Picture 1 – Surface run conduit
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What you see here are surface run conduits inside a fan room of an office building.
For some reason that I do not know, personally I like surface run conduits, trunking, etc.
I also like to see things exposed like the exposed structural elements of a covered stadium.
Being able to see the things myself make me feel comfortable.
In the case of these conduits, having them exposed on the wall like that leave no room for any contractor to play with their tricks.
There is no doubt that concealed conduit works help keep the electrical system away from interference and possible accidental damage.
This is important where the space is a public space like a public corridor of a building.
Residential buildings are also not a good place to use surface run conduits like shown in the picture, even though surface run wiring (without conduit) used to be a standard practice in house wiring.
Why do we need these conduits?
The conduits are used to protect the wiring cables that carry the electrical current.
Electricity is dangerous. Even though the electric conductors that carry the current is insulated by some PVC covering materials or whatever, the covering material is not strong enough to protect the cable from damage.
When the covering is damaged, the electrical conductor inside may be exposed to touch or it can also unintentionally come into contact with things that can carry electric current.
This scenario would present a high risk of electric shocks.
Therefore, we protect the electric cables so that they do not present the dangers of electric shocks to people (or animals).
A second reason to protect the cables is to make sure the electrical system stays reliable.
When the cable PVC covering is damaged, the conducting metal can come into contact with other metals that are in contact with earth.
When this happens, the leakage protection of the electrical system will automatically trip the circuit breaker controlling the circuit whose cable has been damaged.
Damaged electric wiring cables can cause this tripping at time we need the electricity most.
Therefore, electric conduits are used to provide this reliability.
Picture 2 – Conduit connection to light switch concealed box
This picture shows how the electrical conduits are connected to the metal boxes.
These boxes are installed to house the rear of wall-mounted light switches. A 13A switched socket outlet mounted flushed to a wall would also have this concealed metal box.
The purpose of the box is to provide a protected space for the wiring cables from the conduit. This space gives enough room for the cables to maneuvers before terminating at the light switch or the socket outlet.
The space also provides the room for the rear portion of a switch unit to sit in.
With this method, we can change a switch unit when it is damaged without disturbing the wiring system too much.
Note the four screw holes already provided by the metal box.
Now, when the electric conduit is cut to suit height of the box, there may be sharp edges at the conduit end. These edges may damage the PVC insulation covering of our wiring cables.
A careful workmanship can easily trim the sharp edges, but controlling workmanship is not easy.
That is why a copper bushing is provided as you can see in the picture.
The bushing also helps to give good contact between the metal box and the metal conduit.
Did I forget to say that these conduits are made of metal?
They are actually galvanized steel conduit.
You can use the cheaper PVC conduit for the same purpose if you like. In that case, you may still use a suitable PVC bushing but not for the purpose of providing e good electrical contact.
The conduit in picture 2 is a concealed conduit for a dry wall partition. So the metal box is a concealed box.
Picture 2Picture 2a below show connection of sufrace conduit to a supface mounted switched socket outlet. The same method of conduit to metal box connection is required here except that the metal box is exposed so it it is a different design.
Picture 2a - Conduit connection to surface mounted 13A switched socket outlet
Picture 3 – Conduit saddle picture
When a conduit is installed exposed like those in Picture 1, it may sag if the distance between two supports exceed certain distance.
An electrical conduit must be installed rigid, permanent and without sag. It should also be able last with integrity for the design life of the building.
What if the building can last 100 years? Should the conduit installation be able to last 100 years also?
Well, if it can, then that is good. However, a wiring system will not be good enough 50 years from the time the building is constructed.
Technology change, how people use electricity change, the personal taste and preference of the people occupying a building space also change. All this will result in the need to renovate the building, and most likely the wiring system also.
The conduit system would not need to last 100 years.
However, it should be able to last 30 years. This is my opinion.
Therefore, once installed, the electric conduit should be rigid, permanent and strong.
In order to satisfy that, electrical installation specifications usually specify the maximum distance between conduit saddles. Some say 900 mm, some say 1100 mm.
The size of the conduit, the materials and the installation environment also are among the major factor in determining the maximum distance between the support saddles that is allowed.
The distance for the orange electrical conduit (the red painted conduit carried fire protection system wiring, not electrical cables) in Picture 1 is around 1.1 meters.
I am guessing only here.
I did not really check the distance. It looked reasonable when I was supervising the job. However, the specifications asked for 900 mm maximum for a one inch diameter conduit, if I remember correctly.
Do not quote me on this. If you are supervising a job, check your own specifications.
If you are doing your office wiring on your own, just keep the saddle spacing to 1000 mm.
Picture 4 – Draw in box
Some people call this a junction box; draw in box, or outlet box.
Whichever name you prefer, this component allows the electrician to draw in cable into the conduit a short length at a time.
If one tries to pull wiring cable into a conduit for too much distance in a single shot, he may find it very difficult to get the job done.
In real installations, the routes available for conduit installation are not straight all the time. There are bends and turns. These bend and turns plus the cable friction with the conduit interior make it hard to pull the wiring cables.
Forcing it too much may damage the cables permanently.
By practice, a maximum of 9 meter distance between draw in boxes on straight run conduit has proved to be practical enough for most wiring works.
If there are bends, then the box should be installed after every two bends.
Okay folks. That is all the time I can spare today.
See you in the next post.
P/S : You can see more pictures at this post, Electrical installation pictures.
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