ECN Forum Forums General Topics of the Trade General Discussion Area dissimilar metals

anyone ever been told by an inspector to isolate mc cable from holes through steel studs? show me the code section. 110-14 is connections, and rmc and emt says to avoid it where practical, but even emt through steel studs without plastic grommets or bushings has been normal. we do isolate emt from copper water lines.

whats correct? emt from copper water and black iron gas lines? emt from steel? mc from all of the above, none of the above?

Cindy,
During my time in the military I was taught that ANY metal to metal contact of different types should be handled with care. I witnessed things that I never thought possible (box openings being oxidized to dust in a matter of weeks due to dissimilar metals). If you can avoid contact it is just a good thing to do. You will save yourself headaches and callbacks.

This also takes into account what kind of environment that the project is located in. The above mentioned example was in Korea where the climate was conducive to corrosion. They (my boss at the time) went so far as to have different types of metal brushes purchased to avoid dissimilar metal contact during routine Preventive Maintenance Inspections.

I am not aware of any code that states it but it would be a good practice to follow.

Hope this helps…

Cindy,

Are you talking about 'Armorlite' or similar Aluminum sheathed MC cables? I was trying to find something in the UL White Book last nite but didn't have any luck with it.

(I thought EMT was steel)

Bill

Would there be issues other than electrical? Like noise? would the drywall be enough to damp the squeak as the building expanded and contracted? I work next to a railroad and the buildings actually shake when the train goes by, so any metal to metal contact would rattle.

oops bill, emt and steel studs, duh, sometimes i type faster than i think [dont make fun, that was my first fopaw, sp?][this would be a good place for a smilie, if i knew how to make a smiley face with crossed eyes and my tongue sticking out the side, kind of like brittany spears commercials], but yeah i suppose what he was talking about was the alum mc and steel. and the corrosive conditions arent an issue in this case, rattling could be though..... anyway what i was wondering about here was how far everybody goes with this. i have put insulation between copper water lines and mc and emt, but have wondered if it should be put between mc/emt and galv sprinkler lines or black iron gas pipe, and if you all even bother to separate the copper waters and mc/emt? and seriously, does anyone separate alum mc from the steel studs because of galvanic action, if thats what its called? [we use mostly mc tuff, and i think that its steel, but also use aluminum mc]

after using the term above, thought i should look it up to be sure it was right.
Galvanic Action: Flow of electrons which occurs when two dissimilar metals come into contact in the presence of moisture which is capable of carrying electric currents, resulting in the corrosion of the more active metal, which is lower on the galvanic scale, without damage to the more passive metal.
this site also helped out some, it bascically says that the "more noble metals" like copper pipes would corrode the "less noble metal" like aluminum mc, and another interesting thing is that the type of steel makes a lot of difference on where it is on the galvanic scale [galvanized, active, or passive steel] so guess it would be good to know what the emt and mc are made of, and the presense of electrolytes, water, etc, is a big part of the answer. http://www.roofhelp.com/galvanicscale.htm
so what do ya think?

I think I wish I were half the electrician you are, Cindy.

Y'all come up with things I've never even pondered...

nice thing to say sparky, but i'm sure i'm a few decades behind most of you guys.
okay..... so lets see, if i'm about half of an electrician now, and you were half of that, then that would make you a plumber, right?

With your permission may I add a thought? If you aren't using the heavy cast connectors and fittings on your conduit, then they are made from aluminum, right? so you would have a steel to aluminum connection there. An experience that I had, I recently had opportuniy to open a wiring trough built into a concrete floor. We were adding a piece of machinery for the first time in a long time, and all of the emt that was in the trough had rusted to nothing! The aluminum connectors were still there, but white and powdery, but the emt runs were just rusty powder in the bottom.

Point of clarification, I guess it wasn't so much a wiring trough, as it was a raceway to run conduit from one end of the building to the other.