ISOLATOR RATING (according to NFPA 79)

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69FIREBIRD

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Hi, I know this might be not quite the place to ask a question like this but I need to confirm how I should rate an isolator for machinery being shipped to the U.S. You guys in the U.S may be able to shed some light on this.

According to NFPA 79 5.3.3.1(6)(b) "where rated in horsepower the horsepower rating shall be at least equal to that which is defined by table 430.251(b) of NFPA 70 NEC for a locked rotor current equal to the largest sum resulting from the locked rotor currents of any combination of motors that can be started simultaneously and the full load current of the remaining motor and non motor loads that can be operated at that time."

So does that mean that my 40HP (45.5A @ 460V) motor (the only motor/load) according to table 430.251(B) having a locked rotor current of 290A @ 460V will require the main isolator to be rated at 290A @ 460V?
 
No, they are not referring to the continuous current carrying capacity. Disconnect switches have several ratings based on different operating conditions. The one this is referring to is the worst case, that of opening a live inductive motor load circuit, know as the "breaking capacity", when the motor is in a locked rotor condition. The basic specs will only show you the current rating and maybe the HP rating. You will need access to more detailed spec sheets to see what the breaking capacity is. In 99% of cases that HP rating will cover the circumstance of breaking a motor in locked rotor condition. This code reference is, however, providing caution to you that what they are really interested in is the ability to break YOUR motor, not a generalized test version. In other words, the switch mfr will have picked an average 40HP motor, but if your motor is above average, the onus is on you to make sure you have the right switch.

But if you have a switch rated for 60HP and you are using it on a 40HP motor, chances are probably 99.99999% that the LRA of that "above average" 40HP motor is still lower than an average 60HP motor. So in essence, the code is saying "When in doubt, oversize".
 
Hi Guys, I have had a look and posted article on Mike Halts forum as well. First response is that the isolator ("local disconnect" mounted in the control panel for the entire machine) needs to have a "breaking capacity" / "amps interrupting" rating to cover isolation during a locked rotor period and that by design the HP rating will take this into account as it will be designed to cover a disconnect at locked rotor currents. so i think I'm onto it. If I rate the isolator as per the regs with respect to HP's of the motors then the "breaking capacity" / "amps interrupting" should be covered for their locked rotor currents.

Sound right?
 
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