PVC expansion coupling

[Logisticsman]

Hello,

I am running 1" PVC from residence to a detached garage. Distance is about 45' and the PVC will be buried the required 18" depth and will have a 90 degree transition to the garage at about the halfway point of the run. A NEMA 4X J-box will be mounted on exterior of residence and will be about 2' from the ground. Is it necessary/recommended to use an expansion joint when connecting the PVC to the J-box? Thank you.

 

[don_resqcapt19]

I don't think thermal expansion would be an issue for that length but you still have to comply with 300.5(J). An expansion fitting is often used for that purpose.

 

[Coppersmith]

Underground the temperature remains fairly constant so it's only the exposed section that is going to have much expansion/contraction. Here in Florida, based on the yearly difference in ambient temperature, I only use expansion joints in straight runs over ten feet.

 

[don_resqcapt19]

Underground the temperature remains fairly constant so it's only the exposed section that is going to have much expansion/contraction. Here in Florida, based on the yearly difference in ambient temperature, I only use expansion joints in straight runs over ten feet.

But you don't have frost heave, so if you properly compact the soil around the conduit you don't have the 300.5(J) issue. The OP is in Michigan, and he could have frost heave issues requiring compliance with 300.5(J).

 

[Logisticsman]

Hello all who answered my question.

(1) Thank you for taking the time to reply and provide me information.​

​

(2) I would prefer to use an expansion fitting for my project.​

​

(3) Because location of the PVC will be fall under the "subject to damage" definition in the NEC, are the expansion couplings considered suitable for locations where this is the case? I only see the expansion couplings available in Schedule 40 rating and I am required to use Schedule 80 in this particular area. Just want to make sure the electrical inspector will not have an issue with code on this part. Thank you.​

 

[ptonsparky]

Hello all who answered my question.

(1) Thank you for taking the time to reply and provide me information.​

​

(2) I would prefer to use an expansion fitting for my project.​

​

(3) Because location of the PVC will be fall under the "subject to damage" definition in the NEC, are the expansion couplings considered suitable for locations where this is the case? I only see the expansion couplings available in Schedule 40 rating and I am required to use Schedule 80 in this particular area. Just want to make sure the electrical inspector will not have an issue with code on this part. Thank you.​

Not sure about your inspector but expansion joints have never been called because of 40 vs 80 here. Honest to goodness though, I don’t know what mine are rated.

 

[Logisticsman]

Not sure about your inspector but expansion joints have never been called because of 40 vs 80 here. Honest to goodness though, I don’t know what mine are rated.

Thank you for your comment.

 

[Coppersmith]

(3) Because location of the PVC will be fall under the "subject to damage" definition in the NEC, are the expansion couplings considered suitable for locations where this is the case? I only see the expansion couplings available in Schedule 40 rating and I am required to use Schedule 80 in this particular area. Just want to make sure the electrical inspector will not have an issue with code on this part. Thank you.

Just put them high enough so they are not subject to damage. On a grassy area this is above the height of a mower deck. On a residential driveway this is above the height of a car bumper. In an industrial area it's above the height of a truck bumper or forklift.

 

[Coppersmith]

But you don't have frost heave, so if you properly compact the soil around the conduit you don't have the 300.5(J) issue. The OP is in Michigan, and he could have frost heave issues requiring compliance with 300.5(J).

You are right that I don't have frost heave issues. I moved to the sunny south to get away the cold . But now I'm wondering two things: should the pipe be buried below the frost line; and isn't frost heave a vertical heaving which would not effect the pipe in a horizontal manner? And isn't it uniformly heaving (vertically) across an area?

https://en.wikipedia.org/wiki/Frost_heaving

 

[Carultch]

(3) Because location of the PVC will be fall under the "subject to damage" definition in the NEC, are the expansion couplings considered suitable for locations where this is the case? I only see the expansion couplings available in Schedule 40 rating and I am required to use Schedule 80 in this particular area. Just want to make sure the electrical inspector will not have an issue with code on this part. Thank you.

Most fittings for PVC are not built to be Schedule 40 vs Schedule 80 specific. You aren't going to find schedule 80 expansion fittings, gendered thread adapters, couplings, conduit bodies, or just about any other fitting. For the most part, SCH40 and SCH80 use the same family of fittings. The two schedules of PVC conduit use the same outer diameter. The inner diameter is less for Schedule 80, which means you'll need to anticipate this in your sizing calculation. The fittings are built to fit over the outer diameter of the conduit, which is designed to be the same in both schedules.

The only kind of fitting that I'm aware of being manufactured as schedule 80 specific, is factory elbows/sweeps. And that is due to the fact that there is a functional application of the SCH80 rating, which is withstanding sidewall pressure during a wire pull. SCH80 end bells would be useful so the conduit termination could better meet the intent of even using one in the first place, but I've never seen such a product.

 

[Carultch]

You are right that I don't have frost heave issues. I moved to the sunny south to get away the cold . But now I'm wondering two things: should the pipe be buried below the frost line; and isn't frost heave a vertical heaving which would not effect the pipe in a horizontal manner? And isn't it uniformly heaving (vertically) across an area?

https://en.wikipedia.org/wiki/Frost_heaving

It's not necessarily uniformly heaving across any given site. Local thermal factors could come in to play. For instance, a location exposed to the night sky could freeze before a shaded location, since shade acts as an "insulator" to the objects cooling at night. Another example of a factor that would be how "black" the ground surface is as applies to radiation heat transfer. Objects that are "black" to infrared, emit more heat than a comparable object with a reflective surface. You might also have different amounts of water saturation in the soil, due to the topography and soil types. Some parts may never freeze due to lacking water in the first place, while other parts of the run pass right through the drainage basin and collect all the surface runoff.

A frost heave is vertical. The idea is that you want the underground conduit to be able to rise and fall independently of any structure to which it is attached, rather than absorbing the difference as a mechanical load. If the frost heave "tries" to raise the conduit, but doesn't raise the structure with it, the conduit will pass that load onto the enclosure/structure where it terminates. Conduit is not strong enough to lift a building, and neither is the enclosure bottom where it is terminated. Either that enclosure bottom could get damaged, or the conduit could rupture, from the load imposed on it by the frost heave.

 

[Coppersmith]

A frost heave is vertical. The idea is that you want the underground conduit to be able to rise and fall independently of any structure to which it is attached, rather than absorbing the difference as a mechanical load. If the frost heave "tries" to raise the conduit, but doesn't raise the structure with it, the conduit will pass that load onto the enclosure/structure where it terminates. Conduit is not strong enough to lift a building, and neither is the enclosure bottom where it is terminated. Either that enclosure bottom could get damaged, or the conduit could rupture, from the load imposed on it by the frost heave.

Based on this, it sounds like an expansion joint would not be required for a horizontal conduit subject to frost heaving, but the vertical section would require one.

 

[Carultch]

Based on this, it sounds like an expansion joint would not be required for a horizontal conduit subject to frost heaving, but the vertical section would require one.

Exactly. It's the vertical section as it rises out of the trench, where the expansion fitting is needed, to comply with 300.5(J).

 

[Logisticsman]

Most fittings for PVC are not built to be Schedule 40 vs Schedule 80 specific. You aren't going to find schedule 80 expansion fittings, gendered thread adapters, couplings, conduit bodies, or just about any other fitting. For the most part, SCH40 and SCH80 use the same family of fittings. The two schedules of PVC conduit use the same outer diameter. The inner diameter is less for Schedule 80, which means you'll need to anticipate this in your sizing calculation. The fittings are built to fit over the outer diameter of the conduit, which is designed to be the same in both schedules.

The only kind of fitting that I'm aware of being manufactured as schedule 80 specific, is factory elbows/sweeps. And that is due to the fact that there is a functional application of the SCH80 rating, which is withstanding sidewall pressure during a wire pull. SCH80 end bells would be useful so the conduit termination could better meet the intent of even using one in the first place, but I've never seen such a product.

Hello Carultch.

Thank you for the information. I have looked at the sizing calculation and I will be okay (going with quantity of (3) #6 THHN/THWN wires and a #10 ground wire.

My concern/question relating to Schedule 80 PVC issue was more along the lines of if the expansion coupling was considered okay due to it being exposed in an area where it was called out that Schedule 80 PVC needed to be used. The advice given to just make sure the expansion coupling was above riding mower deck height (for my particular situation) satisfies my concern with this potential issue.

One thing I am not certain of/familiar with is the securing of the PVC to the wall. I know that when using an expansion coupling the PVC needs to be somewhat loosely mounted to the exterior wall so that the expansion joint can move freely, but since there is not an offset fitting made for connecting the PVC to the bottom of the NEMA 4X junction box, is it acceptable to use a type of PVC clamp on the exterior wall. Guess what I am asking is if it is okay to not have the PVC tied directly against the wall as it feeds up to the junction box, but have its distance from the exterior wall be about the thickness of any clamp I would be using? Thank you.

 

[infinity]

If an expansion fitting is required you have to use whatever is made which means good luck finding one that is specifically listed as sch80. Arlington makes a slip fitting where you actually use two different sized raceways (the larger slips over the smaller) as an expansion fitting.

 

[ptonsparky]

Arlington has nice straps for pvc. The only ones we use now. Use strut or whatever to space it out from the wall.

 

[electrofelon]

This is, IMO, the best practice for PVC coming out of the ground. In this case, I couldn't recess the nose of the LB into the wall because even with a box adapter I didn't have enough space. For cases with the nose recessed, it works nice because the extra thickness of the expansion coupling is the same thickness as the LB, so everything is straight. I usually put a screw through the back of the LB and a strap, next size up from the pipe size, over the body of the expansion coupling. Some people like to use sched 80 where it transitions from below to above ground.

P.S. in the pic, I'd like to get another strap higher up. This is my house and I haven't gotten to it yet.

 

[Logisticsman]

Arlington has nice straps for pvc. The only ones we use now. Use strut or whatever to space it out from the wall.

Thank you sir. I will take your advice.

 

[Logisticsman]

This is, IMO, the best practice for PVC coming out of the ground. In this case, I couldn't recess the nose of the LB into the wall because even with a box adapter I didn't have enough space. For cases with the nose recessed, it works nice because the extra thickness of the expansion coupling is the same thickness as the LB, so everything is straight. I usually put a screw through the back of the LB and a strap, next size up from the pipe size, over the body of the expansion coupling. Some people like to use sched 80 where it transitions from below to above ground.

P.S. in the pic, I'd like to get another strap higher up. This is my house and I haven't gotten to it yet.

Thank you for taking the time to provide a photo, and thank you for your advice.

 

[infinity]

This is, IMO, the best practice for PVC coming out of the ground. In this case, I couldn't recess the nose of the LB into the wall because even with a box adapter I didn't have enough space. For cases with the nose recessed, it works nice because the extra thickness of the expansion coupling is the same thickness as the LB, so everything is straight. I usually put a screw through the back of the LB and a strap, next size up from the pipe size, over the body of the expansion coupling. Some people like to use sched 80 where it transitions from below to above ground.

P.S. in the pic, I'd like to get another strap higher up. This is my house and I haven't gotten to it yet.

Looks good with PVC nice and straight.
Me being very anal I probably would have bent an offset in the pipe to keep it tight to the wood but that would be a lot of extra work for zero benefit other than to look at it.