FMC/Greenfield: Aluminum or steel

1998 Views 4 Replies 3 Participants Last post by seharper, Mar 26, 2020

ablodneyget

Discussion Starter · Mar 25, 2020 (Edited)

Is there a big difference in durability or strength between reduced-wall aluminum greenfield vs. steel?

All I can find locally in 3/4" is AL, but for an extra 20 bucks I can order steel.

This would be run from the subpanel in a low attic alongside many other MC cable runs between a pair of 2x3's for protection, but that wouldn't stop an errant foot, knee, or hammer from smacking it while someone maneuvers in the space. It is NOT a place where it would ordinarily be subject to physical damage, but there could be traffic near it if I upgrade insulation next year. This is the last run I'm doing for this remodel and I'd like to use a raceway to allow an extra few wires to be pulled from the panel if needed later.

The MC cable used for the other circuits has held up extremely well in that regard. (At one point I caught roofers standing on it.)

Thank you!

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surferdude2

· #2 · Mar 25, 2020

Yes, steel is considerably stronger. EMT is stronger yet and considerably cheaper if you can do it.

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ablodneyget

Discussion Starter · #3 · Mar 26, 2020

surferdude2 said:
Yes, steel is considerably stronger.

That's what I would have guessed, but then a lot of the strength is in the shape of the construction.

The runs would be roughly 25 feet and 30 feet, respectively, of 3/4" raceway with a 4-11/16 box in the middle and at the end. Each run would have two 90-degree bends. (The boxes will be mounted down between the truss bottom chords with mud rings installed, while the conduit will run atop the bottom chords.)

surferdude2 said:
EMT is stronger yet and considerably cheaper if you can do it.

Since you mention that, here's the situation. This will be running through the attic where clearance is about 3 feet (4 ft max at peak), so there isn't really room to bend conduit up there. I can bend in the garage and bring pieces up if I can do it right without a lot of trial-and-error. If I have to make adjustments then it will eat up a lot of time due crawling back and forth. (I've only ever bent conduit in the open where it was easy to check and adjust, so I might be selling myself short here.) I would need to jog toward the truss chord after leaving the box, then angle up, then a 90-degree bend. Nothing too hard, just in an awkward work location.

If the effort isn't too bad, then the flex would save me hours of horsing around installing the raceway vs. EMT. (+1 for Rotosplit/Rotoflex)

Perhaps I'm being overly optimistic about the task of pulling through flex. Am I trading easy of raceway installation for a miserable time pulling the cable? The initial fill would be six #12 stranded. I was just hoping to have a way to pull an additional one or two circuits down the road without having to rip the drywall out.

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surferdude2

· #4 · Mar 26, 2020

All that considered, go with the 3/4" steel flex. Pull it tight and keep the bends gracefully long radius and it'll pull easy. Don't forget the ground conductor. You can run aluminum but since you think it may get abused, I'll defer to your judgment.

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seharper

· #5 · Mar 26, 2020

ablodneyget said:
I can bend in the garage and bring pieces up if I can do it right without a lot of trial-and-error. If I have to make adjustments then it will eat up a lot of time due crawling back and forth.

What I do is make the bend on the bench, but leave myself a few inches of excess pipe, then test-fit, mark and cut it right there.

I also don't spare the couplers. If one 10' stick requires bends that are too complex or annoying, I'll just cut it so I have 2 pipes each with a simple bend. Mind you, I also don't spare the straps.

Perhaps I'm being overly optimistic about the task of pulling through flex. Am I trading easy of raceway installation for a miserable time pulling the cable?

I suspect you might. In my limited dealings with flex, when you want the wire to move, the flex moves instead. So you end up having to push/pull so hard that you put the flex at its absolute limit of motion (i.e. kinked as far as the straps will allow) ... and now the bends are tight. Also it's ribbed for your displeasure.

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