I have been trying to find some examples on disconnect calculation and selection by the CEC, a few years out of school and after a year in Europe I am rather rusty.
CEC 28-714 outlines selection of disconnects for refridgeration compressors a little differently than motors in general which are addressed in 28-612. I would like to first consider refridgeration compressors but discussion of motor disconnects is welcome as well.
Most roof top units will have some fans in addition to a compressor, so I am more interested in 28-714 (2) for one or more motor-compressors together with other loads. I seem to understand from the rule that, for a 40A RLA (which appears to be "rated load amps" often misinterperated as "running load amps") with 3 1-1/2HP fans with FLAs of 2.8A, we should have 40x1.15+3x2.8=54.4A correct?
For the interrupting capacity, take the LRA of the largest motor-compressor, say, 200A, and add the remaining FLAs, so 200+3x2.8=208.4A, correct?
The refridgeration technician installing the system said he would normally he would feed those RTUs with a 70A CB and wire and use a 60A disconnect, and looks like that would be just fine. The prints called for 90A CBs and 90A wire, and he said in that case, he wasn't going to tell me to put a 60A disconnect on a 90A circuit. That is a principal of the code, if 60 amps is exceeded, the upstream circuit breaker will not prevent the disconnect from being overloaded, but in theory, the RTUs will only over amp so much before it's own protection opens to prevent it from destroying itself. And even the other tech uses a 70A CB with a 60A disconnect, +10A is ok, but not +30? I also know the code allows wire size to be reduced a certain amount below the ampacity of the CB for a maximum 7.5m distance from a t-stat, because the load is fixed and doesn't change, an example where the CB does not strictly require everything downstream to match it in ampacity.
One other issue I have is the interrupting capacity. If there are 2 or more motor compressors, we only appear to be required to take the LRA of one, and add the FLA of the other loads. First, if there are more motor-compressors, it will be RLA, and FLA will not be available. Second, if they start together, and someone opens the disconnect at the moment where they happen to be starting, simultaneously, which is not impossible, the disconnect will likely be called upon to interrupt more than the current calculated by this method, would it not?
CEC 28-714 outlines selection of disconnects for refridgeration compressors a little differently than motors in general which are addressed in 28-612. I would like to first consider refridgeration compressors but discussion of motor disconnects is welcome as well.
Most roof top units will have some fans in addition to a compressor, so I am more interested in 28-714 (2) for one or more motor-compressors together with other loads. I seem to understand from the rule that, for a 40A RLA (which appears to be "rated load amps" often misinterperated as "running load amps") with 3 1-1/2HP fans with FLAs of 2.8A, we should have 40x1.15+3x2.8=54.4A correct?
For the interrupting capacity, take the LRA of the largest motor-compressor, say, 200A, and add the remaining FLAs, so 200+3x2.8=208.4A, correct?
The refridgeration technician installing the system said he would normally he would feed those RTUs with a 70A CB and wire and use a 60A disconnect, and looks like that would be just fine. The prints called for 90A CBs and 90A wire, and he said in that case, he wasn't going to tell me to put a 60A disconnect on a 90A circuit. That is a principal of the code, if 60 amps is exceeded, the upstream circuit breaker will not prevent the disconnect from being overloaded, but in theory, the RTUs will only over amp so much before it's own protection opens to prevent it from destroying itself. And even the other tech uses a 70A CB with a 60A disconnect, +10A is ok, but not +30? I also know the code allows wire size to be reduced a certain amount below the ampacity of the CB for a maximum 7.5m distance from a t-stat, because the load is fixed and doesn't change, an example where the CB does not strictly require everything downstream to match it in ampacity.
One other issue I have is the interrupting capacity. If there are 2 or more motor compressors, we only appear to be required to take the LRA of one, and add the FLA of the other loads. First, if there are more motor-compressors, it will be RLA, and FLA will not be available. Second, if they start together, and someone opens the disconnect at the moment where they happen to be starting, simultaneously, which is not impossible, the disconnect will likely be called upon to interrupt more than the current calculated by this method, would it not?