×
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

Motor Feeder size

Motor Feeder size

Motor Feeder size

(OP)
Per CEC,the motor feeders need to be sized not less than 1.25FLA. But I have seen a lot of cases where motor feeders have been sized not less than 1.15FLA  but smaller than 1.25FLA, especially for some large motors, which are protected by electronic devices. Apparently it is against code requirements.  But as an engineer, we can make our own judgement (OBIEC). SO long as the cables are not higher than the overload relay settings, the cables should be protected. Do we need to get the deviation from the client to do so?

RE: Motor Feeder size

Codes, like the CEC, are there to protect against fire and other electrical hazards. They are not necessarily design tools. If you choose to ignore them and a fire happens, you are taking a big risk. But if you are in that business, then maybe that is acceptable to you. Asking an end user if it is OK to violate local codes however is not likely to endear them to you.

Also, be careful of what you think you see. If you observed smaller conductors on a piece of factory assembled equipment, understand that there are different rules for that. In Canada, that would fall under CSA listing and conductor sizing (if CSA is anything like UL in this regard) is not the same as the installation code (CEC/NEC). The equipment can use smaller conductors because it has been (presumably) engineered to an overall specification and listed by that agency.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Motor Feeder size

In the US, I've never been involved in any situation where the 125% requirement was violated or ever heard of anyone asking for a variance.  It's hard to imagine an inspector granting a variance on this, at least in the US. From a practical standpoint, after commissioning, the first time the motor trips the O/L, they will put in a larger O/L based on the motor nameplate and starter manufacturer's tables.  They won't bother looking at the conductor size.  At that point, all of your sage engineering judgment becomes irrelevant.   

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

RE: Motor Feeder size

(OP)
This is API541 Special designed motor(s.f=1.0). It will never run higher than 1.15xFLA, VFD won't allow it.

RE: Motor Feeder size

What part of no don't you understand?  Installations under the CEC are inspected by the AHJ. If the conductors are under sized, you will change them for adequate conductors. If the conduit is now too small for the larger conductors, you will also change the conduit to the proper size.

Quote:

It will never run higher than 1.15xFLA, VFD won't allow it.
Duh. How many times have I seen someone panic and start changing settings when a new piece of equipment doesn't meet his expectations. There are several experts here and possibly more in the field who have no problem reprogramming a VFD to supply more than 1.15% of rated current, passwords notwithstanding.
With your attitude that you know more than the code, you may be the first to decide that it will be OK to set the current limit higher than 1.15% when you have a problem during start-up.
You may find it humbling to check the CEC and see the backgrounds of the committee members who write the code. In many instances the committee members are in a position to be exposed to or be aware of a very much higher number of incidents than most designers.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

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