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There are several types of conduit. Of course, you know that. But in this article, we're only going to discuss metallic conduits. PVC conduit may be a topic for another day. Because of their somewhat fragile composition — particularly in commercial and industrial settings — they don't fit in with this article.<br><br>Of course, the main two families of conduit, if conduits had families, are steel and aluminum.

Conduit Comparisons… and a Surprising Fact

Steve Maurer, IME
There are several types of conduit. Of course, you know that. But in this article, we're only going to discuss metallic conduits. PVC conduit may be a topic for another day. Because of their somewhat fragile composition — particularly in commercial and industrial settings — they don't fit in with this article.

Of course, the main two families of conduit, if conduits had families, are steel and aluminum.

Steel includes RMC, EMT, and stainless steel. Now EMT is somewhat in a class of its own, with assembly made by either setscrew connectors, or compression fittings, often with a designation of concrete tight.

There are transitional connectors that allow the installer to start with one type, such as EMT, and transition to a threaded type of conduit.

Enough about EMT… let's transition back to RMC, stainless steel, and aluminum.

Steel Versus Aluminum
There are several advantages of using aluminum conduit over steel. Of course, cost per foot is one of them. However, ease of installation is another advantage of aluminum. It's often much easier to work with because of its lighter weight and malleability.

In the smaller sizes, aluminum is much easier to form and thread with hand tools. Because it is a softer metal, the wear and tear on dies is less. You probably won't need to change the blades or dies nearly as often as you would with rigid metallic conduit.

Weight is also another factor to consider. It takes less "horsepower" to move around aluminum conduit. Believe me… this is important to an old guy like me. But for you, it means that the additional workers needed to horse around steel conduit can be put to work on other projects. According to one chart I've read, the labor saving of aluminum over rigid steel can run as high as 70% for 6-inch conduit installed on a 20-foot-high ceiling.

To compare, the weight of 100 feet of rigid steel comes to approximately 79 pounds. The same 100 feet of aluminum weighs in at around 28 pounds. Just the weight of the rigid steel can slow down installations significantly.

It also can mean a lot for rooftop installations. The lighter weight means less stress on roof support members such as bar joists and purlins. During inclement weather such as heavy snowfalls, this may be the difference between making it through without roof collapse.

Rigid steel may be stronger. But a properly support aluminum conduit will do the job just as well.

Corrosion resistance
Now, you can't beat stainless steel for corrosion resistance. And both the 304 and 316 alloys provide a high degree of protection. But… it's rather expensive. If stainless steel conduit isn't necessary, the aluminum might be better.

Granted … most RMC is galvanized, which affords some protection against corrosion and oxidation. But remember, the galvanization process uses zinc, a sacrificial metal. And we all know what sacrifice means! When corrosives hit, the zinc gives up some of its coating. And that means if it's subjected to a lot of corrosive chemicals, it will eventually fail.

Aluminum, on the other hand, has a self-healing property that replenishes the outer "skin" of the pipe. Sure, it will eventually fail under extremely harsh conditions. But in many cases, it has a longer lifespan than rigid steel.

Sure, rigid steel can be PVC-coated to protect it from the elements. But that requires additional equipment and processes for proper installation. And consider this: an imperceptible nick or scratch in the PVC coating can allow corrosion and oxidation to pervade the encased conduit. There have even been instances where – upon close inspection or during replacement – it was found that the only thing restraining the conductors was the PVC.

News to me!
Here's that interesting and somewhat surprising fact I mentioned. Now, you might have heard this … but it was news to me. I've always known that the length of the conductors affected voltage drop. That's why you need to calculate it.

What I didn't know was that the type of conduit also affects voltage drop. Rigid steel induces a hysteresis because of its "magnetic personality." Do some research on the topic and it may surprise you.

There are also some other safety factors to compare between rigid steel and aluminum conduit. Using due diligence when engineering your next project could save you time and money in the long run.
Photo courtesy of Patriot Industries
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