When to use PVC conduit expansion joints?

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Jim22

Member
Location
Illinois
Should these be used for risers from buried conduit? Not talking about a stub into the ground and then direct burial cable with no conduit but where there is a 90 at the bottom and a horizontal buried conduit. We get freezing to 30" max here as I recall.
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
Should these be used for risers from buried conduit? Not talking about a stub into the ground and then direct burial cable with no conduit but where there is a 90 at the bottom and a horizontal buried conduit. We get freezing to 30" max here as I recall.
Yes, I think they should be used, and in my case they are required by local amendment. Plus they make it easy to finish the run.
 

iwire

Moderator
Staff member
Location
Massachusetts
Should these be used for risers from buried conduit? Not talking about a stub into the ground and then direct burial cable with no conduit but where there is a 90 at the bottom and a horizontal buried conduit. We get freezing to 30" max here as I recall.

In my opinion the NEC requires you do something about the ground movement and expansion fittings are a good choice.

This is from the 2011 NEC but the intent has not changed.


300.5(J) Earth Movement. Where direct-buried conductors,
raceways, or cables are subject to movement by settlement
or frost, direct-buried conductors, raceways, or cables shall
be arranged so as to prevent damage to the enclosed conductors
or to equipment connected to the raceways.


Informational Note: This section recognizes “S” loops in
underground direct burial to raceway transitions, expansion
fittings in raceway risers to fixed equipment, and, generally,
the provision of flexible connections to equipment subject
to settlement or frost heaves.
 

goldstar

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
It's not my intent to pirate this thread away from the OP but as long as this thread was started I have a question regarding PVC and expansion fittings. I recently had a situation where I had to add a 100A outdoor sub-panel to handle 3 ductless AC unit condensers. Prior to my installation another EC had installed a generator that was about 75' away from the transfer switch. They had installed Versabar ( http://www.globalindustrial.com/g/storage/struts-and-framing/versabar/beam-clamps-101694 ) along the bottom of the building and strapped their PVC to it using the appropriate Versabar straps. There was enough room on the Versabar to install my PVC's. I did not use any expansion fittings (nor did the EC who installed the generator PVC's).
  • Am I asking for trouble ?
  • Will I pass an inspection ?
  • Do you have to anticipate the PVC expansion and use PVC straps directly to the masonry wall instead of Versabar straps on the Versabar ?
Thanks.
 

electricalist

Senior Member
Location
dallas tx
It seems like I read that every 30' expansion fitting were required with pvc where certain climant changes in degrees were present.

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Smart $

Esteemed Member
Location
Ohio
Will you pass inspection? Depends on how savvy the inspector is.

Yes, you have to anticipate the expansion or contraction, which is dependent on temperature when installed. Many overlook this requirement when the run doesn't seem worthy of consideration. However, the length of a outdoor straight run between secured points requiring an expansion fitting is surprisingly short. Where there is a 100°F temperature swing, that length is about 6' 2"... and it varies inversely with respect to temperature swing.

You can support with strut and straps, but the straps must allow longitudinal movement (i.e. permit the conduit to slide lengthwise).
 

goldstar

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
Will you pass inspection? Depends on how savvy the inspector is.

Yes, you have to anticipate the expansion or contraction, which is dependent on temperature when installed. Many overlook this requirement when the run doesn't seem worthy of consideration. However, the length of a outdoor straight run between secured points requiring an expansion fitting is surprisingly short. Where there is a 100°F temperature swing, that length is about 6' 2"... and it varies inversely with respect to temperature swing.

You can support with strut and straps, but the straps must allow longitudinal movement (i.e. permit the conduit to slide lengthwise).
Thanks. Does that mean I should loosen the straps to allow for expansion ? Not sure how long the generator (at my installation) has been in place but there doesn't seem to be much movement or warping of that PVC from what I saw.
 

electricalist

Senior Member
Location
dallas tx
Thanks Smart for clarifying that.
If the temp fluctuates pvc will bow and twist and make "me" look like a fool for not believing it can happen.

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Smart $

Esteemed Member
Location
Ohio
Thanks. Does that mean I should loosen the straps to allow for expansion ? Not sure how long the generator (at my installation) has been in place but there doesn't seem to be much movement or warping of that PVC from what I saw.
Loosened straps also allow easier side movement. Not ideal, but time will tell whether it suffices. The best way is to use straps designed for longitudinal slip.

FWIW, here's a link to a spreadsheet calculator I put together for the purpose:
https://docs.google.com/spreadsheets/d/1CtAEegLVSKgCab-HQSJeqrEusBcZsW2UFZILm1gb9xs/edit?usp=sharing
(View only. You have to make a copy before you can edit.)
 
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kwired

Electron manager
Location
NE Nebraska
Whether expansion fittings are needed or not depends on amount of length change to be expected which is directly related to amount of ambient temp change to be expected. Also depends on whether or not the ends of the run in question are secured or if they are allowed to move.

A run with a fixed box on each end needs to have an expansion fitting if expected movement is more then 1/4 inch according to NEC.

A run of the same length with an elbow on each end may not need an expansion fitting at all because the ends are free to move.


I have debated the need for expansion fitting for conduits emerging from grade are needed to protect from frost or settling, but not everyone agrees.
IMO the fitting is there to protect the raceway from length changes due to thermal expansion the distance between boxes, fittings, etc otherwise remains constant, but frost heave or earth settling is not only changing the length of raceway it is also changing the length between say a meter socket and the buried elbow - an expansion coupling will keep from breaking the raceway or other connected items, but will not allow the contained conductors to change length and still can strain the terminals of equipment they connect to. Extra conductor in the meter socket has been suggested to me - but when you consider how hard it is to bend conductors in extreme cold is that really sufficient to protect the lugs from strain they may encounter?
 

Smart $

Esteemed Member
Location
Ohio
Whether expansion fittings are needed or not depends on amount of length change to be expected which is directly related to amount of ambient temp change to be expected. Also depends on whether or not the ends of the run in question are secured or if they are allowed to move.

A run with a fixed box on each end needs to have an expansion fitting if expected movement is more then 1/4 inch according to NEC.

A run of the same length with an elbow on each end may not need an expansion fitting at all because the ends are free to move.

...
Not because the ends are free to move...

Don't forget that enclosures, boxes, and such are required to be securely fastened (unmovable). The run with the elbow may not need an expansion fitting because the run between terminations is not straight... and requirement to "be fastened so that movement from thermal expansion or contraction is permitted" should allow the elbow to move... but if the elbow is supported too rigidly and close to its tangent points, it may not allow enough movement and one, perhaps two expansion fittings may be required.
 

kwired

Electron manager
Location
NE Nebraska
Not because the ends are free to move...

Don't forget that enclosures, boxes, and such are required to be securely fastened (unmovable). The run with the elbow may not need an expansion fitting because the run between terminations is not straight... and requirement to "be fastened so that movement from thermal expansion or contraction is permitted" should allow the elbow to move... but if the elbow is supported too rigidly and close to its tangent points, it may not allow enough movement and one, perhaps two expansion fittings may be required.
Ok, that is basically what I was thinking I just didn't provide as much details. Be careful some people don't like it when you give too many details and make the post too long, apparently they can't keep awake long enough to read all those details.;)
 

Smart $

Esteemed Member
Location
Ohio
Ok, that is basically what I was thinking I just didn't provide as much details. Be careful some people don't like it when you give too many details and make the post too long, apparently they can't keep awake long enough to read all those details.;)
No worries there. I try to keep my posts as succinct as possible. Not for those you mention, but simply because I don't want to be typing up an essay to explain a simple concept. :eek:hmy:
 

kwired

Electron manager
Location
NE Nebraska
No worries there. I try to keep my posts as succinct as possible. Not for those you mention, but simply because I don't want to be typing up an essay to explain a simple concept. :eek:hmy:
Well sometimes I don't chew things up enough before swallowing I guess:)

Add: Then again maybe it's the other way around, I chew too much and should swallow sooner.
 

Smart $

Esteemed Member
Location
Ohio
Well sometimes I don't chew things up enough before swallowing I guess:)

Add: Then again maybe it's the other way around, I chew too much and should swallow sooner.
Continuing the analogy, I think it depends on the "food". But either way is fine as long as your "health" don't suffer. But please, please do not "regurgitate". :D
 

Sabri Rama

Member
Location
RI
So do you need expansion fitings in this application?
I say yes
 

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