National Electrical Installation Standards

Standards as High as Your Own

Wednesday, March 11, 2020


Hello, Code Question of the Day. I enjoy reading the questions and responses each day. Thank you. Here is my question: What is the minimum depth of working space required from a switchboard supplied by an ungrounded 480-volt closed delta system. There is a concrete wall directly in front of the switchboard. I believe the minimum depth of the working space required is 36 inches. What is your opinion? Thanks for any assistance.

Harry Cannaday



Hi Harry, thanks for the comments and thanks for participating. Your question is a little involved but here is our opinion. Minimum depth of working space is provided in Section 110.26(A) and Table 110.26(A). Table 110.26(A) is based on nominal voltages to ground. The voltage to ground for an ungrounded 480-volt delta system must be taken at 480 volts, based on the definition of "voltage-to-ground." Note that the actual voltage to ground read by a instrument is usually 0 or very low and going to vary based on the effects of distributed leakage capacitance among other factors. These systems are not solidly grounded, meaning there is no solid connection to ground from any phase conductor of the system. Back to the question...Since 110.26(A) Condition 2 addresses minimum working space depths to grounded surfaces, such as concrete, Condition 2 would be applicable. The minimum distance required, based on the information in the question, is 42 inches. Sections: 110.26(A), Table 110.26(A) - Condition 2, and Article 100 definition of the term "Voltage to Ground." Hope this information is helpful. 

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