Anybody know about expansion fittings?


Old 01-06-06, 12:20 AM
Thread Starter
Join Date: Jul 2005
Posts: 41
Anybody know about expansion fittings?

I am helping a friend run an underground electrical line to her guest house. The contractor who built her main house ran 1 1/4" PVC about half way and ended at a stub out. I want to continue the conduit to the guest house and bring it up to a subpanel. When I looked in Practical Electrical Wiring I noticed a diagram showing an "expansion fitting" where the PVC conduit comes out of the ground and connects to a structure. The text says this is important to relieve the stress on the junction box from differential thermal expansion between the PVC and other builiding materials to which it is attached. The problem I am having is that since I live in southern California, even the guys in the electrical supply houses look at me like I am from Mars when I ask if they sell expansion fittings for PVC conduit. I guess they are not generally used around here, but my friend's house is located higher up in the mountains where it does freeze and even snow during the winter. The book also mentions using an elliptical hole to allow for expansion, but since mice are a big problem in the area, I would rather not make it any easier for them to get into the structure. I like to do things as much by the book as possible, but I feel stymied since no one else I have talked to seems to care, or even know about expansion fittings around here.
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Old 01-06-06, 01:10 AM
Join Date: Aug 2004
Location: NA
Posts: 1,065
Hello Gregor, only pvc rigid non-metallic electrical conduit (RNC) is subject to a required need for expansion fittings by the NEC. Generally if the conduit is direct buried you will not need them. The standard is 1/4 " of expected expansion /contraction in a straight run between securely mounted equipment such as sub-panels and the like, then expansion fittings are required. This quarter inch is determined by the amount of temperature change expected in the region where the installation is made. For instance if you have a 100 foot straight run between two electrical cabinets with pvc conduit and there is a 50 degree F change possible in that area your pvc can be expected to change in length by 2 inches. Therefore expansion fittings would be required by the NEC.

It sounds like your conduit is direct buried so it wont be much concern. Occassionally you will see expansion fittings on the conduit risers when they exit grade level to enter an LB or cabinet. This is generally done when your region has a problem with frost heave in the soil.

If you want to cover your bases just contact your local building codes department and they can tell you if ground movement due to frost heave will require expansion fittings on your risers in your area. If you need to install these on your risers remember to secure them on the BELL (fixed end) and not the end that moves.

There is a chart in the NEC on this link

that will give you more info. You need to click agree (free viewing) then open the 2005 NEC, then let the load complete and click on the index icon I believe the article of code you want is 352.44 or there abouts but if that isnt correct you want the article that addresses Rigid non-metallic conduit. There will be a table that allows you to compute the expansion if you have an idea of the expected temperature change in your region where the installation is made.

Last edited by Roger; 01-06-06 at 06:49 AM.
Old 01-06-06, 06:31 AM
Join Date: Oct 2005
Location: Milwaukee WI
Posts: 1,338
Originally Posted by Gregor
The problem I am having is that since I live in southern California, even the guys in the electrical supply houses look at me like I am from Mars when I ask if they sell expansion fittings for PVC conduit.
My "shorties" are Carlon E955G. They come in a short and long size for 1-1/4" and which one depends on your own calculations and configuration. The Carlon catalog provides formulas & installation practices for figuring things out. Follow them closely to reduce water infiltration and based on the temperature at which installation takes place.

Anyway, google 'carlon expansion' and you will see some hits if you want to buy online. At Graybar they took day or two to get them from the IL warehouse and they cost me $10-20 each.

I lubed the seals with 100% silicone grease from NAPA. After installation mark an inch scale on the PVC going into the expansion side of the fitting. Mine expand about 1/2 - 1 inch at the height of summer and that's for Polyethylene underground innerduct on distances of less than 60 & 75 feet.

I used to drive by a parking ramp where they used PVC along the outside face for security lighting. At irregular intervals the conduit was pulled loose from the concrete and dipped or bubbled out. My guess was they installed it in the winter here.
Old 01-07-06, 12:59 AM
Thread Starter
Join Date: Jul 2005
Posts: 41
Thanks for the info. That is what I was looking for. I actually did find an electrical supply house that had one 2" expansion fitting today. They gave it to me for free just to get it out of the way. It was nice to see that the part actually does exist. Thanks again for the help.

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