Lets give this one more shot at getting to the issue, because most of the discussion has missed it I think. This is a very rear situation that is being discussed between an AHJ and a utility representative. I was asked to give my view as a code panel member and based on my experience, and because the utility didn't agree with the AHJ's interpretation. The utility doesn't agree with my view either, so the discussion broadened in scope into this forum.
To Smart $, no, I didnt' miss that important aspect of the calculated load being reduced. That's exactly the issue; the calculated load is still the same, but the utility is requesting that the overload portion of the breaker be set below the calculated load - way below.
pfalcon's comment that this is a "nuisance" trip is, in my opinion, incorrect. I hear around the country talk about nuisance tripping, usually in reference to AFCI or GFCI devices. That is in the context of a device opening a circuit when it's not supposed to or not designed to. If you set the breaker for a long time trip of 1200 amps, and it opens at 1200 amps, it's doing what you wanted it to do. That not a nuisance trip, that's functioning as it is intended. What we're talking about here is setting a device well below the normal expected load of the building that is operating without any problems. Your reference to a faster trip being safer in the event of a fault is on the money, but not the issue; we are dealing with the long time setting that picks up on overload.
Remember, every overcurrent device in question, protects from three different, distinct problems: overload, short circuit, and ground fault. Because this is a 480 volt, solidly grounded wye system, equipment ground fault protection is also provided (230.95) to protect from high impedance arcing faults that the device might see as just a load and not otherwise respond to. The short circuit, ground fault, and arcing fault are not at issue here. Certainly a device that clears these faults quickly is preferred over one that doesn't act as quickly. But at fault levels, the OCPD is into the instantaneous range. I think instantaneous means pretty quick.
In the case of our adjustable trip breaker, turning the long time setting down does not affect the instantaneous trip. That can be set to pick up at various levels, but once it sees that level of fault, it will trip. So yes, there are different levels of fault clearing times based on different fuse types, and different breaker settings, and they will be able to limit peak let through currents in the event of a short circuit or ground fault, protecting downstream equipment better than a slower acting device. But the relative speed here is fractions of a second/fractions of a cycle. Remember, our breaker has the same internal components regardless of the trip setting, meaning that the opening time, once the threshold is met, will not change: the contacts have a fixed mass, the spring has a fixed tension, and the design of a modern breaker is to route the fault current so the magnetic field helps to blow the contacts apart. But the time that takes will not change, regardless of the trip setting, so adjusting the long time trip will not make the installation any safer or less safe from a short circuit or ground fault; the same amount of time will elapse once the fault threshold is met.
So that leaves just the long time setting to consider. That protects against overload, defined as operation of equipment in excess of normal, full load rating. If the equipment is all rated for the full load, as calculated per Article 220, and will operate just fine at that load, but you set the overload device to trip at 2/3 of that load, what have you accomplished? The fault current interruption hasn't changed, but you can't operate the load as intended. As an example, lets look at a motor circuit. You can provide short circuit and ground fault protection at the source of the circuit to protect the wire and equipment from those excessive currents. Then you can provide overload protection at or near the motor, to protect the motor from damaging overload. Lets say the motor must handle a varying load, like a conveyor belt that has different amounts of material at different times, but it is designed to draw a maximum of 10 amps at full design load, and the circuit is properly sized to handle that load. Then someone says, lower is better, so lets install overload protection (heaters) at 7 amps. The system operates great when there is less material being conveyed, but when you're really producing, the system shuts down, even though the motor was drawing just 9 amps and is operating as you wanted. I guess this is just an inconvenience, and the solution is to never operate the system at over 2/3 of the capacity safely designed in.
In essence, that's the situation under discussion. The contention is that a disorderly, unintended, unexpected shutdown of an entire building is not a safety concern and not addressed in the NEC. But the NEC, and the people who make up various code panels are increasingly requiring selelective coordination in buildings to isolate a faulted or overloaded circuit or feeder to make sure the entire building doesn't suddenly go dark. Articles 620, 700, 720, 708all contain requirements for selective coordination, and others are under consideration. So, obviously the code panel members consider a building suddenly and unexpectedly shutting down to be a safety issue and not just an inconvenience or it wouldn't be part of the NEC. As a member of CMP-1, it is my opinion that 90.1 indeed should be interpreted to mean that a whole building shutdown should only occur to protect from a short circuit, ground fault, or dangerous overload. The system in question protects from the first two problems regardless of the long time setting of the adjustable trip breaker.
Now, as for protection from overload: all of the equipment has been selected, sized and installed to safely handle the full calculated load of the building indefinitely with no adverse effects. So, by selecting a trip setting below the calculated load, I ask again, what additional safety are we gaining? As for the ideal of setting it just below what has been called the "nuisance" trip level (and what I call the unnecessary and dangerous building shut down under normal conditions), I agree. And since we can't know exactly what the load will be once the building is fully operational, we need to make a decision based on the best available information. In my opinion, that would be the calculation based on Article 220 of the NEC. While certainly not perfect, it has been used, reviewed, challenged, and revised over the 115 years the National Electrical Code has been in existence, and represents the best we have to judge where that threshold should be. And while I have said many times that the calculations are probably somewhat conservative, I Don't think it could reasonably be argued that it is wrong by 33%.
So, here are the basics of this real life situation as I understand them, in round numbers:
- Extensively remodeled commercial space.
- Existing service gear is rated 3000 amperes, 480v, 3 phase, 3000 ampere rated conductors, with a 3000 ampere adjustable trip breaker as the single main disconnect. The breaker has provisions for covering and sealing the trip adjustment and the AHJ has accepted this as being limited access per 240.6(C)
- The calculated load is in the range of 1800 amperes
- The customer requested a 2000kva utility transformer (2400 FLA - provides capacity for future expansion)
- The utility initially agreed to provide a 1500kva transformer (1800 FLA - capacity to serve the calculated load: no additional capcaity)
- The utility installed a 1000kva transformer (1200 full load amperes, 2/3 of the calculated load of the building)
- The utility engineer says the customer needs to have the trip set at 1200 amperes in order to coordinate with the utility transformer.
- The AHJ says the trip cannot be lower than the calculated load per 230.79.
- The utility rep says that 230.79 refers to the rating of the equipment/breaker frame size, and not the trip setting, and that the overcurrent protection only needs to be sized to protect the equipment/conductors, and that the ?current rating? in 240.6 is different than the ?equipment rating? referenced in 230.79, and that the lower trip setting offers better protection. He also says that having the building go dark because the main tripped even though there are no electrical problems and the building is operating as designed is not a concern of the NEC and is only a convenience issue, not a safety issue.
- The AHJ says that a sudden, unplanned power outage poses a safety concern and is an NEC issue, and the main device can not be set to trip at less than the calculated load.
OK, that's about it. I'm done. Have at it. Cheers!
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