×
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

Please check me

Please check me

Please check me

(OP)
  New piece of equipment coming soon, total amp draw will be 100 amps. Please check my figures.
100 amp x 1.25 = 125amp x .8 derating the wire = #2 THHN, #8 ground
100 amp x 2.5(inverse timed breaker)= breaker rated 250amps

using NEC 2002 430.52 for breaker calculations.

RE: Please check me

The breaker x 2.5 is a MAXIMUM size, not a recommended size. If you use that breaker, you must then size the wire for 250A x 125%. Also, I don't know where you got the .8 derating factor on the original wire size you did.

If this is for a dedicated motor branch circuit, the scenario is different by the way.

JRaef.com
"Engineers like to solve problems.  If there are no problems handily available, they will create their own problems."   Scott Adams  
For the best use of Eng-Tips, please click here -> FAQ731-376

RE: Please check me

D'oh!

Just noticed the reference to 430.52, which means it must be a motor branch circuit. In that case you can use the 250A CB as long as you have an overload relay in your motor controller.

JRaef.com
"Engineers like to solve problems.  If there are no problems handily available, they will create their own problems."   Scott Adams  
For the best use of Eng-Tips, please click here -> FAQ731-376

RE: Please check me

If this is a single motor then the wire should be OK. You didn't specify a distance. The breaker should never need to be larger than 2X the motor FLA. I would use 150A or 175A.

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