125% for transformer and motor feeders

[mull982]

I know that whe sizing the service to a group of motors you all the FLA of all the motors and then take 125% of the largest motor. Do you still need to take 125% of largest motor even if motors are not continuous?

Do you need to size conductors 125% for feeder to transformer or only 100% of transformer primary rated current? Is it good practice to size transformer feeders at 125% of transformer load? I have seen this done sometimes.

 

[iwire]

I know that whe sizing the service to a group of motors you all the FLA of all the motors and then take 125% of the largest motor. Do you still need to take 125% of largest motor even if motors are not continuous?

Yes, the 125% of the largest motor is required regarless of it being continuous or non-continuous.

Do you need to size conductors 125% for feeder to transformer or only 100% of transformer primary rated current? Is it good practice to size transformer feeders at 125% of transformer load? I have seen this done sometimes.

You need to size the conductors based on the size of the over current protection you have chosen to supply the load.

In most cases it is good practice to supply the transformer with over current protection between 125% and 250% of the primary current rating.

 

[augie47]

My opinion is you need to watch the difference in the Code definition in "continuous load" and "continuous duty". From there 430.22(A) tells us continuous duty motor branch circuits are sized at 125%, where 430.22 (E) addresses branch circuits for other duties.

 

[nakulak]

sizing motor feeders, conductors, overloads & ocp can be complicated sometimes. It's a good idea to read article 430 now and then to familiarize yourself with all the variables, since it is not always so simple of an answer (especially if you are supervising these installations)