterations.
As all the initial designs are drawn up from plans, there are always unforeseen requirements when the job is underway. It is much cheaper to slightly over specify in the first order than to put in small subsidiary orders when the project is in progress. However, if prefabricated wiring systems are to maintain a good reputation for supplying fast-track projects, the manufacturers must be able to supply products on time at the start of the project, and to supply extra parts at short notice.
With no standards currently covering prefabricated wiring, it is also important that individual manufacturers maintain supplies of particular wiring systems to allow affordable upgrading of lighting designs in the future.
Busbars for adaptability
Bill Wright of JLP is always looking for ways to make his department’s wiring projects smarter, quicker and cheaper. In the John Lewis department stores such as the new development at West Quay, Southampton, busbars have a number of advantages.
The shop floors of large department stores like West Quay are very large, open plan areas. However, the arrangements of displays and till positions are by no means static. When they are moved, this has to be done very quickly to minimise disruption to the store operations. JLP uses a grid system of busbars for both the luminaire power supply in the ceiling and for the underfloor power socket supply. The underfloor supply consists of two independent circuits within the same busbar – one "clean" circuit for till and computer use, and one general purpose supply.
The busbars are spaced at 4.8 m intervals across the shopfloor and have connection points at 0.5 m spacing. This means that wherever the luminaire or floor socket box is situated, it can connect to a busbar using a 3 m cable.
JLP has added to the system’s flexibility by using luminaires and ceiling tiles that are totally and easily interchangeable. The ceiling tiles can also be hinged down to give easy access to any part of the circuit without removing the tile completely. The floor tiles and power socket boxes are equally easy to exchange.
With such large power supplies all needing to be connected to the main store distribution boards, the main plantroom could contain considerable amounts of very heavy-duty cables, which all require at least 0.8 m radius bends to change direction. A considerable amount of space is saved in the plantroom by using heavy-duty busbars (0.30 m by 0.15 m, 4 kA) with prefabricated right-angled joints.
JLP estimates that its busbar solution is 30% simpler to design and draw because it is only necessary to draw the busbars, not individual circuits and socket outlets. It is also easier for a contractor to price and to assess the tender returns, simpler to install, is robust to other parts of the construction process, and offers a net minimum saving of 20% on combined capital and installation costs.
Summing up the benefits
Testing of both busbar and flexible prefabricated wiring plug and play systems is simplified as much of the testing is done off-site and the modularity of the approaches makes on-site testing more straightforward.
Prefabricated systems have been shown to offer significant advantages over conventional methods in terms of on-site time and overall cost. When well designed, they can offer an ease and elegance of construction not offered by conventional systems with a timescale almost impossible to match.
Source
Building Sustainable Design
Postscript
Hilary Graves is a research scientist with the BRE’s Environmental Engineering Centre. Further reading Roche L, ‘Only connect’, Building Services Journal, 3/2000. The author would like to thank all the owners, designers, contractors and manufacturers for their help and co-operation with the case studies. The research was commissioned by the DETR as part of a programme of research and innovation to improve the construction process.
