When is the largest motor not the largest motor?

[finhead]

When using the General Method to size the service of a single family dwelling, it is a requirement to increase the largest motor by 25 percent. The question is, which is the largest motor? For example:

A 1 hp motor at 230 volts has a FLC of 8.0 amps and a VA of 1840
A 1/2 hp motor at 115 volts has a FLC of 9.8 amps and a VA of 1127

Section 430.17 requires the FLC to be used in determining the largest or smallest rated motor, but I don't think it applies to Part III of Article 220.

It is a common practice to do service calculations using VA. It seems to me that the motor with largest VA should be inserted into the calculation.

Any input would be appreciated
Brian Dolan

 

[charlie b]

430.17 is concerned with the amps drawn by conductors, and the ampacity of those conductors. When you add 25% for the purpose of making sure the conductors are big enough, you need to be looking at the amps drawn by each motor.

As you suggested, 220 is all about load, and load is all about VA. When you calculate the load on a service, a feeder, or a branch circuit, you are adding up VA. So when you pick the largest motor, you look for the largest value of VA.

 

[finhead]

Thanks Charlie, we are in complete agreement.

Brian