National Electrical Installation Standards

Standards as High as Your Own

Tuesday, December 11, 2018


Charlie, The 2017 NEC code reference at 501.15(C)(3) for sealing and drainage of Class I, Division 1 and 2 installations tells the user of the Code that the thickness of the sealing compound installed in completed seals, other than listed cable sealing fittings, shall not be less than the metric designator (trade size) of the sealing fitting expressed in the units of measurement employed; however, in no case shall the thickness of the compound be less than 16 mm (5∕8 in.).

This language seems to be telling the installer and inspector that for a 19 mm (¾ in) seal off fitting the sealing compound shall be ¾” but in no case less than 5/8 in.  Therefore, is a 5/8 in. installation of sealing compound meet the intent of this code requirement?  If not, can you explain the intent of this language?

Adolphe Menjou



Hey Adolphe thanks for your question, your name seems vaguely familiar, do you have a mustache?

There are two concepts to the rule in 501.15(C)(3). One - the sailing compound thickness must be not less than the trade diameter of the sealing fitting so for a 2-inch sealing fitting the compound must be 2 inches thick. The sealing compound also cannot be less than 5/8 inch so that would need to be the minimum thickness for a 1/2-inch sealing fitting.

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ABOUT CHARLIE: Charles M. Trout, better known as Charlie, was a nationally known NEC® expert and author. He served on several NEC® technical committees and is past chairman of CMP-12. In 2006 Charlie was awarded the prestigious Coggeshall Award for outstanding contributions to the electrical contracting industry, codes and standards development, and technical training. Charlie was also a member of NECA’s Academy of Electrical Contracting. Charlie’s experienced team of industry experts keep the CQD dialogue and discussions active and informative in the spirit of the man himself, as he wanted.

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