National Electrical Code

Minimum Wire Bending Radii and Conduit Fittings

June 4, 2018

Conduit fittings may be required to be oversized.

Something as simple as installing conduit bodies (such as LBs) should require little thought as far as sizing goes, especially in cases where there are no splices involved. Surprisingly enough, this is one area of the NEC that is often violated. The issue is the minimum bending radius required for the conductors. Let’s run through an example to explain this situation.

Say we have a ¾-in. rigid metal conduit (RMC). We installed 20 ft of conduit, including a couple of ¾-in. LB conduit bodies along the way. Inside the conduit we’ve installed one, 600V, 7/C, 12 AWG, Type TC-ER cable. The individual conductors are type XHHW-2. We looked up the manufacturer’s data on the cable and find the O.D. is 0.59 in.

Just to be sure that we’ve sized the conduit adequately, let’s check the conduit size. We’re allowed a 53% fill maximum as per NEC Chapter 9, Table 1, and Notes to Tables [Note (9)]. We’re going to use a cable that has an actual O.D. of 0.59 in. (area = 0.273 in2) in accordance with Note (5). Looking at Chapter 9, Table for Art. 344, we see that the maximum conduit fill allowed is 0.291 in2. Great! Our conduit size should be large enough for a short pull.

Next, we check the bending radius from the cable manufacturer, and find that they require a minimum bending radius of four times the O.D. of the cable: 4 x 0.59 in. = 2.36 in. radius (minimum).

Now we check the minimum bending radius that the fitting will allow. Fitting bend radius and fill information is available from the conduit body manufacturer and is also typically found on their web site or through the local manufacturer representative. In this case, we’re going to use a Form 8 LB, malleable iron, which is much larger than a Form 7. However, even the Form 8 condulet only has an allowable bending radius of 2.1 in. per the manufacturer literature that we’re using in this example. The next trade size larger is a 1-in. LB with a minimum bending radius of 2.5 in.

So, based on our example scenario, we would need to install a 1-in. LB. Finding something like this after the conduit is installed can result in either damaged cable or a costly rework situation. It’s also important to note that the minimum bending radii for different brands of “Form 8” fittings vary.

About the Author

Eddie Guidry | Senior Fellow

Eddie Guidry is a senior fellow with Fluor Enterprises, Inc., Sugar Land, Texas. He is highly skilled in electrical and control systems for industrial construction, design, and engineering (both domestic and abroad). The majority of Eddie’s 38 years of experience includes upstream and downstream portions of petrochemical plants and refineries. He is also heavily experienced in water and wastewater treatment facilities. Eddie is very active in the development of U.S. national (ANSI) and international codes and standards, and has been a principal member of the National Fire Protection Association (NFPA) National Electrical Code (NEC) committee since 1999 and NFPA’s National Advisory Committee on Electrical Safety Research since 2008. Eddie, who has also developed and conducted many electrical courses and seminars over the years, currently holds the corporate Master Electrician license for Fluor Corporation in the State of Texas and is also an ICC/IAEI certified electrical inspector.