expansion joints for emt

[electro7]

Hi,

I am installing long conduit runs on a standing seam metal roof where I am fastening in place strut to secure the conduit. Is there a NEC rule on the max length before I need to install an expansion joint? Any other code references I should refer to?

The plans are saying to keep strut straps loose around the expansion joints. First time installing them so your help is appreciated.

Thanks ahead of time.

 

[hbiss]

How long is long and what is the temperature variation? I have only seen expansion fittings used where a run crosses a building expansion joint.

-Hal

 

[electro7]

500ft maximum, some are less. Estimated temp range is 40 deg. C.

 

[hbiss]

So you are saying that the difference between lowest temperature and highest temperature is 104 deg F? What size conduit? 500 feet uninterrupted by boxes and bends? Somebody smarter than me can figure the coefficient of expansion and come up with how much the length of a straight run will change over that temperature range. I suspect though that it will be minimal and a "Z" in the run will handle it.

-Hal

 

[electrofelon]

Are you attaching to the standing seams? It seems that the roof will expand and contract with the conduit.

 

[gadfly56]

So you are saying that the difference between lowest temperature and highest temperature is 104 deg F? What size conduit? 500 feet uninterrupted by boxes and bends? Somebody smarter than me can figure the coefficient of expansion and come up with how much the length of a straight run will change over that temperature range. I suspect though that it will be minimal and a "Z" in the run will handle it.

-Hal

More about steel conduit than you ever wanted to know, here. From Table 3, the expansion per 100 feet for EMT is 0.82 in. at a delta T of 105°F. So, for 500 feet total length, the swing is 4.1 inches.

 

[GeorgeB]

But won't the metal roof structure expand at about the same rate as the conduit? I'm assuming the structure is steel, even if the roofing is aluminum.

 

[gadfly56]

But won't the metal roof structure expand at about the same rate as the conduit? I'm assuming the structure is steel, even if the roofing is aluminum.

Unless the entire roof rests gently, unattached, on the roof joists, that would be a "no". Everywhere you drive a nail to connect the tin to the sheathing you constrain the movement of the metal.

 

[electrofelon]

Unless the entire roof rests gently, unattached, on the roof joists, that would be a "no". Everywhere you drive a nail to connect the tin to the sheathing you constrain the movement of the metal.

But whatever the nails go into will move too. Granted there may not be as extreme a temperature swing on the substructure.

 

[gadfly56]

But whatever the nails go into will move too. Granted there may not be as extreme a temperature swing on the substructure.

Looking at the Engineer's Toolbox, it appears that the coefficient for wood is about half that for steel, assuming the substrate is, in fact, wood.

 

[JoeyD74]

Use an expansion coupling.