×
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

Sizing Disconnect/Breaker for main feed

Sizing Disconnect/Breaker for main feed

Sizing Disconnect/Breaker for main feed

(OP)
This may be a remedial question, but I can never seem to find exactly what I am looking for in the NEC.

I have a control panel that has the main 480VAC power feed into a breaker - (ABB Tmax)  Power then branches into three motors and a transformer.  Each motor and transformer is protected per NEC and manufacturer recomendations.  I am trying to find how to size the main breaker.  Is there a rule of thumb or a very specific formula/table?  I am trying to master the process, not just answer this one situation.

Thanks in advance.

"To be or not to be, that is the question" - William Shakespeare via Hamlet

RE: Sizing Disconnect/Breaker for main feed

See NEC 430.63.  This should be what you are looking for.  It references some other sections you'll have to go to, but it should answer your question.

Post back if you have questions.

Cheers,

dpc

RE: Sizing Disconnect/Breaker for main feed

I think it would be easier if you add up the individual current demand of each load to get the main breaker current rating.

But then you would also have to consider the maximum current each motor can draw when there is a voltage sag of about 10%

You might also have to consider the diversity factor of your loads

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