×
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

Fuse sizing in a 3 phase circuit

Fuse sizing in a 3 phase circuit

Fuse sizing in a 3 phase circuit

(OP)
I had an interview that had a test. One of the questions I supposedly got wrong was sizing each line (1L1, 1L2, 1L3) in a motor branch circuit. The nameplate on the motor was a given 3A FLA. I was asked to size the fuses by 300% using a class CC fuse. I took the 3A and divided by the square root of 3 and then took 300% of that. The EE manager told me I was wrong and that the 3A is across each line. I had thought the 3A on the nameplate was across all 3 phases and to find the particular current on each line you had to take into account the square root of 3. I went along with what he said. But, am I wrong and have I been undersizing my fuses or have they been oversizing them. I haven't had any problems in the past.

Any thoughts?

Thanks.

RE: Fuse sizing in a 3 phase circuit

If you are deriving current from total watts or total KVA or horsepower, then you may need to use the root 3 factor.
The nameplate current of a motor is the actual current in each phase. You have been very lucky with your fusing.
Using the root of three resulted in your fuse sizing being 173% instead of 300%

RE: Fuse sizing in a 3 phase circuit

By  code you are allowed to size class CC fuses at 300 percent of FLC.  9 amps

175% percent if using Dual Element(Time Delay)


RE: Fuse sizing in a 3 phase circuit

(OP)
Thanks. At least now I know.

RE: Fuse sizing in a 3 phase circuit

sangit:

I undestand your confusion, being just out of the school. The term phase and lines are often loosely used in field, but are well understood by experienced persons.

When someone says I measured current in each phase of a panel or a motor, he/she usually means he/she measured the line current.

All equipment nameplate current invariably refer to line currents. For fuse sizing, only line current that matters.

For some insight refer to the following threrad:
thread238-118444


RE: Fuse sizing in a 3 phase circuit

Generally speaking, isn't 300% used for fast blow and 173% used for slow blow as far as fusing goes?

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