Motor Branch Circuit Conductors 1

[Timewaster9]

I have never been able to get a good explanation as to how to use table 430.22(E)to size branch circuit conductors for other than continuous duty. The table has "Classifications of Service" which are defined in article 100, but it refers to 5, 10, 15, 30, and 60 minute rated motors. Even after speaking with people from a motor manufacturer, I could not get an answer on the motor rating or what it meant. I have never seen this rating (other than continuous)on a motor nameplate. Does anyone know how to use this?

 

[MakeItSpin]

Time rated motors are common on cranes and hoists.

 

[uno]

Hi

I think NEC has this table to match the Concept of IEC. IEC and European standards allow motors to have a rating for short time/Intermittant duty such as S2, S3 etc.It is followed by the indication of the duration of the duty. These ratings are mentioned on the nameplate of the motor if it is designed for ratings other than the Continuous running duty S1.
The duty of US motors are usually expressed in minutes (15min, 30 min etc) and standard continuous duty is for 24 hrs a day.These are based on short time load tests.The duration of the test is for the time given in the rating such that the temperature rise does not exceed those of the designated class of insulation.

In IEC an intermittant duty motor never reaches equilibrium temperature of the insulation class in thermal tests when applied with specified loading, but is permitted to cool down between operations.

Hope the above helps. Why have you chosen this name for yourself? I think it was a good question?

 

[uno]

In short the above implies that the motor and the branch circuits need not be oversized for full load when only an intermittant operation of the machine is required in a process.