The importance of using a torque wrench on electrical connections
Every electrician has a responsibility for installing conductors and making connections according to the manufacturers’ instructions. When a particular vendor designs a connector, it’s manufactured, tested, and listed to work with a prescribed conductor(s) and a specific installation method — whether it’s of the set-screw or compression variety.
Set-screw connectors are common in electrical equipment, including panelboards, switchboards, and meter bases. For field installation, the connector manufacturer typically publishes a set of installation instructions on the packaging. For terminations in equipment, the manufacturer labels the equipment with the recommended installation torque value and any other special instructions. If for some reason the recommended torque is not available on the connector or equipment, there are published tables of “default” torque values included in NECA and UL standards, as well as in manufacturer catalogs. (Note: Default torque values are not meant to override manufacturers’ recommended torque values. Instead, they are to be used in the absence of manufacturers’ recommended values.) NEC Code-Making Panel 1 recently accepted a proposal to include torque tables in an Annex in the 2011 NEC.
It is common practice for most electricians to tighten mechanical set-screw connections without a torque wrench. Many qualified installers believe they know how much to tighten set-screw connections without using a torque wrench, because everything works fine once the system is energized. What they rarely see is what happens a year or a decade down the road when a connection that was not properly installed begins to overheat and fail (Photos). Then, the service electrician comes in, repairs the problem, and tightens the new connector — once again, without a torque wrench. These electricians aren’t intentionally doing inferior work; they simply don’t understand how important correct torque is to the safety and long-term integrity of the electrical system.
It’s estimated that 90% of electrical failures occur at connections. To determine how accurately installers tighten set-screw connectors without a torque wrench, a test assembly was designed using aluminum bar stock and aluminum tin-plated mechanical terminal lugs.
Putting torque to the test
The specific lugs used had a recommended tightening torque of 275 lb-in., or
22.9 lb-ft. Participants were given a choice of tools — either a 12-in. ratchet drive or a folding Allen set. The tightening torque applied was measured with a transducer and read from a remote digital meter, so only the test administrator could see the peak torque applied.
Results from testing at four different events were collected in 2009: the NTI Show in Ann Arbor, Mich.; the NECA show in Seattle; an IAEI chapter meeting in Las Vegas; and the IAEI Southwestern section meeting in Honolulu. Data was also collected from Mojave Electric service electricians in Las Vegas. In all, there were 402 participants in the study.
Although many of the participants predicted they would over-tighten the connections, very few did. The Table (click here to see Table) summarizes the results of the study. The breakdown assumes that set-screw connections tightened to within ±20% are adequate, based on a study performed by the Georgia Power Research Institute (now known as NEETRAC) and published in a Wire Journal International 1998 paper, “Reliability of Connections: A Comparison of Aluminum Alloy Stranded Conductors and Electrically Equivalent Copper Conductors,” authored by R. Ganatra and T. McKoon.
Not all of the test participants were electricians. To find out if electricians did any better as a specific group making connections, participants at three of the events were asked if they were employed or trained as an electrician. Of the 208 participants who answered the question, 172 responded affirmatively. The results of the subset of participants who were employed or trained as electricians are also shown in the Table.
While the electricians were slightly more likely to over-tighten connections than the entire group of participants, the study revealed they were no better at achieving a proper connection than someone with no experience as an electrician. Anecdotally, it was obvious during the test that people who had never made an electrical connection before had as much chance of getting it within 20% as an electrician with decades of experience. This indicates that achieving recommended torque is nearly independent of training and experience — and requires the use of a torque wrench to properly tighten a connection.
The connectors used for the test were listed for use with copper and aluminum conductors. To determine if the conductor used would make any difference, the test was performed with electrically equivalent 2/0 AWG copper and 4/0 AWG aluminum alloy building wire. These conductor sizes would typically be used for a 200A residential service according to the 2008 NEC Table 310.15(B)(6). The results using each conductor material are also shown in the Table. It appears that the tendency to under-tighten the connection for both materials is virtually the same, and it is slightly more likely that copper conductors will be over-tightened.
According to the 2007 paper, “Connector Theory and Application: A Guide to Connection Design and Specification,” produced by FCI USA, Inc. and authored by Gary Di Troia, Kenneth Woo, and Gaylord Zahlman, over-tightening a set-screw connection does not increase its reliability. It can, however, damage the lug or the equipment to which it is attached. It can even break conductor strands. Under-tightening connections can lead to overheating and failure at the connection.
While this testing was performed on 2/0 AWG copper or 4/0 AWG aluminum conductors, smaller connections (such as those found on receptacles) also have a recommended installation torque. For example, UL Standard 20 and UL Standard 1567 covering snap switches and receptacles call for binding wire screws to be tightened to 12 in.-lb.
Torque screwdrivers are available, but very few electricians use them. Electricians indicated they were more likely to use a torque wrench on larger conductors, such as those found on
transformers or switchboards, but not on branch circuit connections. Only 36 of the 402 participants indicated they always use a torque wrench or screwdriver to make electrical connections, while 247 answered they sometimes use a torque wrench or screwdriver to make electrical connections.
So why don’t we have more failed connections if most electricians appear to be doing such a poor job of tightening them? The fact is that our electrical system has built-in safety factors, and circuits are rarely fully loaded. Lightly loaded circuits put less stress on weak connections, so they may perform satisfactorily for many years before experiencing a problem, if ever. However, this is no justification to ignore basic installation instructions. The bottom line is a torque wrench is an easy and inexpensive way to improve the reliability and safety of an electrical system.
Hunter is a senior engineer with Alcan Cable in Atlanta. She can be reached at Christel.Hunter@Alcan.com.