Conduit supports on metal roofs

JpSolar

New member
If you are installing IMC conduit for PV DC source circuits for long runs on a large standing seam metal roof (commercial), should you use conduit support straps that allow the conduit to move within the strap (for thermal expansion/contraction) if the support is attached to the roof via a standing seam clip, or should the strap be tight fitted so that there is no movement allowed (assuming the conduit and the roof expand/contract at the same rate)?
 

pv_n00b

Senior Member
Location
CA, USA
Shouldn't the movement be allowed for by using conduit expansion joints? NEC 342 requires that IMC be securely fastened but does not say anything about allowing for expansion. Unlike PVC where it calls out the requirements. It's up to the contractor to determine if expansion will be an issue and plan accordingly.
 
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jumper

Senior Member
Shouldn't the movement be allowed for by using conduit expansion joints? NEC 342 requires that IMC be securely fastened but does not say anything about allowing for expansion. Unlike PVC where it calls out the requirements. It's up to the contractor to determine if expansion will be an issue and plan accordingly.
See 300.7(B) and the Index for other applicable code sections.
 
If you are installing IMC conduit for PV DC source circuits for long runs on a large standing seam metal roof (commercial), should you use conduit support straps that allow the conduit to move within the strap (for thermal expansion/contraction) if the support is attached to the roof via a standing seam clip, or should the strap be tight fitted so that there is no movement allowed (assuming the conduit and the roof expand/contract at the same rate)?
IMO no expansion joint is needed. Not only is the IMC and roofing the same material so it will expand at the same rate, the standing seem clips will have plenty of flex anyway.
 

Carultch

Senior Member
Location
Massachusetts
Expansion joints eat up and release conduit as it expands or contracts. They don't help its displacement relative to the roof surface. Or am I picturing the question wrongly?
On a related note about thermal expansion planning. A lot of PV rail systems specify expansion joints, yet rigidly fasten each support point to the structure below. It would appear that this doesn't allow the expansion joint do its job, if the run is only able to freely expand and contract in one span length, while 9 spans out of 10 are rigidly bolted at both ends.

Yes, standing seam systems can flex a little at each L-foot/attachment, but systems rigidly bolted to a beam system or rafters cannot do the same.

Does anyone else understand how this is supposed to work?
 

MAC702

Senior Member
Location
Clark County, NV
...Not only is the IMC and roofing the same material so it will expand at the same rate...
I would not have expected that. There is a small difference in the type of steel used, but there is a huge difference in material shape and volume or cross-section, and a huge difference in heat sinks.

Steel roofing seems to be completely fastened to prevent any expansion movement. My limited experience with steel roofing is residential, and it's simply screwed down every couple feet.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I would not have expected that. There is a small difference in the type of steel used, but there is a huge difference in material shape and volume or cross-section, and a huge difference in heat sinks.

Steel roofing seems to be completely fastened to prevent any expansion movement. My limited experience with steel roofing is residential, and it's simply screwed down every couple feet.
Some larger metal roofs are secured with "T" shaped clips that allow the roof to move through them as it expands and contracts. They are troublesome to mount solar on because you cannot put an S-5 clip on top of or very near a roofing clip because compressing the seam at that point can restrict the movement of the roof.
 

pv_n00b

Senior Member
Location
CA, USA
Expansion joints eat up and release conduit as it expands or contracts. They don't help its displacement relative to the roof surface. Or am I picturing the question wrongly?
In theory, the conduit mount can take a certain amount of force from the expansion and contraction of the conduit without movement or damage. When the amount of force exceeds a given point the mount will be damaged or pulled out of the roof. The amount of force due to expansion is related to the length of the conduit and the temperature change. The temperature change and length of the conduit run will dictate the number of expansion joints required. The NEC does not provide a table for IMC so you have to do it on your own. You can get the expansion in length per deg from the conduit manufacturer. The NEC does not give a requirement for how much change in length is allowed for IMC, but the 1/4" given for PVC is probably not a bad number or get a number from the conduit mount manufacturer.


To bring it all together, get a temperature change range for the site, using that and the change in conduit length per deg, find the length of conduit you can have where you get maximum change in length you are designing for. That's the longest straight run you can have before you put in an expansion joint.
 
Read carefully what it says in 352.44. An expansion coupling is only needed in select spots - only where you have two rigidly supported ends without 90's between them. Pretty much every other situaion you have enough flex in the system.




Does anyone else understand how this is supposed to work?
No. Had a cable tray system on durablocks on a roof. IT had expansion joints in it. It was totally ridiculous and un-necessary.
 

pv_n00b

Senior Member
Location
CA, USA
Read carefully what it says in 352.44. An expansion coupling is only needed in select spots - only where you have two rigidly supported ends without 90's between them. Pretty much every other situaion you have enough flex in the system.


No. Had a cable tray system on durablocks on a roof. IT had expansion joints in it. It was totally ridiculous and un-necessary.
Floating blocks are great. Just make sure you put something between the block and the roof membrane to protect the membrane from abrasion. Also, don't float the blocks an inch above the roof hanging from the conduit, I've seen that many times.

Unlike PVC, IMC is probably not going to buckle or pull apart, it's just going to rip the mounting off whatever it's screwed too if it moves too much.
 

MAC702

Senior Member
Location
Clark County, NV
...make sure you put something between the block and the roof membrane to protect the membrane from abrasion...
What would be normal things to recommend for this? I'm starting to see more and more of these types of roofs, and it's only a matter of time before I'll be running conduit on one.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
What would be normal things to recommend for this? I'm starting to see more and more of these types of roofs, and it's only a matter of time before I'll be running conduit on one.
Most roofing companies that supply bitumen and TPO roof membranes require that slip sheets of the same type material as the roof membrane be placed under ballasted racking and conduit supports in order to preserve the roof warranty. A few even require that the slip sheets be exactly the same as the membrane (i.e., their product).
 
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