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#31339 - 11/18/03 05:19 PM Expansion Joints
NorthernLights Offline
Member

Registered: 03/12/03
Posts: 10
Loc: Massachusetts
Where do the inspectors you guys deal with require expansion joints. I don't feel they are necessary on Service Drops of 15' or less. If there is expansion, it can go in an upward direction, as long as you have expansion clips on the PVC. What do you guys think?

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#31340 - 11/18/03 05:49 PM Re: Expansion Joints
Electricmanscott Offline
Member

Registered: 01/12/02
Posts: 1478
Loc: Holden, MA USA
Maybe someone could do an example of the calculation as to where they are needed according to the NEC. In our area some inspectors insist on them regardless of the pipe length. Some others don't seem to care either way.

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#31341 - 11/19/03 03:38 AM Re: Expansion Joints
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
i dislike any raceway out of plumb, so it becomes a judgement call....for instance the last 400A service i did in 20' of 3" sch 80 is not going to move, whereas a 3/4" sch 40 horizontal run of 50' will move....

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#31342 - 11/19/03 04:02 AM Re: Expansion Joints
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
I will give it a try it is pretty straight forward.

First here is the rule, notice it says "securely mounted items" so if the weather head is not attached to the building an expansion joint is unnecessary.

 Quote:
352.44 Expansion Fittings.
Expansion fittings for RNC shall be provided to compensate for thermal expansion and contraction where the length change, in accordance with Table 352.44(A) or (B), is expected to be 6 mm (1/4 in.) or greater in a straight run between securely mounted items such as boxes, cabinets, elbows, or other conduit terminations.


Table 352.44(A) tells us the expansion characteristics of PVC.

I will figure here in New England we could see temperatures of -10F to 100F in between summer and winter.

The table tells us PVC will change 4.46" per 100' of length for a 110F temp change.

NorthernLights said 15' of length, 15' is .15 of 100'

4.46" x .15 = .669" of change

That is close to 3 times the code allowed .25" of change.

For 15' of length the largest temp change allowed without an expansion fitting needed would be only about 40-45F.

[This message has been edited by iwire (edited 11-19-2003).]
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#31343 - 11/19/03 04:38 AM Re: Expansion Joints
George Corron Offline
Member

Registered: 05/16/01
Posts: 728
Loc: Lorton, Va USA
For the most part, I always required an expansion joint when the building included one. IOW, if the engineer calculated the building needed one, all other trades had to accomodate that need.

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#31344 - 11/19/03 02:33 PM Re: Expansion Joints
Electricmanscott Offline
Member

Registered: 01/12/02
Posts: 1478
Loc: Holden, MA USA
I don't use them on a service if it is a straight run. If there is a bend, the run between the meter and the bend would require one. I have had inspectors ask why I used an expansion coupling a few times. Shouldn't they know?

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#31345 - 11/19/03 04:07 PM Re: Expansion Joints
walrus Offline
Member

Registered: 07/25/02
Posts: 671
Loc: Bangor Me. USA
iwire

When the conduit is installed its the right length, cut to fit. So unless it was installed at 100 degrees or at -10 you won't get that much expansion??

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#31346 - 11/19/03 04:17 PM Re: Expansion Joints
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Walrus I understand what you are saying but the code article does not talk about degrees of change from installation day.

Lets say you install it at 50F any temp below that results in tension on the joints and most likely will pull them apart.

Any temp above that will result in pressure which will cause bowing.

No matter what the temp was on installation day in my example above you will .669 from summer to winter.

At least that is how I understand it.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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