×
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

size breaker

size breaker

size breaker

(OP)
I met a problem of sizing a three phase breaker. the current in each wire is measured around 50A, and it is well balanced in three phases. The breaker using now is 225A. Is it too large for the current load? how can I choose the breaker according to the current in wiring? Can I just use 50A or do I need multiply a factor?

RE: size breaker

Suggestion: There are many factors involved in sizing breakers. More information is needed, namely:
1. Is there any motor load?
2. What is the largest motor in HP?
3. What is the starting arrangement of the motor?
4. Expected load growth?
5. Ambient temperature derating?
6. Altitude derating?
7. Sizes of conductor?
8. The largest rated circuit breaker or fuse downstream?
9. Does the breaker have a sensor?
10. What type of 225A circuit breaker is it?
11. Etc.

RE: size breaker

The C/B rating is based on the cable, and the cable size is based on the load. If the system is already installed, there is usually no problem with leaving it alone if the only problem is 50A on a 225A circuit.
To size a new circuit, the NEC specifies several procedures, but mostly that the circuit is sized for at least 125% of the calculated load.

RE: size breaker

Breakers for motor loads are usually sized differently from other loads -- you need to know what load the breaker is serving....

As DanDel poins out the NEC specifies several procedures... take the time to review these...

RE: size breaker

a better question is Why do you want to reduce the breaker size?

RE: size breaker

Suggestion: Except for some fire protection equipment such as fire pumps, the circuit breaker should be protecting conductors, load, etc. downstream.

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