Does spending more money for copper lugs offer extra security and reduced maintenance?
During the early 1960s, the electrical industry began installing aluminum wire in many locations, and wire termination failures were commonplace. Installers used aluminum wire lugs (today called "terminators") on almost all wire terminations, knowing little about the thermal properties of aluminum. Due largely to a lack of experience in terminating aluminum wire and using aluminum products, many residential fires occurred, and the commercial and industrial wire termination failure rate increased.
At that time, most electrical professionals were still learning about the larger coefficient of thermal expansion of aluminum compared to that of copper. The result was aluminum-bodied lugs used to terminate copper wire operated successfully, while copper-bodied lugs used to terminate aluminum wire failed. Three entire series of branch circuit device termination types were tried and failed: copper (C), copper/aluminum (CU/AL), and copper/aluminum/revised CU/ALR) before a workable design, copper/aluminum/rev. II (CO/ALR) was achieved. Larger aluminum wire sizes terminated in copper lugs almost always presented problems, while the use of aluminum-bodied lugs reduced those problems, except when moisture entered in the formula.
Unprotected aluminum lugs corrode, forming aluminum oxide, which in turn causes failure modes with all sizes of aluminum wire. The past history of the problems resulted in the development of a viewpoint within the electrical and insurance industries that the most safe and reliable approach to using wire and lugs was to install an all-copper system. Thus, specifiers demanded that conductors, lugs, and bus bars be copper.
I researched what vendors build and sell in several countries worldwide (Egypt, Singapore, Jakarta, Tunisia, England, France, Australia, Germany, Trinidad, and the United States). After speaking to many vendors, engineers, and construction employees in these locations, I discovered what they think about different types of lugs. The review shows where electrical continuity really counts, "copper throughout" is most-often the specification still today.
Although most manufacturers of electrical equipment still tend to use copper lugs, the review also revealed manufacturers are providing equipment with brass lugs, copper-plated steel lugs, tin-plated copper lugs, tin-plated brass lugs, and aluminum-bodied lugs. In fact, some manufacturers build whatever is specified, while others only build copper, aluminum, brass, or copper-plated steel lugs.
In some locations, using compression lugs is a requirement, while authorities forbid the use of mechanical lugs of any metal type (having setscrews). Further, many industrial specifications, and even some country standards, require compression lugs with full tinning of copper conductors prior to insertion into the lug, followed by the crimping of the lug using a full circumference die in a hydraulic crimp tool - trusting that a compression copper lug is stronger during a short-circuit and less vulnerable to corrosion/galvanic action.
A look around the electrical industry within the United States paints a different picture. The majority of the lugs we use have aluminum bodies, yet today's electrical systems continue to improve in reliability.
What has changed? Is an all-copper system still the most reliable way to go? Aluminum-bodied lugs still have some failures. But when you use them within their design parameters, statistically they have no more failures than copper lugs. In fact, the only aluminum lug failures I found involved cracking of the sidewalls of the lug and threads at the setscrew during initial installation - the result of not properly using the required torque wrench.
The thermal expansion properties of copper and aluminum dictate that installers use only aluminum-bodied lugs to terminate aluminum conductors. Likewise, installers can use copper- or aluminum-bodied lugs to terminate copper conductors.
This research turns up some other surprising findings, too. Manufacturers today do not make aluminum-bodied lugs of pure aluminum. Instead, they produce them with a continuous copper or nickel flashing or coating, that they cover with tin or silver. The flashing is necessary because the tin coating will not adhere to the aluminum. You can see this copper coating on lugs you use today by erasing the thin plating from the lug with a pencil eraser, revealing the copper underlayer.
When manufacturers don't provide these coatings, the formation of a high-resistance aluminum oxide powder occurs soon after installation and even sooner in moist conditions. However, the coatings impede aluminum oxide formation for a long time - even in moisture-laden conditions.
When installers use an oxide-inhibiting compound while inserting an aluminum wire (not needed for copper wires) into the lug, and at the lug area where it's connected to the bus bar, the survey shows galvanic action and oxide formation almost ceases to be a problem. When an aluminum lug surrounds the wire (instead of a copper lug surrounding an aluminum wire), it can expand and contract with current flow (and the corresponding temperature change) without causing deformation of the wire, regardless of whether the wire is copper or aluminum.
Manufacturers used to fit aluminum-bodied lugs with steel setscrews that could rust. Recently, UL 486B effectively stopped the use of steel setscrews for wire sizes of No. 3 and larger, so aluminum 6262T9 setscrews are now the "norm" for large aluminum-bodied lugs. When you thread aluminum into aluminum, the aluminum galls or wears away. A polymer wax applied to the threads prevents galling of aluminum lug to aluminum setscrew.
Because aluminum-bodied lugs are more prone to cracking during installation than copper or steel lugs, the use of a torque wrench is a must. When cracking occurs, it usually takes the form of thread stripping that prevents the Allen-head setscrew from securing the lug to the conductor. It might also occur as a crack along the vertical wall of the lug where the setscrew resides.
Discussions with the manufacturers of copper and aluminum lugs revealed another surprising issue: Once the lug setscrew is torqued to specification and the bolt with a Belleville washer is torqued to specification, leave them for life (except after a short-circuit event).
This is the opposite of the normal maintenance techniques where lugs are typically re-torqued every 2 yr to 6 yr. Manufacturers now recommend a thermal scan with repairs and re-torque only where the thermal scan shows a heating problem.
We usually find heating because the bolt holding the lug to the bus bar was over-torqued, which simply resulted in stretching the bolt. Deformed bolts lack the strength needed to maintain good contact between the lug and bus bar and hold the lug in place during a short-circuit event.
Many people still don't trust anything but all copper installations. However, experience in the United States shows no statistical increase in failures with modern aluminum-bodied lugs than with copper terminations. This is provided the aluminum lugs have a copper or nickel flashing, tin or silver coatings, and installers use the correct torque wrench values during installation and an antioxidant compound with aluminum wire.