×
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

Calculating Motor Full Load Rating

Calculating Motor Full Load Rating

Calculating Motor Full Load Rating

(OP)
Can anyone tell me why the full load amps listed in the NEC do not equal the motor power rating divided by the motor voltage rating?  For instance, a 10hp 230V single phase motor is rated at 50Amps according to NEC, but using I=V/P, you get 32Amps.

RE: Calculating Motor Full Load Rating

So that you can replace you 32FLA motor with any other 10hp motor and not run into trouble in terms of wire sizing or overcurrent protection.

RE: Calculating Motor Full Load Rating

Don't forget to include power factor and efficiency in your calculations, too.  If you assumed 0.8 pf and 0.9 efficiency that would get you up to 45A for a 10hp 230v motor.

=====================================
(2B)+(2B)'  ?

RE: Calculating Motor Full Load Rating

Cables heat up faster than motors too so if your load occasionally eats into the the overload margin but stays below the motor protection trip level, you don't want your cable being damaged. The motor should normally be the limiting factor, not the cable and controlgear ratings.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Calculating Motor Full Load Rating

In addition to what electricpete said, the pf of single phase motors are generally lower than 0.8 and efficiencies are also lower compared to 3 phase motors.

Try comparing NEC to the nameplate of an actual 10 HP, 1 Ph, 230V motor.

Rafiq Bulsara
http://www.srengineersct.com

RE: Calculating Motor Full Load Rating

Good point Nick.
I looked at Reliance's site and I couldn't find any single-phase 10hp motor....
But here is a 5hp single phase 230vac motor:
http://www.reliance.com/pdf/pdf/aced/A00020-A-B001.pdf
FLA of the 5hp motor is 30.
If you doubled that for 10hp motor, you'd expect 60A.  
If you allow for better efficiency and power factor on the 10hp than the 5hp, 50A might be reasonable.
 

=====================================
(2B)+(2B)'  ?

RE: Calculating Motor Full Load Rating

Sorry, I don't know where Nick came from... I mean Rafiq.

=====================================
(2B)+(2B)'  ?

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