×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

NEC Article 430.53 question

NEC Article 430.53 question

NEC Article 430.53 question

(OP)
I am confused with article 430.53(C)(4) and (C)(5), I want to size a branch circuit short circuit ground fault protection device for a three motor installation.  Two motors are 5hp and the third is 3hp on a 480Vac 3ph system.  Tables say 5hp is 7.6amps and the 3hp is 4.8 amps
So do I multiply 7.6x250%+7.6+4.8 to get the breaker, or do i have to size the breaker for the smallest motors overload relay according to 430.53(C)(5).  Any help or hints would be welcome thank you.

RE: NEC Article 430.53 question

Size feeder breaker based upon the size of the largest motor branch breaker plus the sum of the other motor FLa. See Example D8

RE: NEC Article 430.53 question

First off, (C) only applies to a factory assembled system or for what is called "Group Fused" installations where you are using products specifically designed and marked (listed) to be used together as a group installation. In that case, you need to follow the manufacturers stated instructions and protection scheme. If that is the case, your component manufacturer will be able to supply you with a specific maximum breaker size that the OL device can be connected downstream of and that is the maximum size you may use as the main. as per (5). In some cases, that size will be far more than what would be allowed in (4), so then (4) would prevail. In other cases it will fall far short of what you need. So you need to start first with your supplier to get the right information from them for the components you plan to use.

If you are doing this in the field with components that do not have specific manufacturer's instructions as to SCPD ratings, you would follow B and D.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources