National Electrical Installation Standards

Standards as High as Your Own

Monday, January 21, 2019


I have noticed a Romex connector to Clamp individual Unjacketed MTW rated wires in a Machine tool Control Panel. Theses conductors will carry 400 volts to and from an Inverter type Motor Drive. Is this a violation of NEC or UL508?

Kurt Adlon



Hey Kurt thanks for your question. The Word “Romex” is a registered trademark of Southwire Company. We have been asked to point that out, the correct term is Nonmetallic Sheathed Cable.

Machine Tool Wiring (MTW) is included in Table 310.104(A) with an intended application as machine tool wiring. The Informational Note refers to NFPA 79.

Nonmetallic-sheathed-cable Connectors (PXJV) are intended for use with nonmetallic-sheathed cable and sometimes other cable types. More information is available at:

If the Industrial Control Panel is listed (UL 508A), ask the manufacturer and certifying laboratory how they determined the cable connector was suitable for that purpose. If the Industrial Control Panel is not listed (more likely) the AHJ is still responsible for approval. A "cable clamp" is generally not suitable for securing individual conductors so it would be not be acceptable based on 110.3(B).

ABOUT CQD: NECA’s Code Question of the Day (CQD) is a leading National Electrical Code® forum for NECA and the industry. The CQD generates a lively dialogue and relative practical and Code-based responses to an ever-increasing and interactive audience.

IMPORTANT NOTICE: All answers are based on the latest edition of the National Electrical Code®, unless the question requests a response based on a specific edition. This correspondence is not a formal interpretation of the NEC®. Any responses expressed to the questions are opinions and do not necessarily represent the official position of NECA, NFPA, or any technical committee. In addition, this correspondence is neither intended, nor should it be relied upon, to provide professional consultation or services.

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.

NECA STANDARDS: NECA publishes the National Electrical Installation Standards™ (NEIS™), a series of ANSI-approved performance and quality standards for electrical construction. Visit for more information. NEISÔ can be purchased in three formats: as paper books, on CD, or as electronic downloads.

NECA SAFETY PRODUCTS: NECA publishes valuable electrical safety books and CDs for the industry. Visit to view or purchase NECA safety products.

 CONTACT US: To submit a Code question, subscribe, or unsubscribe from this list, please e-mail: