Code Q&A

Sept. 8, 2011

Q. How do I size the conductors for an arc welder?

A. The supply conductors for arc welders must have an ampacity not less than the welder nameplate rating. If the nameplate rating isn't available, the supply conductors must have an ampacity not less than the rated primary current as adjusted by the multiplier in Table 630.11(A), based on the duty cycle of the welder.

Let's work an example problem to help hit this point home.

A non-motor/generator arc welder has a primary current rating of 40A with a duty cycle of 50%.
Calculated Load = Primary Rating x Multiplier [Table 630.11(A)]
Calculated Load = 40A x 0.71 = 28.40A

Therefore, the branch-circuit conductor for the welder must not be sized less than 30A. As per Table 310.15(B)(16), a 10 AWG conductor has an ampacity of 35A at 75ºC.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.