Code Change Summary: Changes were made regarding the compatibility of fittings used with Intermediate Metal Conduit (IMC).

The previous code language in 342.14 allowed aluminum fittings to be used generally with IMC and did not mention if the IMC could be steel or aluminum. As well, the previous code language did not address stainless steel IMC at all.

In order for galvanic corrosion to occur, the following conditions must exist:

• There must two dissimilar metals in contact with one another; an anode (the less noble metal that will corrode) and a cathode (the more noble metal that will be protected through the corrosion process).
• There must be different corrosion potential between the anode and the cathode (see illustration).
• There must be an electrolyte (electrically conductive fluid) in contact with the two metals.

When the above conditions exist, the less noble metal (anode) deteriorates at an accelerated rate and deposits portions of itself onto the cathode as the anode tries to return to its original state before it was refined (ore).

In the 2017 NEC^®, this code section changed to recognize the significance of the differences in potential between the most commonly used materials for IMC which are galvanized coated (zinc) steel, aluminum and stainless steel.

Aluminum and galvanized steel are reasonably compatible with one another (see illustration) unless a corrosive agent is introduced which will accelerate the corrosion process. The closer the metals are together on the dissimilar metals chart, the slower the corrosion process will be when an electrolyte is introduced.

Stainless steel however, is pretty far up the chart when determining the corrosion potential of dissimilar metals. Because of the greater difference in potential between stainless steel and aluminum or stainless steel and galvanized (zinc) coated steel, stainless steel IMC shall only be used with stainless steel fittings and approved accessories, outlet boxes, and enclosures.

Similar code changes occurred in 344.14 for Rigid Metal Conduit and 358.14 for Electrical Metallic Tubing.

Below is a preview of Article 342. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC^® edition of NFPA 70.