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Installation of electrical components.

posted Mar 19, 2012 23:22:15 by JohnGorman
Is there an NEC citation for the answer to question #6 of the electrical component installation set?
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4 replies
Jen said Mar 20, 2012 01:14:39
The closest thing I could find is NEC article 384.30 (A) for Uni-strut. If you use those specifications I come up with 10 plus one at each end so 12, one every 2.3 feet... Its a pretty vague question.
SarahRaymer1 said Mar 20, 2012 17:53:41
Problem #6 is the one about the drill bit, right? "Of the choices below, what would be the appropriate drill bit to use to make a pilot hole for a 5/16" lag screw?"
a. 1/8"
b. 3/16"
c. 1/4"
d. 5/16"
I looked it up online and I read a lead hole/pilot hole for a lag bolt into wood should be 75% of the screw diameter. Mathematically I did this:
5/16= 5 divided by 16= .3125
75% of that is figured by: .3125 x .75= .2344

So- I say c.- 1/4" because"
1/8-.125 too small
3/16=.1875- too small
1/4= .25- close to 75%
5/16=.3125- too big

I don't know if this is in the NEC code book, but we can't assume everything on this test will be. :\
JohnGorman said Mar 20, 2012 21:07:19
Thanks for your response. Did you have a solution for question #3 regarding the number of attachment points?
CathyRedson said Mar 21, 2012 16:49:41
The answer was based on using 70% as the guide, which is consistent with the Dunlop text, that says 60-75% as the range. .3125 x .7 = .21875 3/16 = .1875 which is slighltly smaller than .21875, but for pilot holes I'd rather go slightly smaller than larger to give more room for the threads to grip. In Johan's presentation, he said 7/32 would be the best, and that was right in between the two.

As puzzling as this is, this is very reminiscent of a NABCEP question and depending on your online reference or your interpretation from Dunlop, or the article is SolarPro from the reading list, without clear reference of the percentage they want you to use, it is challenging. Remember, at the end of the test you will have a comment form to address ambituities, and this would be one of them.

From SolarPro Article: "Lag Screws in Residential Installations" June/July, 2010, Issue 3.4, page 70: "Pilot holes. Lag screws need pilot holes. For composition shingle roofs, use a 5/32-inch diameter pilot hole for 1/4-inch lag screws, a 3/16-inch diameter pilot hole for 5/16-inch lag screws and a 1/4-inch diameter pilot hole for 3/8-inch diameter lag screws. To minimize wood splitting, drilling to a depth that is 0.50-inch shorter than the lag is ideal. In addition, the lag screw should be at least 0.50-inch shorter than the combined thickness of the mount, shingle, plywood and truss to help prevent the wood at the bottom of the rafter or truss from splitting, as shown in Example 7."

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