Multiple motor circuit breaker undersized?

[Jraef]

Some motor mfrs make 208V motors. The utilization voltage rating issue is for an EXPECTED voltage drop from the service point to the load. But Bill Cowern (in the Cowern Papers from Baldor) said that it was expected that typical 208V systems were small enough that the distance would not be much of a factor, so the motor mfrs make the 208-230/460V motor ratings under the assumption that they don't need to go down to 200V - 10%.

When this approach is taken the motor manufacturer is essentially saying that this motor can be successfully operated on voltages as low as 208 minus 10% or 187 volts. This approach usually works very well since 208 volt power systems are normally used in small buildings with relatively short distances between the incoming power service and the utilization equipment. These short runs tend to make 208 volt power systems quite stable so that the limit of the motor’s low voltage capability is seldom tested.

Link: pages 49-50 (PDF pages 51-52)

 

[Sberry]

The utilization voltage rating issue is for an EXPECTED voltage drop from the service point to the load.

I agree with this, I see it repeatedly said that service voltage is somehow measured at the pole and that the calcs begin there and that is not the way it is expressed in voltage tolerance boundaries that guide service requirements for the utilities. They reference the NEC for "acceptable" or allowable losses.

 

[don_resqcapt19]

Some motor mfrs make 208V motors. The utilization voltage rating issue is for an EXPECTED voltage drop from the service point to the load. But Bill Cowern (in the Cowern Papers from Baldor) said that it was expected that typical 208V systems were small enough that the distance would not be much of a factor, so the motor mfrs make the 208-230/460V motor ratings under the assumption that they don't need to go down to 200V - 10%.



Link: pages 49-50 (PDF pages 51-52)

But where supplied from a 208Y/120 volt system, the voltage could easily be less than 200. The Illinois commerce commission says this about the permitted voltages supplied by a utility in Illinois.

Allowable voltage variations. For service rendered at the standard service voltage, voltage variations as measured at any customer's point of delivery shall not exceed a maximum of 127 volts nor fall below a minimum of 113 volts for periods longer than two minutes in each instance. For service rendered at voltages other than the standard voltage value, voltage variations as measured at any customer's point of delivery shall not exceed 10% above or below the service voltage for a longer period than two minutes in each instance.

If you consider the 208Y/120 volt system as a system supplied at 120 then, the 208 could be 195 volts and be in compliance with the requirement. If you see it as a 208 volt supply, then it could be as low as 187 volts. The rule specifies that this voltage is at the service point, so does not include any voltage drop on the load side of the service equipment.