Sizing motor conductors

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mstrlucky74

Senior Member
Location
NJ
Just want to be clear that even if the FLA is given on a motor nameplate you go to the FLC in the code book for the ampacity correct?

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victor.cherkashi

Senior Member
Location
NYC, NY
I use following rules:

  1. Small 120V motor with thermal overload protection, 15A circuit breaker is usually ok. Check the manufacturer recommendations.
  2. Motor without thermal overload protection, you will need a standalone overload protection, I use the current in code book to size branch circuit conductors and branch circuit protection.
  3. When motor is connected thru VFD, the branch circuit conductors and protection should be sized based on VFD requirements (NEC.2017-430.122)
 
kwired

kwired

Electron manager
Location
NE Nebraska
Nameplate FLA is still necessary for motor overload selection.

If the motor is not a motor in the tables at the end of art 430 then you don't have much choice but to use nameplate for branch circuit calculations.

That does happen with specialty/OEM type motors. General purpose NEMA designed motors are usually going to be in those tables.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The only caveat I use is if the actual motor nameplate FLC is HIGHER than the NEC charts, then I use the motor nameplate FLC. That can happen when you have high pole counts in the motor (low RPM) motors. For example I did a pumping plant project recently where they said the motors were 250HP, everyone sized everything expecting the FLC to be 302A or less (per the NEC table). But before we started I asked for the motor data sheets and sure enough, they were 12 pole (550RPM) motors so the nameplate FLC is 395A! That is actually closer to a 350HP in the table...
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Jraef said:
The only caveat I use is if the actual motor nameplate FLC is HIGHER than the NEC charts, then I use the motor nameplate FLC. That can happen when you have high pole counts in the motor (low RPM) motors. For example I did a pumping plant project recently where they said the motors were 250HP, everyone sized everything expecting the FLC to be 302A or less (per the NEC table). But before we started I asked for the motor data sheets and sure enough, they were 12 pole (550RPM) motors so the nameplate FLC is 395A! That is actually closer to a 350HP in the table...
Click to expand...
Just so others know, that is addressed in 430.6 with the first sentence that says the table values apply for
other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors ...
Click to expand...
 
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