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#31339 - 11/18/03 09:19 PM Expansion Joints  
NorthernLights  Offline
Member
Joined: Mar 2003
Posts: 10
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 09:49 PM Re: Expansion Joints  
Electricmanscott  Offline
Member
Joined: Jan 2002
Posts: 1,457
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.


#31341 - 11/19/03 07:38 AM Re: Expansion Joints  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,303
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....


#31342 - 11/19/03 08:02 AM Re: Expansion Joints  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
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

#31343 - 11/19/03 08:38 AM Re: Expansion Joints  
George Corron  Offline
Member
Joined: May 2001
Posts: 717
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.


#31344 - 11/19/03 06:33 PM Re: Expansion Joints  
Electricmanscott  Offline
Member
Joined: Jan 2002
Posts: 1,457
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?


#31345 - 11/19/03 08:07 PM Re: Expansion Joints  
walrus  Offline
Member
Joined: Jul 2002
Posts: 680
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??


#31346 - 11/19/03 08:17 PM Re: Expansion Joints  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
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. [Linked Image]


Bob Badger
Construction & Maintenance Electrician
Massachusetts


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