PVC Expansion Joint Question

[JFletcher]

Per NEC 352.44, expansion fittings are required when the change in length is expected to be a quarter inch or greater.

In runs that would have a very large amount of expansion, my question is how much does the wiring inside expand and contract? If it's roughly the same as the PVC, then there would be no issue. I was thinking though in longer runs that the difference between the expansion rates of the wiring and PVC could cause issues, such as stress on terminals or the PVC itself.

Is this ever an issue, and if so how would one resolve it? With smaller wire I could see that a J- box in the middle of the run with a loop of wiring in it would probably solve any issues, but what about larger cables like 4 / 0 aluminum or 500 MCM copper?

I ask because recently I saw an underground parking garage within a very long run of PVC, probably 250 ft, with no expansion joints at all. It was suspended and supported buy all thread hanging from the ceiling and clamps that are typically used for sprinkler piping. The conduit could obviously move in the clamps (eta: apparently negating 352.44) however I imagine there would be a pretty severe distortion of the 90s at the ends on a really hot or cold day, to say nothing of the wiring inside.

Nothing was broken nor appeared repaired, so I guess it works. I also followed a few pieces of EMT and stopped counting at 9 quarter bends, with no pull points between, as the conduits disappeared behind a wall

 

[kwired]

Per NEC 352.44, expansion fittings are required when the change in length is expected to be a quarter inch or greater.

In runs that would have a very large amount of expansion, my question is how much does the wiring inside expand and contract? If it's roughly the same as the PVC, then there would be no issue. I was thinking though in longer runs that the difference between the expansion rates of the wiring and PVC could cause issues, such as stress on terminals or the PVC itself.

Is this ever an issue, and if so how would one resolve it? With smaller wire I could see that a J- box in the middle of the run with a loop of wiring in it would probably solve any issues, but what about larger cables like 4 / 0 aluminum or 500 MCM copper?

I ask because recently I saw an underground parking garage within a very long run of PVC, probably 250 ft, with no expansion joints at all. It was suspended and supported buy all thread hanging from the ceiling and clamps that are typically used for sprinkler piping. The conduit could obviously move in the clamps (eta: apparently negating 352.44) however I imagine there would be a pretty severe distortion of the 90s at the ends on a really hot or cold day, to say nothing of the wiring inside.

Nothing was broken nor appeared repaired, so I guess it works. I also followed a few pieces of EMT and stopped counting at 9 quarter bends, with no pull points between, as the conduits disappeared behind a wall

Copper expands and contracts with temp change but not nearly as much as PVC does. If you don't put expansion fittings on the PVC raceway then when it expands it will try to take conductor with it, if no slack in conductor then it can strain connectors the conductors terminate in.

I have many times tried to tell people that expansion fittings for ground settling or frost heave only benefits the conduit - the conductors inside will still be either compressed or pulled on if the ground is what is moving and there is no slack somewhere to make up for it, seems to fall on deaf ears most of the time though.

 

[GerryB]

Per NEC 352.44, expansion fittings are required when the change in length is expected to be a quarter inch or greater.

In runs that would have a very large amount of expansion, my question is how much does the wiring inside expand and contract? If it's roughly the same as the PVC, then there would be no issue. I was thinking though in longer runs that the difference between the expansion rates of the wiring and PVC could cause issues, such as stress on terminals or the PVC itself.

Is this ever an issue, and if so how would one resolve it? With smaller wire I could see that a J- box in the middle of the run with a loop of wiring in it would probably solve any issues, but what about larger cables like 4 / 0 aluminum or 500 MCM copper?

I ask because recently I saw an underground parking garage within a very long run of PVC, probably 250 ft, with no expansion joints at all. It was suspended and supported buy all thread hanging from the ceiling and clamps that are typically used for sprinkler piping. The conduit could obviously move in the clamps (eta: apparently negating 352.44) however I imagine there would be a pretty severe distortion of the 90s at the ends on a really hot or cold day, to say nothing of the wiring inside.

Nothing was broken nor appeared repaired, so I guess it works. I also followed a few pieces of EMT and stopped counting at 9 quarter bends, with no pull points between, as the conduits disappeared behind a wall

What is a quarter bend?

 

[infinity]

What is a quarter bend?

A 90° bend or the 1/4 of a circle.

 

[JFletcher]

Thanks K wired. GerryB, a quarter bend is a 90* Bend in the conduit, 1/4 of a complete circle.

 

[electrofelon]

I think there is enough extra conductor to account for expansion of the conduit. Usually pulling and pushing in the conductors will reveal plus and minus a few inches. For potential frost heave or settling where the movement could be greater than just thermal expansion, I will make a loop, s curve, or some other strategy to give it more "flex". Often I like to install lugs or breaker up for certain bottom fed stuff. That gives all the flex you'll ever need.

 

[GerryB]

What is a quarter bend?

9 quarters, lot of overtime in that run. (always called em 90's 30's 45's) I don't know are expansion joints needed if out of the sun? I know you don't need then underground, I've never seen them, just coming up out of the ground to say a meter can.

 

[kwired]

9 quarters, lot of overtime in that run. (always called em 90's 30's 45's) I don't know are expansion joints needed if out of the sun? I know you don't need then underground, I've never seen them, just coming up out of the ground to say a meter can.

The rule (which was referenced in OP) is if the conduit is expected to change length by more then 1/4 inch where installed between fixed points (like between two boxes) you must use expansion fitting. If you don't have two fixed points - like at an elbow that is free to move, then you wouldn't need an expansion fitting on that portion of the run.

 

[JFletcher]

I think there is enough extra conductor to account for expansion of the conduit. Usually pulling and pushing in the conductors will reveal plus and minus a few inches. For potential frost heave or settling where the movement could be greater than just thermal expansion, I will make a loop, s curve, or some other strategy to give it more "flex". Often I like to install lugs or breaker up for certain bottom fed stuff. That gives all the flex you'll ever need.

Great idea regarding installing a panel with the lugs on top for a bottom feed set up

 

[electrofelon]

Great idea regarding installing a panel with the lugs on top for a bottom feed set up

I often hate in when people do that, but sometimes its worth it for the piece of mind and easy make up of the incoming feeder. The last solar project we did, I wanted to install the combiner panel boards breaker up for these reasons. They got installed down, and these panels had particularly little clearance and they were a total bear to make up. Just sweeping up and over into the top would have been so much easier. For this install, there were only 12 inverters coming in so its not like it would have been a total mess having the feeder in the side gutter.

 

[don_resqcapt19]

.... The conduit could obviously move in the clamps (eta: apparently negating 352.44) however I imagine there would be a pretty severe distortion of the 90s at the ends on a really hot or cold day, to say nothing of the wiring inside. ....

The fact that the conduit can move in the clamps does not change the requirement to provide expansion fittings. 352.30 says "shall be fastened so that movement from thermal expansion or contraction is permitted." That really requires all straps used for PVC to permit the conduit to move.

 

[FionaZuppa]

The fact that the conduit can move in the clamps does not change the requirement to provide expansion fittings. 352.30 says "shall be fastened so that movement from thermal expansion or contraction is permitted." That really requires all straps used for PVC to permit the conduit to move.

??
but that does not necessarily mean that the "straps" must be loose. it all depends on the mounting system.

 

[don_resqcapt19]

??
but that does not necessarily mean that the "straps" must be loose. it all depends on the mounting system.

I don't know how it can be tightly strapped and still allow the required movement.

 

[GoldDigger]

I don't know how it can be tightly strapped and still allow the required movement.

Rollers as part of the clamp/strap?

Sent from my XT1585 using Tapatalk

 

[Fatty]

I ask because recently I saw an underground parking garage within a very long run of PVC, probably 250 ft, with no expansion joints at all. It was suspended and supported buy all thread hanging from the ceiling and clamps that are typically used for sprinkler piping.

I am not saying they don't exist but i have never seen PVC run above ground that looks any good after a year or more. As for the expansion fitting i wonder if they assumed that because the structure was underground the ambient temprature would not be so extreme, minimizing expansion. Just my 2 cents.

 

[romex jockey]

According to Table 352.44, 250' @ 80F might be an annual 8" of expand/contract , that amount of wire slack wouldn't be a hardship to install imho.... the table just addresses the ambient , i suppose a sunlight and/or brick outside install might rate higher?

Further, PVC straps come in a variety of pvc , up to i think 3" after which vinyl coated strut clips are listed for use >

~RJ~

 

[don_resqcapt19]

According to Table 352.44, 250' @ 80F might be an annual 8" of expand/contract , that amount of wire slack wouldn't be a hardship to install imho.... the table just addresses the ambient , i suppose a sunlight and/or brick outside install might rate higher?

Further, PVC straps come in a variety of pvc , up to i think 3" after which vinyl coated strut clips are listed for use >

~RJ~

I would red tag PVC conduit installed using that type of strap and it would not permit the required movement. That strap is intended to be used in corrosive areas to extend the life of the strap.

 

[FionaZuppa]

I don't know how it can be tightly strapped and still allow the required movement.

if the base is a rubber that allows movement, the strap can be tight to the base. there is no prescription for "free movement".

don_resqcapt19
The fact that the conduit can move in the clamps does not change the requirement to provide expansion fittings. 352.30 says "shall be fastened so that movement from thermal expansion or contraction is permitted." That really requires all straps used for PVC to permit the conduit to move.

 

[don_resqcapt19]

if the base is a rubber that allows movement, the strap can be tight to the base. there is no prescription for "free movement".

A rubber base does not allow the amount of movement that is required for PVC conduit. The movement required at any given support could be a number of inches.

 

[kwired]

I would red tag PVC conduit installed using that type of strap and it would not permit the required movement. That strap is intended to be used in corrosive areas to extend the life of the strap.

I agree.

Will also say that the coating could be compromised if you over tighten and it will rust out anyway. If corrosion is going to be a problem why stainless strut and clamps?