Multiple motor circuit breaker undersized?

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Tainted

Tainted

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Location
New York
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Engineer (PE)
I encountered something that doesn't make sense to me in the field in a cooling tower room.

There is (1) 25HP blower motor and (2) 15HP pump motors being powered via a 3-phase 208V controller panel. Inside the control panel is a 3-pole 225A circuit breaker feeding all the motor loads mentioned. But a 3-pole 150 circuit breaker feeds the 3-pole 225A circuit breaker so in reality the whole circuit is limited by the 3-pole 150 amp circuit breaker.

Inside the control panel the 25HP is controlled by a VFD, the other (2) 15HP motors is not connected to the VFD and just being controlled by the contactors.

To size the breaker I take 2 times the largest motor FLC plus all the other FLC:

2*(74.8) + 48.3 +48.3 = 246.2A

Based on the calculation above a 250A circuit breaker is needed but how come the whole system is being powered by a 150A circuit breaker?
 
Remember, the 2x-the-largest is for the maximum breaker size. It probably works well enough because the drive acts like a soft-starter. I would be suspicious of the wire size though. That system needs 3/0.

Are the two 15HP non-coincident? That gets you under 150A for the wire size.

BTW, your math uses the 208V amps for the 25 and 200V amps for the 15's. Fun fact; table 430.250 gives system voltage ranges for all the motor rated voltages - except 200V and 208V!
 
JoeStillman said:
Remember, the 2x-the-largest is for the maximum breaker size. It probably works well enough because the drive acts like a soft-starter. I would be suspicious of the wire size though. That system needs 3/0.

Are the two 15HP non-coincident? That gets you under 150A for the wire size.

BTW, your math uses the 208V amps for the 25 and 200V amps for the 15's. Fun fact; table 430.250 gives system voltage ranges for all the motor rated voltages - except 200V and 208V!
Click to expand...
Ah thinks for pointing out my silly mistakes.

The whole system is currently powered by 1/0 wires. I think it's safe to assume the motors are non-coincident, I will have to ask my mechanical engineer. We are replacing and renewing the whole system so I just want to make sure I know the existing conditions before work is being performed.

So if the 25HP motor is a soft start we can size the motor circuit breaker at 100% of the total FLC?
 
Tainted said:
So if the 25HP motor is a soft start we can size the motor circuit breaker at 100% of the total FLC?
Click to expand...
Not a silly mistake! I just thought the code table was silly with voltage ranges for all but the 200/208V system voltages.

The drive manufacturer will have a recommended breaker size.
 
JoeStillman said:
Not a silly mistake! I just thought the code table was silly with voltage ranges for all but the 200/208V system voltages.

The drive manufacturer will have a recommended breaker size.
Click to expand...
How come the 150A CB feeds the 225A CB? isn't 225A breaker redundant and overkill?
 
How
JoeStillman said:
Not a silly mistake! I just thought the code table was silly with voltage ranges for all but the 200/208V system voltages.

The drive manufacturer will have a recommended breaker size.
Click to expand...
do you obtain motor FLC if it’s not in the code book chart for 208 volt systems? You obviously are not allowed to use motor name plate full load amperes?
 
ptonsparky said:
Table 430.250 includes 208 volt motors.
What am I missing?
Click to expand...

The note right at the top of the table...

The following values of full-load currents are typical for motors running at speeds usual for belted motors and motors with normal
torque characteristics. The voltages listed are rated motor voltages. The currents listed shall be permitted for system voltage ranges of
110 to 120, 220 to 240, 440 to 480, and 550 to 600 volts.
Click to expand...
 
I’m wondering if there is just a typo in that note, as in it should have read “200-240V” and someone bounced on the 2 key instead of the 0 key.

Also, 45 years in this business, I never noticed that before!
 
JoeStillman said:
Remember, the 2x-the-largest is for the maximum breaker size. It probably works well enough because the drive acts like a soft-starter. I would be suspicious of the wire size though. That system needs 3/0.

Are the two 15HP non-coincident? That gets you under 150A for the wire size.

BTW, your math uses the 208V amps for the 25 and 200V amps for the 15's. Fun fact; table 430.250 gives system voltage ranges for all the motor rated voltages - except 200V and 208V!
Click to expand...
Not sure why there is a 208 volt column as that is the nominal system voltage for equipment rated at 200 volts. All of the others are the utilization equipment voltages.
115 for a nominal 120 volt system
230 for a nominal 240 volt system
460 for a nominal 460 480 volt system.
Not completely sure about the 575, but believe that is the equipment voltage for a nominal 600 volt system.

edit to correct voltage
 
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don_resqcapt19 said:
Not sure why there is a 208 volt column as that is the nominal system voltage for equipment rated at 200 volts. All of the others are the utilization equipment voltages.
115 for a nominal 120 volt system
230 for a nominal 240 volt system
460 for a nominal 460 480 volt system.
Not completely sure about the 575, but believe that is the equipment voltage for a nominal 600 volt system.
Click to expand...
 
wwhitney said:
PI submitted!

Also, why are there columns for both 200V and 208V?

Cheers, Wayne
Click to expand...
Those columns were added in the 1990 code with the following substantiation.
Due to the common use of 200V and 208V motors in different areas of the country the readily accessible information in this table wou'Id be of benefit to electricians and inspectors alike.
Click to expand...
There were no comments on the change.

Prior to 1990 those voltages were addressed by a table note that said to add 10% and 15% to the 230 volt currents for 208 and 200 volt motors respectively.

It still appears to me that there should only be a 200 volt column as the is the utilization voltage for a nominal 208 volt system
 
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