NEC requirement

[Babu Hirapara]

I have a clean room AHU designed with 8KW heater and 10HP fan motor on VFD set at 70%.
208V, 3PH, 8 KW heater wired to 30A, 3P circuit breaker and 208V, 3PH, 10HP motor is wired to separate 30A, 3P circuit breaker in panel. Panel is feed from normal/emergency generator power thru Automatic Transfer Switch.
At Normal Power load, AHU runs fine with heater and blower motor works without tripping circuit breaker but when normal power is out and emergency generator starts thru ATS, BOTH 30A, 3P circuit breakers for heater and blower motor trips and AHU goes completely off.
After checking load on generator panel, we found total load of 40 KW on panel and size of Generator is 40 KW.
Is it possible due to overload on generator, high AMP makes VOLTAGE drop on generator which overloads BOTH circuit breakers on AHU and trips both circuit breakers?
Is there any NEC requirements to correct this problem?

 

[steve66]

The breaker for the motor seems to be too small. My chart shows a 60 A for a 10 HP motor.

Your explination would also work for the motor (voltage drop causes higher current for a motor trying to start), but would not explain the heater breaker tripping.

I'm not sure why the heater breaker is tripping.

Steve

 

[aaatraker]

Curious, if on house power and you turn off main breaker to unit, then turn it back on do the breakers trip? What about adjusting the time delay on the breakers? Is the gen backing up just that unit or a panel with other equipment? Have you hooked up test equipment to see what voltage and amps are happening at gen and AHU? Wire size? Design flaw? Interesting problem, good ones always are.


kurt












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[nakulak]

I could be mistaken, but table 430.250 shows 30.8 amps flc for the 10 hp, which puts the breaker size at 40 min ? (not sure is this applies to the vfd - what does the literature say ?) could it be that the breaker for the fan is blowing, then the heater element is overheating and blowing ? is this an optional standby generator ? why is it sized too small ?