MOPD - how much lower can i go

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DdaveM

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New York
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HVAC Contractor
I am fitting a new control package (KE2-Therm Evap efficiency) to a condensing unit. 9hp unit, Compressor RLA of 33.2 with two compressor fans which together total a FLA of 5.4. (the spec sheet for the unit says MCA 46.9 MOPD 80A) the question is, can I use a 63A C or D curve breaker ahead of the compressor /fan contactor so I can use a comb busbar for all the breaker distribution? (SE Multi9 series only goes to 63A in 9mm pitch)
 
Wow. So they are seeking ever more ways to complicate what should be simple with layers of solid state and wireless.....
So this is a NON-OEM retrofit and you are saying you have limitations on your chosen re-wire of the line voltage side of the unit at 63A?
I would have to ask what is the current breaker rating feeding the condensing unit and what kind of inrush as captured instant does the compressor draw on starting onder the heaviest load of the year? There are all kinds of terminal blocks and DIN rail mount breakers out there to do things cleanly. You are taking about adding circuit breakers into the unit control panel that were not previously there, correct?
I remember when retrofits did not take an encyclopedia of a manual to configure and get up and running.
 
StarCat said:
Wow. So they are seeking ever more ways to complicate what should be simple with layers of solid state and wireless.....
So this is a NON-OEM retrofit and you are saying you have limitations on your chosen re-wire of the line voltage side of the unit at 63A?
I would have to ask what is the current breaker rating feeding the condensing unit and what kind of inrush as captured instant does the compressor draw on starting onder the heaviest load of the year? There are all kinds of terminal blocks and DIN rail mount breakers out there to do things cleanly. You are taking about adding circuit breakers into the unit control panel that were not previously there, correct?
I remember when retrofits did not take an encyclopedia of a manual to configure and get up and running.
Click to expand...
Might not be the easiest to install, but sure gives you about all you could ask for as far as oversight and email alarm notification etc which is what my customer wants. Currently the system is fed from the main dist. panel with a 80A breaker. I am adding breakers into the control panel that were not there previously as the evaporator control (defrost heaters and fans) is moving from the evaporator itself (Beacon II control unit) to the condensing unit panel. I understand that this is probably not required but the customer would like them. The LRA on the compressor is 215A. I can use a 80A breaker ahead of the compressor contractor, but it would make a cleaner install if I can use a 63A (as above that voltage is a different product line that won't work as well as an 80A breaker would). Just wondering if that would give nuisance trips or not...
 
"C or D curve" and 63A implies IEC MCBs, which would not qualify here in North America as OCPDs all by themselves. Those are UL1077 listed breakers, so they need to have another legal OCPD ahead of them, either a UL489 listed circuit breaker or fuses. That makes them essentially pointless to use.
 
so with one 80A UL489 MCB on the main feed inside my panel I can then feed the control circuit etc with UL1077 MCBs because I have the UL489 upstream?
 
all the control wiring will be in the condenser panel - easier for maint. and trouble shooting etc. (and warmer).
 
DdaveM said:
so with one 80A UL489 MCB on the main feed inside my panel I can then feed the control circuit etc with UL1077 MCBs because I have the UL489 upstream?
Click to expand...
Probably not. UL1077 devices cannot be used as feeder or branch circuit protective devices. So your 80A upstream breaker would only qualify as a Branch Circuit Protective Device is the ENTIRE branch circuit was rated for 80A, i.e. the wire to the end devices. The simpler way to think of how UL1077 devices can be used is to just NOT use them for any circuit that leaves your panel, because those MUST be either Feeders or Branches.

I'm rescinding my earlier statement though that IEC style MCBs are not UL 489 listed AND have C or D trip curves; SOME of them are (including the Schneider one posted earlier) and those would be fine. Still, another aspect to be wary of is their Interrupt Rating. In some cases, they are only rated for 5kAIC, maybe 10kAIC (although that's the same as the QOU breakers mentioned earlier). Plus, if you are talking about 480V, ALL of the ones rated for 480V are always 480/277V rated (called "slash rated"), so you CANNOT use them on 480V delta systems. If your application is 240V this is irrelevant of course.
 
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