National Electrical Installation Standards

Standards as High as Your Own
 
Menu

NECA Online Help Desk
Please leave a message
Submit your Code Question | Subscribe to CQD | Un-Subscribe To CQD | Print | Share This | RSS
?
Friday, May 7, 2021

Question:

Comment on CQD February 26, 2021 Friday, February 26, 2021

Friday's Question:

Hello, My question is what is the size overcurrent protection for a 75 kVA transformer, 3 phase, 4-wire, 480 primary, and 208Y/120 secondary?

Thomas Gentry

Friday's Answer:

Greetings Thomas,

Section 450.3 and Table 450.3(B) with Notes are used to determine the overcurrent protection size. This question presumes the OCPD is installed on the primary side of the transformer. Based on the Standard Ampere Rating Table in 240.6(A), 125 amperes would generally be required (112 amperes round up to the next higher size).  

Follow up Question:

Wouldn't this installation require primary and secondary protection?   Reference 240.4(F) and 240.21.

Respectfully,

Paul Briggs 

A

Answer:

Hello Paul,

Thanks for the comment. The question was only seeking the "required overcurrent protection for the transformer," not the conductors supplying it or connected to the secondary side of it. Section 450.3 provides the general rule. There is an informational note following that section that refers to 240.4, 240.21, 240.100 and 240.101 for protection of the "conductors." So you are on the right track, that the transformer once protected in accordance with the NEC, often requires additional conductor protection rules to be applied, such as Section 240.21 which you mentioned in the follow up. Often this is achieved using the same overcurrent protective device.

Thanks for participating. 

ABOUT CQD: The Code Question of the Day (CQD) is NECA and ELECTRICAL CONTRACTOR Magazine’s flagship National Electrical Code (NEC®) public forum for the industry, sponsored by EATON. The daily distribution of Q&A generates a lively dialogue and shares relative Code-based practical responses.

SUBMIT YOUR CODE QUESTION: Click here to submit a question to for inclusion in an upcoming edition of the Code Question of the Day, or email codequestion@necanet.org. 

CHARLIE TROUT: 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. Even though Charlie passed away in October of 2015, his work continues in spirit. NECA continues to maintain this question forum for its many subscribers in memory and recognition of all his significant contributions to making the NEC what it is today.

NECA STANDARDS: NECA publishes the National Electrical Installation Standards™ (NEIS™), a series of ANSI-approved performance and quality standards for electrical construction. NEIS can be purchased in the NECA Store in three formats: a printed or  PDF download of a standard or, as an  annual subscription service.

NECA SAFETY PRODUCTS & PUBLICATIONS: NECA produces electrical safety publications and products for the industry including jobsite safety guides, handbooks and resource kits. View a full listing of available resources and products »

IMPORTANT NOTICE: Unless the question requests a response based on a specific edition, all answers are based on the latest edition of NFPA 70® National Electrical Code®.

This correspondence is not a formal interpretation of the NEC® and any responses expressed to the questions are opinions and do not necessarily represent the official position of NECA, NFPA, the NEC Correlating Committee any Code-making panel or other electrical technical committee. In addition, this correspondence is neither intended, nor should it be relied upon, to provide professional consultation or services. 

UPDATE YOUR SUBSCRIPTION PREFERENCES: Subscribe or Unsubscribe from this list.

Submit your Code Question | Subscribe to CQD | Un-Subscribe To CQD | Print | Share This | RSS

© 1999 – 2014 National Electrical Installation Standards. All rights reserved.
Brought to you by NECA. NECA is the voice of the electrical contracting industry NATIONAL ELECTRICAL INSTALLATION STANDARDS, NEIS and the NEIS logo are registered trademarks of the National Electrical Contractors Association.

Created by Matrix Group International, Inc.

Follow Us on: