question on installing expansion joint in conduit

[James23912]

I am having the underground wire replaced from the meter to the garage, about 20'. using 4 wire and conduit, my electrician is on vacation and i do not want to bother him so I am running the new conduit now and got an expansion joint for the garage side. There are two on the meter post conduits already installed , each joint is tight together, IOW, fully compressed, so I am curious as to why they are not put in halfway open, or if perhaps the ground had pushed them up tight? I was assuming I would be putting the new one halfway open, there is a midpoint on the piece. I will probably just leave the end part off for the time being but am just curious, YouTube videos seem to show them compressed also, I would think that frost is more likely to push up rather than down. I am not doing the connections themselves,

 

[wwhitney]

There's two main reasons to use expansion joints:

1) You are installing buried conduit in backfill that you expect to settle in the future, causing the conduit to move downward, but the conduit also connects to a structure you don't expect to settle, like a house on footings on undisturbed soil. For this case you only need to accommodate the conduit lengthening, so you might install your expansion joint fully contracted. But you'll also need to leave slack or a loop in the wires, so they can also handle the downward motion.

2) You are installing PVC conduit (or a very long run of metal conduit), and you need to accommodate thermal expansion. In this case you set the expansion joint to be full contracted if you happen to install at the expected lowest ever temp; fully extended if you happen to install at the expected highest ever temp; and otherwise proportionally according to the actual installation temp.

If both (1) and (2) apply to your install, then you should calculate the expected thermal strain (change in length) on the exposed length of riser, and allocate that much of the expansion range to the thermal expansion, and set the expansion joint accordingly as per the installation temperature. The rest of the expansion range would be fully contracted to allow for settlement.

Cheers, Wayne