Thursday, April 7, 2011

Distance between conduit draw box

The following few photos and images shows the distance between two draw-boxes in a surface-mounted conduit system. Click here to go back to electric conduit installation.
Photo 1 – Wall-mounted surface conduit

The above photograph shows two metal conduits run on the surface of a wall.
Here I just wanted to show the distance between the two draw boxes on the upper conduit. This distance should not be 9 meter if the requirements of the specifications are to be strictly followed.
No. I did not measure the actual distance while taking this photo.
However, it can be seen that there are three fixing saddles there. The standard distance between saddles in general practice is between 750mm to 1200mm approximately. While the maximum distance of a nearest saddle from a draw box is usually around 300mm. So the distance between the two draw boxes here is around 2.0 to 2.5 meter.
I am just throwing a wild guess here.
There is not really any need to be so strict on the maximum distance between two draw boxes because the purpose of the requirement is to make sure it is possible to do maintenance on the wiring inside the conduit in the future. If the distance is too long, it may be difficult to re-draw the wiring cables in future. The 9 meter conduit may also include two 90-degree conduit bends. That is also a general practice.
Observe that the lower conduit does not have any draw box on the left side while turning downward. The overall length of the conduit run of the lower conduit is still less than 9 meters and it only has one 90-degree bend.
So it is still acceptable. Why the double standard? Why the different practice between the two conduit runs? I do not wish to make this a long post. So I will just give the short answer now. The full answer will be much longer because it involves some issues of contract management. I will dedicate a separate post for this topic in future.
The short answer: The upper conduit is part of the lighting and power wiring for the room. While the lower conduit is the wiring for the room ventilation fan. This room is actually a mechanical room. It houses the smoke spill fans for the fire protection system of the 20-storey office building.
That is why you can see the air duct on the left of the picture. The lighting and power wiring is inside the scope of the electrical subcontractor. The installation of the electrical system need to comply to the specifications of the electrical subcontract.
On the other hand, the upper conduit is the wiring to one of the room exhaust fans. Wiring of the exhaust fan system from the exhaust fan control panel to exhaust fan units is part of the mechanical services contract. The wiring works therefore need to follow the mechanical contract specifications.
Here lies one of the most common sources of problems in the electrical installations of a building of significant size. There is the middle ground between the mechanical and electrical scope of works. This topic deserves and need a dedicated topic to address properly. Click here to get back to electrical conduit installations.
Note: This anchor post, Free electric installation pictures, may contain a summary of the materials you are looking for. It can be faster than clicking through each post title at the Blog Archive. I started it long ago but never actually got around to finish it.
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