The nameplate FLA

BachPhi · May 21, 2019

" The nameplate FLA is used to select the correct wire size, motor starter, and overload protection devices necessary to serve and protect the motor. "
The above statement is from the web site: https://www.ecmweb.com/motors/understanding-induction-motor-nameplate-information

What's your take on this?

rupej · May 21, 2019

I would have to agree with that statement. Also include using the FLA to size overcurrent protection.

widelto · May 21, 2019

@Bachphi:
For a complete info please download cowern papers from :
https://www.baldor.com/mvc/DownloadCenter/Files/9AKK107303

jraef · May 21, 2019

Not entirely true in North America though, at least from a code compliance standpoint...

The correct wire size is selected using a table in the NEC (similar in the CEC) that is based upon the motor rated HP. The motor nameplate FLA is usually less than those values so if you used the nameplate FLA, you may possibly end up with wire that does not meet code, especially on larger motors.

But another interpretation could be that since there is an EXCEPTION to the above, i.e. if the nameplate FLA is HIGHER than the table values, then you would be using the nameplate value. And since you will not know unless you read it, then I suppose you are "using" the nameplate value in some way no matter what...

BachPhi · May 21, 2019

widelto said:
@Bachphi:
For a complete info please download cowern papers from :
https://www.baldor.com/mvc/DownloadCenter/Files/9AKK107303

OK, I read that darn link you gave.

What's your opinion regarding the above statement?

willxfmr · May 21, 2019

There are a fair few details that need to get added, such as sizing a starter to meet the minimum load is usually not a good long term plan, and overloads will vary based on the conditions that the motor is exposed to in normal use. Wire sizing is also not as straight forward as it appears on the surface.
However, the quoted statement is true as far as the basics are concerned.

Bubba.

Rpgunnoe · May 22, 2019

The short answer is that, nameplate current is used to determine overload size and Horsepower (HP) of the motor is used to determine everything else.

There are many factors to consider and the National Electrical Code (NEC) covers these in Article 430, Motors, Motor Circuits, and Controllers.

A good reference and learning tool is a book by Stephen L. Herman titled, "Electrical Transformers and Rotating Machines." Unit 21 of this book is called Motor Installation and does a good job explaining this.

widelto · May 22, 2019

@BachPhi:
Always based on FLA I make the rest of calculations.

Rpgunnoe · May 22, 2019

https://www.youtube.com/watch?v=2cprO8ZdT1U

Mike Holt video above also gives a good explanation on the distinction between FLA (nameplate amps) and FLC (full load current from the NEC tables based on HP).

Rson · May 22, 2019

The motor nameplate value should be used to size the appropriate MCB or OL for protection. Whichever device you choose, it should be set to the motor’s FLA.

If you are fusing, determining wire size, or contactor size, then you do NOT use the FLA of the motor, but the charts in NFPA 70.

The reasoning behind this (from what I understand) is due to possible maintenance and to meet conductor codes. Someone may come by and replace a bad motor with a new one that doesn’t have the same FLA, but is the same HP rating. Equipment should be built (to code) to accommodate any motor in that HP rating (with a possible change of overload/circuit breaker)

BachPhi · May 22, 2019

Rson said:
The motor nameplate value should be used to size the appropriate MCB or OL for protection. Whichever device you choose, it should be set to the motor’s FLA.

If you are fusing, determining wire size, or contactor size, then you do NOT use the FLA of the motor, but the charts in NFPA 70.

The reasoning behind this (from what I understand) is due to possible maintenance and to meet conductor codes. Someone may come by and replace a bad motor with a new one that doesn’t have the same FLA, but is the same HP rating. Equipment should be built (to code) to accommodate any motor in that HP rating (with a possible change of overload/circuit breaker)

Right on , Rson! The statement in my first post was partially correct. FLA should be used for OL size. But Fuse and wire size should be using FLC & NEC 430.250 .

NetNathan · May 22, 2019

Wouldn't you also want to take into consideration SF (Service Factor) when sizing wire and protection?
I use some custom motors that are rated for 1.25 SF continuous.
A lot of motors are rated 1.15 SF, but not necessarily continuous. But this would be 1.15 x FLA.

jraef · May 23, 2019

If you use the NEC tables and rules to size the conductors based on HP, you are covered for the 1.15SF, but there is a note about "unusual conditions" at the bottom that would apply to 1.25 SF or other atypical situations.

The nameplate FLA

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