The underfloor trunking system has been around for a long time. The first time I saw it was inside an application guide published by a public telecommunication company.
The technical manual was already very old and I was in the first year of real professional work after my graduation. That was 23 years ago.
Picture 1 – Underfloor service box installation in progress
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Why do we need an underfloor trunking system?
An underfloor trunking system is an alternative way of providing the dedicated routes to run electrical cables, telephone cables or any other wiring cables you can think of.
I said wiring cables. An 11KV cable is not a wiring cable. A 25 sq.mm electrical feeder cable to supply an 11 KW fire pump panel is not a wiring cable.
(Note: 25 sq.mm means 25 millimeter square. It is a measure of the net cross-sectional area of the electrical conductors of an electric cable).
The 1.5 sq.mm cables that are used for wiring the office lights are categorized wiring cables.
The 2.5 sq.mm or 4 sq.mm cables used in socket outlet wiring are also categorized as wiring cables.
That is on the electric power cables.
On the telephone side, the telephone wiring cables going to each telephone outlet in an office area are also called wiring cables.
But an incoming 100-pair telephone cable from the public telephone company into a multi-storey office building is not a wiring cable.
Likewise, a Cat 5 computer network cables going to the office computers from the server rooms can be called wiring cables.
However, a multi-core fiber optic cables connecting two computer buildings cannot be called wiring cables.
So those cables categorized as wiring cables can be run inside the underfloor trunking system.
In fact, the underfloor trunking has been invented specifically for this purpose.
Why the special treatment is given to the underfloor trunking?
Why not use the normal conduit and trunking? (See the conduit and trunking pictures here: Conduit installation pictures; Electric trunking pictures.)
The underfloor trunking system was developed long before I started my career in electrical engineering.
However, I think I can guess why there was a need for this system.
The need arose because of the popularity of open office layout system in the design of buildings.
There is no doubt that many residential buildings also use underfloor trunking systems. However, these buildings do not really have to use this system. The normal conduit and trunking system would serve the purpose perfectly well.
However, in an open office system, it is difficult to bring the wiring cables to the working tables in the middle of an office space (i.e. away from the walls) without running the cables inside the floor.
With many tables away from the walls, then many trunking and conduit need to be cast into the concrete floor.
Furthermore, different types of cables (eg. electrical and computer network cables) need to be run in different trunking and conduits totally segregated from each other.
In the end there were many trunking and conduit running all over the place inside the concrete floor slab of an office building with the open office concept of design.
So gradually the underfloor trunking concept developed, naturally.
Picture 2 – The underfloor trunking installation in progress
This picture shows a stretch of underfloor trunking installation in progress.
Notice that the floor reinforced concrete slab has been cast. The underfloor trunking was laid onto the already completed structural slab of the floor.
After the underfloor trunking components that need to be cast in have been laid out and fixed, a layer of concrete (called screeding) is poured to the floor to a thickness of about 50mm.
This additional two inch of concrete would cover the trunking parts, but the junction box would be exposed for access.
The thickness of the concrete screeding should be enough to give strength (and therefore would not crack) at the thinnest areas, which are the areas above the trunking parts.
Observe the notes that I gave in the picture.
During installation, there is always some time lapse between the installation pf the underfloor trunking parts and the pouring of the screeding concrete.
During this time, the trunking, junction boxes and service boxes need to be firmly held in place temporarily.
Steel bands and lean concrete are used for this purpose.
The temporary cover for the junction box opening is installed to prevent the fluid concrete from flowing into it during the concreting work. This temporary cover is made of soft metals and is usually supplied together with the junction box or the service box unit.
Notice also that there are three lengths of trunking installed together. So it is a 3-compartment trunking system. It could have been a 4-compartment or 2-compartment.
Here the trunking material is made of high-impact PVC trunking. However, an underfloor trunking can also be made of metals.
What is the difference between a service box and a junction box?
I should have explained this earlier so that beginners do not get confused.
A service box is a box along the underfloor trunking where the user can connect to the power outlet, telephone socket and computer socket.
It is the point of “service”.
Picture 3 below shows how a service box looks like.
Picture 3 – An underfloor service box
While a junction box is provided to facilitate the drawing in of cables during installation and maintenance.
It is also provided where a trunking need to make a bend and where it branches off.
That is why it is called a “junction” box.
Picture 4 – A completed underfloor junction box
Observe that the completed junction box cover is firmly fixed with 4 mounting screws at the corners.
On the other hand, the cover of the service box is designed so it can easily be open frequently.
That is because the service box is designed for user access. This is where users plug in their appliances just like the wall sockets.
Picture 5 – Vertical access box
The underfloor trunking resides at the floor. However, the cabling inside the trunking must come from the distribution panels somewhere.
If the distribution panel is located at the wall, then there must be a connection between the trunking inside the concrete floor and those at the walls.
That is the purpose of the vertical access box in Picture 5 above.
Sometimes, the electrical distribution panel is located inside the electrical riser which is some distance away. Usually the most convenient method of running the main trunking by hanger brackets above the ceiling.
Then at convenient locations, a set of droppers are installed to connect to the underfloor trunking. This is shown in Picture 6 below.
Picture 6 – Vertical access connection to trunking above ceiling
Picture 7 below shows another view of a junction box and underfloor trunking installation is progress, before the floor concrete screeding was poured in.
Picture 7 – Junction box and underfloor trunking picture
Construction works are never free from errors. Picture 8 below shows that a finished floor had to be hacked in order to extend the wiring from a junction box to the dry wall.
Picture 8 – Wall socket wired from an underfloor junction box
This was actually a last minute decision that was made to add another general purpose electrical outlet to the wall.
Theoretically it is best to wire general purpose electrical sockets on walls from separate circuits (better still, from a separate section of the busbar inside the electrical DB) than those inside the underfloor trunking, which supplies the work computers and other high priority equipment.
This is because the general purpose sockets are those used for such things as electric kettle, vacuum cleaners and other similar appliances.
Defects and faults in these appliances can cause trippings of the earth leakage circuit breaker (ELCB) at the electrical distribution panel, which can cause annoyances and other more genuine problems. (See pictures of ELCB at this post, 1-phase ELCB connection pictures.)
You can see more pictures of electrical installations at this post, Electrical installation pictures.
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