EMT - set screw vs. comrpession fittings

[alehman]

I have a client who's standard specification requires comperssion fittings for EMT. I think they could save considerable money by allowing set screw type fittings for indoor applications. I'm curious about opinions as to the relative advantages of one type vs. the other.

I've heard the argument that compression type are more rugged and less likely to come apart. Is there any evidence to support that?

 

[dpc]

I don't have published evidence, but I've worked for two large consulting firms and in both cases, we never allowed the use of setscrew fittings. I'm sure the setscrew fittings are cheaper and faster to install, because, given a choice, that is what the contractor will use. I would really question the continuity of a set-screw connection after many years of corrosion and thermal cycling.

But I don't much to back that up other than general comments from other engineers over the years..

 

[RonShap]

If the compression coupling is steel (not die cast), then I agree that its longevity for electrical continuity is superior to set screw.
Both set screw and compression are worthless if not installed properly.

 

[BJC]

After the Loma Prieta earthquake there was a couple of attical written about how the set screw connectors held up better than set screw connectors. Some of the crimp fittings may be better but a crimping tool is required.

 

[itsmoked]

Hey BJC where you located? I was only 3 miles from the epicenter of that bugger.

 

[EEJaime]

We do a lot of hospital, school and detention work. Facilities that are designed for longevity. As the others have stated, there doesn't appear to be published data, but in the 25+ years I've been doing this, steel, compression, gland ring type fittings have been the sturdiest, longest lasting installations. Often times we have found that contractors try to shorten installation times by over-using power tools. I can't recall all the times I have found set screw fittings tightened to the point of deforming the emt. Other than cost driven light commercial work, we never allow them to be used.

 

[alehman]

I appreciate the feedback. We've always requried steel compression fittings. Contractors will nearly always try to get by with setscrew type. It seems to be a problem, no matter how carefully noted on the plans and specs. By the time it's caught on the job site, it's too late to re-do.

Ron - why would there be a difference in longevity for steel and not for diecast?

 

[RonShap]

When a steel coupling is made-up properly it is stronger.

 

[EEJaime]

Alehman,

That's why I find it critical to get into the field as each part of the construction process is begun. When the first part of the underground work is layed, when rough-in is started, right before any conductors are pulled, etc..., so that I can catch trends and non-compliant installations early and have it all ripped out and corrected before it gets to the point where the construction schedule is adversely impacted. We can't catch it all, but if you catch it early enough, you can set the direction for the installation and assure that the majority of the installation will be as specified.

 

[resqcapt19]

If properly supported, I don't really see a difference in breakage between the two types. In my opinion, the set screw coupling is a superior fault return path compared to the compression type. The only other issue, is that if the run is very long and if I will be using a "vacuum" fish type, I will always use the compression type. There is too much air leakage on the set screw type to use the vacuum over longer distances.
Of course, if it is a "spec" job, I will install the material that is required by the contract.
Don

 

[RonShap]

Since a set screw coupling has metal to metal contact for ..... maybe 30-40% of the circumference, and compression has circumferential metal to metal contact, I think (I've never tested) compression would have reduced impedance and also carry fault current if called upon to do so.

 

[alehman]

UL514B requires that fittings listed for grounding use be fault current tested. Does anyone know about that test?

 

[geso52]

For indoor applications my preference is steel set screw fittings and that is what I use on my specs. The set screw is more fault proof regarding installation as you can install it properly using just two hands. To install a compression fitting correctly you need four hands; two to keep the sections of EMT close together at the union and one for each wrench as the two sections of EMT sometimes separate as you are wrenching (I worked 15 years in the field before becoming an engineer). I seen many instances where coupling compression fittings were not done right and about to come apart and some where the union had actually come apart. I find the die cast fragile as I seen many of them broken in the field and I only allowed them in remodeling work where the die cast type is already existent. If it is an exposed outdoor application my preference is IMC as I have seen rust on EMT in coastal whether here in Florida.

 

[advidana]

Why not called out rain tight fitting . They are easy to install, require no special tools and hold up better than the screw type or compression fittings. Yes they do cost pennies more but they are superior to the other types. I also perfer mc cable if allowed. My clients are usually satisfied with them too.