Let’s Have a Torque Talk: Electrical Connections and Torque Requirements

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When it comes to electrical installations, perhaps one of the most violated sections of the National Electrical Code has been and continues to be proper torquing of electrical connections.

In training across the United States, and even abroad, I have found that this failure to follow manufacturer’s torque specifications has proven to be the source of equipment failures, industrial arc flash accidents, and house fires.

Lessons Learned from a Mobile Home Fire

As a firefighter, I was introduced to this reality by the Indiana State Fire Marshall as I assisted him on an investigation of a mobile home fire.  A contractor had been updating the main feed wiring with new pedestal mounts and feeders to 100 mobile homes in a large mobile home park. The source of this fire turned out to be a recently-installed 200 amp panel board  with its aluminium service wires seriously under-torqued. The required torque spec was 20 foot-pounds (250 inch-pounds). The connections were  tightened to only around 12 foot-pounds (144 inch-pounds).

I was a young firefighter and licensed master electrician, and I was fascinated with the array of investigative tools the marshal had at his disposal to determine causes of electrical failures. The methods to discover causes of fire due to electrical failure  range from colorimetric analysis to something as simple as back-checking of torques. In this particular case, the panel main conductor lugs were back-torqued and found to be only half of the required torque value. When copper wire is used, colorimetric analysis is often able  to determine an exact cause. But even with aluminum, indicators like electrical spatter pattern(s), melting conductors versus insulation burn-back, and a host of other clues can nail down an exact cause of the electrical failure.

In this case, the neutral and one of the hot leads from the outside supply disconnect switch showed obvious signs of arcing. Some spatter was evident around what was left of the conductors, as well as subsequent post-fire melting of the remaining conductors. This was shown by the aluminum that had run down the left hand edge of the cabinet. The neutral was completely missing about 5 inches of its 4/0 conductor.

The contractor was subsequently cited for the fire. His insurance company paid a large sum for the complete loss of the mobile home and for the pain and suffering of the family. In addition, his electrical license was pulled. I’m sure this was not what he expected to have happen when he accepted the original contract.

Standard and Statistics

The NEC (2017) requires that “listed and labeled equipment shall be installed and used in accordance with any instructions included in the listing and labeling” in 110.3(B). A new addition in 2017 goes further in requiring “where a tightening torque is indicated as a numeric value on equipment or in installation instructions provided by the manufacturer, a calibrated torque tool shall be used to achieve the indicated torque value, unless the equipment manufacturer has provided installation instructions for an alternative method of achieving the required torque.” (NEC 2017 110.14(D))

The International Association of Electrical Inspectors published an article in July-August 2010 and in January-February 2015 stating that their research indicated that only 25% of connections performed without a torque wrench were within +/- 20% of the manufacturer’s recommended torque value. This statistic is confirmed over and over in my classes when I ask the class, “How many of you have AND use a torque wrench or a torque screwdriver to make up your electrical connections?” The average response in my unscientific survey is 5% or less – and unfortunately, usually closer to 1%.

Of course, I then ask the students how many of them actually READ installation instructions, and their answers point to the lack of torquing. Almost no electrician or engineer actually opens the paperwork and reads the instructions, and even those who do will confess that they did not FOLLOW the instructions because of lack of proper tools (calibrated torque wrench or torque screwdriver).

The going joke is, “If tight is good, then tighter is better. Tighten it until you hear it start creaking!” This usually results in either a stripped-out thread on the tightening lug, or a damaged (over-torqued) conductor, which subsequently can lead to failure. These failures, unfortunately, can be catastrophic in nature, especially when the improperly-torqued wire is a ground wire feeding a swimming pool, which could easily result in a shock or electrocution. We simply cannot afford these types of errors in our industry.

What To Do

Please take the time to read the manufacturer’s instructions, understand the various requirements, and of course, follow-through. Torque specs are often listed on the manufacturer’s literature, on the actual lug or terminal, or even on the front or side of the circuit breaker.

Here are a few examples:

 

Ken Sellars

Ken Sellars

Ken Sellars is an instructor of electrical safety, NEC, Grounding/Bonding and Arc Flash Safety courses nationwide. Read more about Ken.

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37 Responses

  1. Where does the tolerance of +/- 20% come from? That seems way too much. UL standards only allow +0/-10%.

    1. Hello Bernie. The 20% was not the tolerance being promoted, but a quote from the article of how close (or far) from the manufacturer’s torque the volunteers were torquing (or should I say not torquing) the connections. I totally support the comments about 10% – I would even prefer 5% with a calibrated tool. Thank you for commenting.

  2. Hi Ken, great post! I wrote the article that was published in the IAEI News in 2010, along with a similar article published in EC&M in August of 2010. Randy Hunter and I designed and built the torque test setup that was used to collect the data, and we are still using that setup to demonstrate to electricians how important it is to use torque tools. Along with several colleagues, we collected data from electricians around the country to support the addition of Annex I in the 2011 NEC and the new 110.14(D) in the 2017 NEC.

    1. Christel, I thoroughly enjoyed your article and use your comments in my training sessions all over the nation and internationally (with proper credit, of course). I loved the test setup idea, and the results always surprise the electricians and engineers. Love it – keep up the great work!

  3. I would make it clear in your original article that +/-20% is not OK. See comments on the LinkedIn post.

    1. Thank you, Margret. It is definitely an ongoing issue. Proper training, auditing, and inspections are the only way I know to correct this issue.

  4. Hello Ken,
    Firstly, thanks for sharing this post.I read your whole post. It’s really so informative. I only ever use a torque wrench for engine rebuilds. Tightening big end and mains cap nuts and bolts etc. Seriously thank you for all this awesomeness!

    1. You make a great point. I have always regarded the seriousness of torques on things like engine bolts, lug nuts, etc. Electrically, these things get overlooked way too often. Thanks for reading and for your nice comments.

  5. I was an electrician for 20+ years. Although I am an IT Manager now, I still remember the shift work that I performed for many years as an industrial electrician and electrical planner. I believe there is a severe shortfall of knowledge in the electrical industry as a whole regarding torquing as your polling suggests. Luckily, I had the benefit of being trained as a Naval Nuclear electrician and was exposed to this concept, but I will also confess that in the many years that I spent as an industrial electrician after my naval career, that I did not always torque down connections properly. I usually adhered to the “if tight is good, then tighter is better” philosophy. In fact, I usually only did so when installing new machinery or new motor drives or working on some special project.

    The article is great. Held my interest throughout entirety of it (which is saying a lot in my case – lol). As a past electrician and as a brother (notice the last name), keep it up!

    1. I think we are all guilty in the electrical trade of the “If tight is good, tighter is better” philosophy. Thank you for the nice comments, Bro!

  6. What is the proper method to tighten a lug with a screwdriver or wrench. For example: after tightening the lugs on a main breaker 200 amp panel if you grab the wires a wiggle them slightly you can tighten the lugs quite a bit more. Also someone earlier talked about engines, on them you are required to torque to a given value then back off and re-tighten to the final value. How does that relate to electrical lugs? I have been a master electrician contractor for 42 years and have heard many times about torque values but always felt tighter was better. We are experiencing a lot of failures of insulated lug connectors on motors lately and are being told by the lug mfgs to TORQUE THEM. I think some of them may be related to over tightening.

    1. Tony,

      Thanks for you reply. I was shocked to find out years ago from assisting a fire marshal in an electrical fire investigation that MOST electrical fires are in fact due to OVER-torque, not loose connections. The idea of tighter is better actually can crush the conductor, causing damage to the strands. A properly torqued connection puts the right amount of pressure to allow the conductor to properly conduct without overheating affects.

      For instance, a Square D 20 amp Homeline breaker, if my memory serves me correctly, requires 22 inch-pounds of torque. This is not a lot of torque, and the connection actually feels loose to most electricians. Anything tighter than this, as mentioned, crushes the conductor and creates a potential hotspot, leading to an overheated connection. As for the proper torque method, I was always taught to make sure the wire going to the lug is not at a bad angle (e.g. overstressing the connection before tightening even occurs), and then use a torque screwdriver or a torque wrench (for larger terminations). Tighten the connection to the correct torque, and you are good to go. I have been told many times that if the wiring is aluminum, a follow-up visit after a few days of operation is warranted to re-torque the connections, although this appears no where in the manufacturer’s instructions. As for grabbing the wires and wiggling slightly and re-torquing, this could be due to larger conductors having stress on them from the bend to reach the termination lugs. I see no problem necessarily in doing this, but again, be sure to follow manufacturer’s instructions, and if unclear, do not hesitate ask Square D or Cutler Hammer, etc.

      Hope this helps.

  7. Great article Ken! So informative and such a good read. Totally agree with you that almost no electrician or engineer actually opens the paperwork and reads the instructions, as a fellow electrician, I agree with this. But that was in the past. I have thoroughly read the instructions now.

  8. I am a firm believer in using manufacture torque requirements I have been in the mining industry as electrical troubleshooter for 30 years and over 90% of failures caused by tightening methods I also will move the wire around and torque again this has proven to help in maintaining torque specs when wires are moved into position to close cabinet, j box etc..

  9. Great Post Ken.
    This article provides highly informative content.i am impressed by the amount of research used in this piece.

  10. What is your opinion on aluminum wire? 1. Torque it once and you are done, or 2. Re-torque the connection after a few days. 3. Re-torque a year later.

    1. Ken,

      First of all, great first name!
      As for aluminum wire, I have heard both arguments, as well as arguments on whether or not aluminum wire compound like Ideal’s Noalox or Gardner-Bender’s OxGard are required. First of all, we must always default to a minimum of following the manufacturer’s requirements. The 2017 NEC requires proper torquing when the manufacturer requires it in 110.14(D), but never mentions the differences and issues with aluminum conductors. A good reference is the American Electrician’s Handbook, but even this book says to follow manufacturer requirements for torque, never mentioning a re-torque at all. The book does, however, list as a requirement, the use of a “suitable joint compound” for any aluminum connector (page 2.106 of the 15th edition).

      I know of several large companies that include annual torque checks on aluminum lugs, especially on main switchgear or main service panels. I am all for rechecking torques, and have usually done so about a month after installation (with power off and lockout/tagout/verify applied)for the few times that I have had to use aluminum conductors (customer request). After that, I have informed the requester that future maintenance is up to them.

  11. as an electrician for a state agency. I’ve seen about everything. I will say aluminum torque is imperative. if you go over then hot spots occur nearly every time. same with under torque. I might mention, that if an aluminum bolt is tightened as tight as you can get it, it tends to bend the threads on the screw and every time it heats up it will loosen then you’ve got a hot spot. it will smash the strands and then it will ark.

  12. Have any tests been done with using a cordless impact driver instead of a manual toque tool?
    Not a dewalt impact driver or any framing impact driver.

    I also believe from experience that the end of the wire needs to be stripped further and then folded clockwise so the screw on the breaker has enough wire surface area to clamp the wire evenly.

    1. My opinion here, but I wouldn’t trust a wire torque to an impact tool, although there are some available that supposedly have torque settings. As for bending the wire clockwise and folding it, that would almost certainly violate the manufacturer’s allowed wire-bending radius and could cause wire strands to break. If you follow the manufacturer’s torque guidelines and wire insertion depths, you should have no problems with overheating or arcing from loose connections.

  13. If you had to buy one or the other ( the wrench or the driver ) could you use just one ( the wrench ) to do small connections to breakers and will the use of adapters skew the torque?

    1. I believe that it would be very cumbersome to use a torque wrench and attempt to properly torque small screws for wire terminations. A bigger concern would be the torque range required. Many small connections, like 12 awg wire, are properly torqued around 22 in-lbs. Most larger connections, like 4/0, may ask for 55 foot-lbs, obviously much higher than any in-lb range. You would be hard pressed to find any tool that could handle such ranges with any accuracy.

      A short adapter typically will not affect torque values. Torque is radius times force, so a typically small adapter will not introduce enough error to be considered an issue. A long adapter will not affect torque as long as the user maintains a straight angle to the item being torqued. Torque skewing comes from any angle change while applying the torque force. This mechanical video is great at explaining this concept: https://www.youtube.com/watch?v=tRR96ILGORU

  14. Hi Ken, can you comment on 2019 NFPA 70 B 8.11.B That states that “After a connection or termination is torqued to the specified value there can be metal relaxation. It is not appropriate to check an existing connection or termination for tightness to the prescribed specified value with a calibrated torque measurement tool. Doing so can result in an improperly terminated conductor or cause damageto the connector and might void the listing”. This would imply that annual checking might cause damage.

    1. Bob, I was taught back in my military days to not perform annual torque checks by simply re-torquing the connection, but to actually loosen the connection, clean it, and re-torque. When I left the military, I was shocked to find preventative maintenance instructions, especially on overhead cranes, that said to perform torque checks as you mentioned above. I saw crane records that had 30+ years of MONTHLY torque checks, and watched electrical maintenance personnel do these checks. Sure enough, it was torque wrenches and torque screwdrivers to the rescue. We took a crane down, locked/tagged/verified zero electrical energy, and then back-torqued several connections to see how tight the connections actually were. We found connections SEVERELY over-torqued, and found cracked and deformed strands of copper under the lugs, so I can verify your last comment from experience. The best option here is not necessarily an annual torque check, but an annual infrared check for improper connections that are showing signs of overheating. If these are found, the connections should be loosened, cleaned, any damage repaired, and then a connection made to manufacturer’s torque specifications.

  15. Not sure which I liked more, the article or the comments. Both are very good and informative. I am not an electrician just a home owner living in New Jersey looking to learn more about Aluminum wiring. My home was built during the early 70’s when aluminum was used to wire housing. I have read about the Purple twist connectors for Copper Aluminum connections. I also read that in Canada it’s NOT recommended to use the Purple twist caps (they are a tempary fix) but to use the purple lugs. what are your thoughts?

    1. Officially, any product that is listed and labeled for the copper/aluminum connection is okay. The twistlocks I have seen are intended for that use, and are so stamped on the package or on the actual twistlock, as are the lugs. A properly-torqued lug will most likely result in a much better connection in my opinion, but most home connections in j-boxes are in fact made with twistlocks. I was taught in my early days to always use an aluminum oxide compound (many brands on the market) to ensure a long-lasting connection with no galvanic action between the metals. Years ago, the Consumer Product Safety Council made a statement that included the following, “In CPSC-sponsored laboratory testing and life tests, substantial numbers of these connections overheated severely.” This was in reference to twist-on connectors. I have included a link to the article that includes some great aluminum wiring repair tips.

  16. One of the commenters referenced a UL standard stating +0%/ -10% for the torque tolerance. What is that UL standard number?

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