The National Electrical Code is the bedrock of the electrical
construction business.
Do you know all the ins and outs of the Code?
NECA and Electrical Contractor magazine are pleased to present their
daily online feature, “Code Question of the Day.”
|
View Archive | Subscribe To CQD | Un-Subscribe To CQD |
Code Question Of The Day
|
|
Monday, February 06, 2012
|
|
|
|
|
Subject: Re: CQD Tuesday, January 31, 2012
What about the Exception in 250.148? It seems that it would not be necessary to splice the individual EGCs together. EGCs brought in and connected to their devices individually seems to satisfy (D) also.
Always good to have this brain stimulation in the morning, thanks Charlie.
Scott
|
|
code question and answer:
Friday, February 03, 2012
|
|
Charlie,
Thanks for the great NEC question service. I have a 460-volt, 3-phase motor rated 50 HP and the nameplate current is 58.5 amperes. The motor will run continuously in an exhaust fan application. I have installed three No. 6 THWN conductors in a 1-inch conduit for this motor. The HVAC installer indicated that on the last job, the electrician installed No. 4 AWG THWN to feed this motor. Table 310.15(B)(16) indicates that No. 6 THWN conductors are good for 65 amperes. Which is the minimum correct size conductor for this motor? Thanks in advance.
Sam Jenkins
|
Answer:
Hey Sam,
Happy motoring, so they say. Well based on the information in your question, it appears the 6 AWG THWN conductors are too small. Section 430.22 requires conductors for a single motor to be sized at not less than 125% of the motor FLA values based on Table 430.250. Table 430.250 indicates that a 50-HP, 460-volt, 3-phase motor draws 65 amperes. Multiply 65 amperes by 125% and the minimum conductor size required has to be capable of handling 81.25 amperes. Using Table 310.15(B)(16), a minimum of 4 AWG THWN copper wire (85 amperes) is required. Also keep in mind this response does not factor in any conductor ampacity correction or adjustment factors that may also be needed. Note that the temperature rating of the terminations can impact the ampacity of this motor branch circuit. The 6 AWG THWN conductors are too small based on the information provided in the question. Hope this is helpful.
|
|
Section: 430.6, 430.22, Table 430.250, Table 310.15(B)(16), 110.14(C
|
|
ANSWERS: Answers are the author’s opinions and do not represent formal interpretations of the National Electrical Code.
All answers are based on the latest edition of the NEC, unless the question requests a response based on a specific edition.
CONTACT US: To submit a question, subscribe to Code Question of the Day, or remove yourself from this list, please send an e-mail to
codequestion@necanet.org.
ABOUT THE AUTHOR: Charlie Trout is a nationally-known NEC expert and author. He served on three different National Electrical Code-Making Panels and is past chairman of CMP-12. He is also a member of the NECA Codes & Standards Committee. In 2006 Charlie Trout won the prestigious Coggeshall Award, given to recognize outstanding contributions to the electrical contracting industry in the technical and training area.
NECA STANDARDS & SAFETY PRODUCTS: NECA publishes the National Electrical Installation Standards (NEIS), a series of ANSI-approved performance and quality standards for electrical construction. Visit
www.neca-neis.org for more information. NEIS can be purchased in three formats: as paper books, on CD, or as .PDF downloads.
NECA also publishes safety books and CDs for the electrical industry. Visit
www.necanet.org/store to purchase NECA safety products.
