Sizing of bolts used for securing mechanical lugs

[j fitzer]

I am presently looking for advice / guidance on sizing bolte used to secure the mechanial lugs to exposed structural steel. I have sized the conductor per table 250.66, its a 3/0. The conductor :-? :-? diameter is .470 inches, and am certain that a minimum size of 1/2 inch steel bolt would be more than adiquate & conservative. However an engineer has raised the question as to where in the code or some other standard are the minimum bolt sizes for a specific lug/cable size specified. Can anyone help? This guy is driving me crazy! Sometimes common sense has to be allowed, along with experience.

 

[Bob NH]

There is nothing in the NEC. It sounds like this is a ground lug. What is the hole size in a 3/0 lug? It is customary to size the bolt at 1/16" less than the hole size.

The bolt has much more area than the net area of the material at the hole in the lug, and is stronger material. The lug will fail long before the bolt.

I'm sure a 1/2" bolt of creditable quality is stronger than the attachment of the cable to the lug. Get something with a grade stamp on the head; not just a piece of junk out of the big bin at HD. Most places will have a Grade 5 or Grade 8 bolt.

Grade 5 bolts have 3 radial head marks; Grade 8 bolts have six marks. A Grade 5, 1/2" bolt will have a tensile strength of around 15,000 pounds.

http://www.rockcrawler.com/techreports/fasteners/index.asp

If the cable lug has a 1/2" hole, then he certainly doesn't expect you to drill it larger for the bolt.
http://www.hechinger.com/web/catalog/product_detail1.aspx?pid=67827

 

[petersonra]

the bolt is just a fastener. it need not conduct any electricity. size it for whatever the hole size is of the lug.

i would not be worried about using a high grade bolt. you don't need to torque them down to the point where you deform the copper or aluminum of the lug.

I seem to recall UL has some torque specs on mounting bolts.

 

[Rockyd]

To keep the your friend "the engineer" busy for an hour, ask him where in the code it says receptacles ground up or ground down(no where in code, but maybe he'll learn that it's not a law if it's not in the book on a lot of situations).

Now that he is out of your hair, here's some help. In the NEC handbook ( I know handbook is not code) at 110.14 there is a torque chart for a lot of screws, bolts, and and split bolt torque spec's.

Hope this gets you to where you need to go.

 

[iwire]

Like the others I size the bolt to match the hole(s) provided in the lugs for mounting.

I use a 'graded' bolt only so it has some standards it has to meet.

An un-graded bolt may be made out of any junk metal the manufacturer chooses.

By the way, as Bob pointed out the bolt is not actually needed to carry current, the nut and bolt is just a clamp that holds the flat surface of the lug to the flat surface of what you are attaching the lug to. It is that surface to surface contact that is capable of carrying the entire current that the lug is designed for. Of course in the real world the bolt likely carries some current but I would not worry about it.

 

[petersonra]

http://www.engineersedge.com/torque_table_sae_ftlbs.htm

 

[j fitzer]

Thanks for the help guys. The engineer is not a friend, but rather works for the custommer we'er installing the distribution systom for. It could be worse, if he knew what he was doing he would have had his electricians do the job & we would have lost the work.