Height and Depth

Above ground wiring must be at the proper height, and wiring that is covered must be installed at the proper depth.  Sounds pretty simple doesn’t it?  However the phrase “safe and legal” transforms that idea into one of many complications and convolutions.

For instance, wiring over the top of a roof has to be of a specific height, depending on the slope of the roof and the distance from the eave.  There is one requirement for the height over your back yard and another for over your driveway.  In fact depending on if your driveway is commercial or residential (in some cases if it is gravel or paved) the height requirement may change.  There are even different height requirements depending if the electrical wiring is going over a street or a highway.

There are different height requirements over a hot tub, or a body of water, or explosive fumes.  If you are placing conduit and electrical wiring on top of a roof, depending on such factors as average ambient temperature, roof construction, etc., there are different requirements for how high OFF the roof the conduit must be run.

If you have a roof covered with tar, the temperatures reflecting off the roof can become very high.  Therefore, the wiring must achieve a certain height off of the roof to provide cooling.  Even the entity doing the wiring installation can have a bearing on the height requirement for a specific wire.

There are just as many requirements for how deep conduit must be buried.  Rigid Steel conduit requires a different depth than PVC, and PVC requires a different depth than direct burial wire.  In fact some conduit is not allowed to be buried at all.

The depth of a ditch for conduit can also depend on what type of cover will be placed on top of the conduit.  If concrete is going to be poured over the conduit the depth of burial will depend on the thickness of the concrete pour.

One of the strangest circumstances I have had to deal with is when a roof is made of a concrete pour mixed with insulation, and conduit, boxes and wiring are encased in the concrete roof pour.  In such an instance, you have to use materials we call “chairs” to keep the conduit a proper distance from the top and bottom surfaces of the pour.

This gets to be a problem when you have the concrete finishers tamping and smoothing the wet cement.  As they perform their work, they can stomp on, step on, or push on the conduit and boxes.  This can result in broken conduit, misplaced boxes, or the whole being too near either the top or bottom surface.  When the concrete pour is finished, you have no way of seeing how well the electrical conduit survived.  Hopefully it all works.

There is a similar puzzle when many floors are being poured in a multi-story building.  There is a wire mesh that is installed in the middle of the pour.  A hook is used to raise the mesh to the proper depth in the wet cement.  However, by doing so, the conduit and boxes can become displaced, or to put it another way, completely out of whack.  I have had to stand and watch my conduit construction while a pour is going on to make sure that the conduit is not pulled out of position or broken.

On a building with no attic, sometimes thick pads of insulation are used and concrete is poured over the top of them.  It is necessary to notch the insulation and bury the conduit, all the while making sure that the conduit is not re-arranged.  On one such job, I got to the job when the concrete pour had already begun.  I had to really work fast. It ended up with some of the boxes and conduit sticking out of the top of the cement.

I couldn’t do anything about it.

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