Maximum Run of EMT Conduit

I'd think the limiting factor in EMT length would't be the temperature extremes, which you don't mention, but the length of the delivery truck.

Its not important. The joints, splices, bends, and sags will take up any expansion that occures.

This is what I think as well, but...

One of the first/biggest fires of rooftop PV system in USA (I guess at Target store in Fresno, CA) was caused by such issue. The couplings were not tightened enough/equally by installers and conduit popped out of the couplings, cut isolation and created short circuit. If there would be an expansion coupling it could prevent this situation.

So, installing an expansion coupling makes sense, there are different sizes available but I am just trying to get to some literature/code or manufacturer recommendation if there is any…

Interesting question. I haven't heard of expansion joints in electrical conduit, but I sure have seen some pulled-apart joints from various causes!

If there are substantial lengths turned 90° at each end of the long run, and if the clips along the run allow some sliding, you might be okay without any other special provisions.

Could use some water tight flexible conduit every so often in a loop to break up long runs. Not sure what electrical codes would say about it. "I need to find my code book"

Charles

There are requirements for compensation of thermal effects on EMT, as well as other metal conduits. If you have a copy of the 2008 NEC, look at 300.7(B). Specifically take note of the footnote. If you don't have a copy, let me know and I'll provide the info.

Thanks for referring me to the NEC. This is what I was looking for to. Of course I do have a copy on 2008 NEC.

So, it seems that I just need to do the math, but I agree with the next post that the 200' to 400' would be the thumb of rules.

They do make expansion couplings for EMT. The UPC number is 0403* (* = 1 for 3/4, 2 for 1", Etc). They should be used any time an expansion joint occurs in the structure where the EMT is rigidly secured to. If the EMT is floating such as hanging from the structure by all-thread, you do not have to install expansion couplings.

Rule of thumb pulling distances in EMT vary from 200' to 400'. The real determining factor is the tensile strenght of the conductors being pulled. For that you need to calculate the friction of the conduit vs the weight and friction of the conductor. Look up a conductor manufacture site on the web they should have the calculations available. Okanite used to have a really nice section on calculations.

I definitely agree with you comments. After doing the math 200' – 400' sounds about right, but in order to get better distribution of the tension I would prefer to use more couplings with small expansion instead of large one. Other words, the shorter the better.

I like Tornado's answer. If you put a 90 degree bend, even two in a straight run the conduit can stretch and shrink back and forth and not pull the conduit out of the connectors.

I see this on steam pipes for the same reason. Have you ever noticed large steam pipes doing an inchworm hump? Betcha it is to allow expansion and contraction without stressing welds. I was looking at the power plant nearby, and the pipes never seem to go straight to their destination, always do a bend or two on the way for no apparent reason.

Here is a copy of the Target fire and what went wrong, Sorry so long. http://www.green-fire.org/content/targetrooftop.pdf