Motor overload sizing question

[Electrogrunt]

Hello, I am getting ready for the Massachussetes journeyman test, and reading about motor overload protection. In the guide I am using, it states that if you have a motor with no nameplate amperage showing, that you can use the FLC tables at the back of the article. I am currently using the 2008 NEC, and can't find the section that specifically allows this. It makes sense, I just don't see it in the code. Any ideas of the section? I am pulling what is left of my hair out. Thank you.

 

[texie]

Hello, I am getting ready for the Massachussetes journeyman test, and reading about motor overload protection. In the guide I am using, it states that if you have a motor with no nameplate amperage showing, that you can use the FLC tables at the back of the article. I am currently using the 2008 NEC, and can't find the section that specifically allows this. It makes sense, I just don't see it in the code. Any ideas of the section? I am pulling what is left of my hair out. Thank you.

I'm not familiar with such a rule. The only similar thing I know pertains to branch circuit conductor sizing where the data plate does not tell you the HP, only amps (430.6(A)(1). But that has nothing to do with overloads. I don't see how there could be such a rule as OL are always sized to the nameplate amps and service factor. Maybe someone will have further comments.

 

[Jraef]

Hello, I am getting ready for the Massachussetes journeyman test, and reading about motor overload protection. In the guide I am using, it states that if you have a motor with no nameplate amperage showing, that you can use the FLC tables at the back of the article. I am currently using the 2008 NEC, and can't find the section that specifically allows this. It makes sense, I just don't see it in the code. Any ideas of the section? I am pulling what is left of my hair out. Thank you.

You are confusing the concept of "can" with the concept of "allow". The NEC makes no provisions for adapting to conditions that are not acceptable, and having a motor without a nameplate is not acceptable. I don't have my NEC here, but there is wording to the effect that a motor, or a machine, must provide the necessary information on it. Either FLC, MCA, MOCP, etc. So you will not find anything "allowing" the use of the FLC tables in lieu of the motor nameplate data, because technically, you should not be using that motor as far as the NEC is concerned. I have seen several situations were AHJs have red tagged a project where old machinery is being installed for that very reason. People take motors to motor shops thinking that the motor shop will "test it and make a new nameplate", but I have never seen one accept that liability, and if one did, I can't imagine they would do it for less than the cost of a new motor anyway.

All that said, it's a very common practice to use the tables if you are very sure of the motor HP rating, number of poles, voltage etc., just be aware that if it is going to be inspected, an AHJ may not agree.