This showed up on the ECN BB (George Little Inspector in Michigan) and I thought it might be interesting to discuss here.

"How is Article 310.15.(B)(2)(c) applied in your area?"

My question
Is there a particular guideline you use or do you just accept the EC/Engineer's calculation?

I am using the guidelines established by the CDA. As indicated in the FPN, the CDA guidelines utilize the average of the 2% ASHRAE design temperatures for each region and major city in the state.

The guildelines and sliding scale card can be obtained FREE from www.copper.org

Thanks That is exactly what George was looking for. I will pass it on.

Bryan,
Could you narrow the link down a little, I cant locate the temps on the web page.
I have been using 34 C for the Tampa area, I got that # from a PV publication, am I close?

Neal Burdick
COT Construction Inspector II

Yea I had to hunt a little and HERE is a page.

Assuming ambient temperature is surrounding air temperature. Rooftop air at 113*F. = 45*C. Add 22*C. for rooftops with sunlight...you get 67*C. which is an correction factor of 0.58 so #12 THWN-2 @ 90*C. is rated 30amps. x 0.58 = 17.4amps...could still be on a 20amp ckt. bkr...only 3 current carrying conductors plus maybe a balanced neutral and E.G...I still think that linear expansion and contraction is a bigger issue on long rooftop runs...

You bet.
I have seen even supposedly properly placed expansion fittings come apart due to excessively long runs.
Which leaves exposed conductors that are subject to uv deterioration.

Expansion Fittings for Rigid NonMetallic Conduit

Rigid NonMetallic Conduit, commonly referred to as PVC, will expand and contract over its length as the ambient temperature around the installation fluctuates. RNC has thermal expansion characteristics three times that of aluminum and six times higher than steel.

The requirement for expansion fittings can be found in Section 352.44 of the National Electrical Code. It’s accompanying Table identifies the change in conduit length based on the maximum temperature change expected during the life of the installation. If this change in length exceeds ¼ in or greater, an expansion fitting is required.

A good rule of thumb is that any straight conduit run exceeding 25 feet and where the temperature change exceeds 25°F, an expansion fitting is probably needed. The temperature change is based on the maximum and minimum temperature expected to occur. Another consideration is exposure to direct sunlight. Most fittings have directions indicating an increase of 30°F to be added to the maximum temperature for proper installation.

For example, assume a 50' run of RNC in direct sunlight with a maximum expected temperature of 95°F and minimum likely temperature of 25°F:

95°F + 30°F – 25F° = 100°F temperature change. Per Table 352.44(A), this would produce a change in conduit length of 4.06 in / 2 = 2.03 in. (The Table is based on in./100ft.)

The best installation practice is to install the expansion fitting in a horizontal position. If the fitting must be installed in a vertical position, proper arrangement must be assured to prevent the entrance of rain water. Care must also be taken to ensure the initial position of the piston within the barrel takes in account the current temperature conditions to allow the full range of expansion and contraction.

Without the use and proper installation of RNC expansion fittings, the conduit system and its supporting structure could be significantly damaged. This may expose the concealed conductors or energized parts. Be sure to read all installation instructions and follow any UL listing requirements.

Way back in my childhood when I did electrical inspections at Clearwater/St. Pete airport and there were long runs of PVC they always used them, and they were always on the plans. Of course, there is vibration to consider as well in that type of construction. But they were used, allright.

Bryan you need to give that info to the DOT.
I can't tell you haw many bad joints on a bridge I have seen.