PVC expansion Exterior

[augie47]

Over the years where expansion joints were not installed as needed the problem I usually saw was busted straps and conduit that looked as crooked as a GA road map.

 

[Fordean]

Good answer ty

 

[Fordean]

This is my possibly failed logic for the maximum permitted length of a raceway without an expansion fitting.

Example:
Two exterior enclosures with concentric KO's with a straight run of PVC in between. The length of the PVC is limited by the NEC because the expansion or contraction of the PVC over 1/4" can damage the enclosures.

100° temperature change so we'll use the maximum length of PVC without an expansion fitting of 7' as Augie stated.

7' of PVC installed when it's 0° F, expands 1/4" when 100° F. Everything good no damage to enclosures.
7' of PVC installed when it's 100° F, contracts 1/4" when 0° F. Everything good no damage to enclosures.
7' of PVC installed when it's 50° F, expands 1/8" when 100° F. Everything good no damage to enclosures.
7' of PVC installed when it's 50° F, contracts 1/8" when it's 0° F. Everything good no damage to enclosures.
14' of PVC installed when it's 50° F, expands 1/4" when 100° F. Everything good no damage to enclosures.
14' of PVC installed when it's 50° F, contracts 1/4" when it's 0° F. Everything good no damage to enclosures.

Good answer ty

 

[winnie]

This is my possibly failed logic for the maximum permitted length of a raceway without an expansion fitting.

Example:
Two exterior enclosures with concentric KO's with a straight run of PVC in between. The length of the PVC is limited by the NEC because the expansion or contraction of the PVC over 1/4" can damage the enclosures.

100° temperature change so we'll use the maximum length of PVC without an expansion fitting of 7' as Augie stated.

7' of PVC installed when it's 0° F, expands 1/4" when 100° F. Everything good no damage to enclosures.
7' of PVC installed when it's 100° F, contracts 1/4" when 0° F. Everything good no damage to enclosures.
7' of PVC installed when it's 50° F, expands 1/8" when 100° F. Everything good no damage to enclosures.
7' of PVC installed when it's 50° F, contracts 1/8" when it's 0° F. Everything good no damage to enclosures.
14' of PVC installed when it's 50° F, expands 1/4" when 100° F. Everything good no damage to enclosures.
14' of PVC installed when it's 50° F, contracts 1/4" when it's 0° F. Everything good no damage to enclosures.

I think that your logic points out a flaw in the wording of the code.

The code says '...when the length change...is expected to be 1/4 in. or greater...'

It isn't clear if that length change is _total_ (minimum expected temperature to maximum expected temperature) or _relative_ to the temperature at installation.

If they intend total from min to max, then your logic is wrong. But if they intend relative to the installation then IMHO your logic is correct.

 

[Fordean]

Augie said any run more than 7' so that would include the two runs at the top along the peak and the run to the right where the two LB's are.

Ty

 

[Strathead]

I was at a high school softball field that had a PVC run from the dugout along the top of the fence to the outfield. I don’t know what it was used for, but on a hot summer day, it looked like strand of cooked spaghetti. No expansion joints.

In almost all cases, I have only needed on fitting in a straight runs, however, I did a greenhouse and found the need of 2 in a single run. I had to argue with the installing "electricians" to even needing any, since the concept eludes most hack electricians.

 

[Strathead]

This is my possibly failed logic for the maximum permitted length of a raceway without an expansion fitting.

Example:
Two exterior enclosures with concentric KO's with a straight run of PVC in between. The length of the PVC is limited by the NEC because the expansion or contraction of the PVC over 1/4" can damage the enclosures.

What makes you think this is any less arbitrary than many other code sections? And I don't know that it is specifically "damage" to enclosures as the popping out or straps, pulling the wire, etc. Remember, we could be talking 2" or more of expansion on longer runs.

 

[infinity]

And I don't know that it is specifically "damage" to enclosures as the popping out or straps, pulling the wire, etc. Remember, we could be talking 2" or more of expansion on longer runs.

The box damage scenario is just one example as it was a non-exhaustive list. The fact is that for some reason it was decided that the PVC raceway installation can handle 1/4" of expansion or contraction without damaging the system. My issue is with how that 1/4" calculated as demonstrated by my example.

 

[kwired]

if I pull a pipe out of a cold shop then install outdoors in warm weather how long does it take that pipe to accumulate- problem long after I am done— the. What temp do I go with?…

Or the opposite- guess we have tk heat scan the pipe before install and right on each pipe- lol

few minutes is probably fine? how long does it take it to cool down when you heat it for bending reasons?

 

[jim dungar]

It isn't clear if that length change is _total_ (minimum expected temperature to maximum expected temperature) or _relative_ to the temperature at installation.

The NEC never addresses this issue. The manufacturers selection charts only show the full range of minimum to maximum.

It is only the manufacturer's literature for expansion couplings that mentions temperature at time of installation, based on the total distance determine by the min-max charts.

 

[kwired]

I was just stating whatever government body picks it temperature because of the ambient temperature does not mean the pipe is that so there’s gonna be a change in shrinkage or gain from the pipe to the temperature outside I was just pointing out the hypocrisy I live in such a mile climate we never use expansion points..,

I’ve been coming to the conclusion that this NEC book is like 10% safety and the rest of is garbage design issue “” not life safety

You don't get situations of uninsulated/unconditioned buildings and high temperature especially near ceiling level on a sunny day?

I prefer to run metal raceway in that situation but maybe it is corrosive environment in there and PVC becomes somewhat a necessity. Here the AHJ says we need to consider that a 140 deg F potential in temperature change.

 

[kwired]

The temperature at the time of installation only applies to where to set the expansion joint. Lets say your your temp range is 0deg-100deg and you install at 50 deg.

So the expansion is calculated at a 100deg change in temp. You install at 50 deg which is half way of the expansion range. So if the total expansion is 1" and you install at 50 deg the joint has to move 1/2" in either direction

If you have long expansion fitting and only 1" of calculated change in length, you have some room to play with and wouldn't necessarily have to install at mid range of the fitting.

If you have more potential length change than the length of the fitting, you are going to need more than one expansion fitting in the run. But you sort of need to secure the mid sections or over time things could move far enough that one of those fittings eventually pulls apart.

 

[Little Bill]

I only see the need for one expansion fitting. It would be between the disconnect and the roof line. All the rest are not fixed points. Unless the drawing is wrong, and the conduit on the side, near the soffit, is up against the soffit, then one would be needed from the soffit down to the LB.

 

[infinity]

I only see the need for one expansion fitting. It would be between the disconnect and the roof line. All the rest are not fixed points. Unless the drawing is wrong, and the conduit on the side, near the soffit, is up against the soffit, then one would be needed from the soffit down to the LB.

What about elbows? The actual code wording says elbows so even if those are 45° bends they're still elbows unless the requirement is only for when an elbow is 90°. Unfortunately it doesn't say anything about the amount of bend.

352.44 Expansion Fittings.
Expansion fittings for PVC conduit shall be provided to compensate for thermal expansion and contraction where the length change, in accordance with Table 352.44, 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.

 

[Little Bill]

What about elbows? The actual code wording says elbows so even if those are 45° bends they're still elbows unless the requirement is only for when an elbow is 90°. Unfortunately it doesn't say anything about the amount of bend.

I don't see an elbow as a fixed point. I see it says "elbow", but logically it will give enough to not cause a problem. The elbow, or bend, at the peak of the gable will not be tight against the peak, as you can't get that tight of a bend without pinching the conduit.

IMO, whoever on the code making panel that wrote this section has never installed one stick of PVC.

 

[wwhitney]

100° temperature change so we'll use the maximum length of PVC without an expansion fitting of 7' as Augie stated.

For 100F delta T, I get 7' as the minimum length where an expansion fitting is required, and 6' as the maximum length without one. The PVC expansion factor of 3.38E-5 / F with 100 degrees F means we get to 0.25" at a length of 0.25 / 3.38E-5 / 100 = 74".

As to your logic about installation temperature, I buy it if the only concern is damage to enclosures. So apparently there are other concerns, like damage to straps or to the pipe itself, which depend on the total range of motion, not just the range of motion relative to the state at the time of installation.

As to the code language you quoted, it refers to when the "length change . . . is expected to be more than 1/4 in." It doesn't say "length change from the time of installation." So it can be the length change between any two points in time, i.e the hottest day and the coldest day.

Cheers, Wayne

 

[wwhitney]

I don't see an elbow as a fixed point. I see it says "elbow", but logically it will give enough to not cause a problem.

I would say the code language quoted above is telling us to consider elbows as fixed points.

The thing about an elbow is that a PVC strap is designed to allow the PVC to move axially (along the long dimension) but not tranversely (side to side). And any change in length of the pipe on one side of the elbow would require a traverse motion of the pipe at the first strap on the other side of the elbow. So it will induce bending in the pipe between the elbow and that first strap.

Cheers, Wayne

 

[Strathead]

I don't see an elbow as a fixed point. I see it says "elbow", but logically it will give enough to not cause a problem. The elbow, or bend, at the peak of the gable will not be tight against the peak, as you can't get that tight of a bend without pinching the conduit.

IMO, whoever on the code making panel that wrote this section has never installed one stick of PVC.

As an electrician that doesn't do residential and wouldn't install PVC more than a short stub up or between panels, I will snobbily say, regardless of the code, there is also what it will look like when you drive by it in a couple years. more expension fittings means less snag and a better looking installation.