this is concerning an old thread by gogod nov 22, and i know its thanksgiving but i need to understand a few basics which i have forgotten. The question was about a rooftop unit 208/230, min circuit amps 28,max rated amps 2304, andmax overcurrent protection 45 amps.now my questions 208/230 means this unit is geared for either 208v or230v yes or no if yes doesnt the amp needs change with one or the other voltages? 2nd what exactly does min circuit amps mean? and why would the max rated amps be higher than min circuit amps and what does it mean? And why do i need overcurrent pritection at a disconnect when my circuit breaker at the panel already protects this circuit? and why is the max overcurrent protection much higher than max rated amps. and why would the load NEVER INCREASE beyond about 25 amps, ive installed disconnects for ac units outside with what seems to be a breaker but is it really?thank you and happy thanks giving.
disconnect sizing
- Thread starter bond
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Re: disconnect sizing
Happy Thanksgiving Bond you asked a boatload of questions but I will take swing at a few in between todays chores.
There is not a maximum circuit amp rating, there is a maximum overcurrent protection rating and sometimes a specification to use fuses, HCAR breakers.
The MCA includes 25% 'extra' above the running load.
How would the load increase unless someone connects more equipment to the circuit.
The units have internal thermal overloads if the compressor starts to fail and draws more than the rating the thermals will cut out and the circuit will not be overloaded.
Bob
[ November 25, 2004, 12:48 PM: Message edited by: iwire ]
Happy Thanksgiving Bond you asked a boatload of questions but I will take swing at a few in between todays chores.
Yes the unit is acceptable for any system voltage between 208 and 230.
Most times the amp ratings will be listed separately for each voltage.
The minimum circuit amps (MCA) is the ampacity the conductors for the branch circuit need to rated for.
There is not a maximum circuit amp rating, there is a maximum overcurrent protection rating and sometimes a specification to use fuses, HCAR breakers.
You don't, the OCP (over current protection) at the panel can do the job...unless the unit tag says you must use fuses and the panel inside is breakers. In that case you have to get fuses in the circuit and the easiest way is a fused disco at the unit.
The MCA is roughly the running load, the MOCP takes into account starting loads.
When I said the load would never increase to more than about 25 amps I meant that for a unit marked with a MCA of about 28 amps.
The MCA includes 25% 'extra' above the running load.
How would the load increase unless someone connects more equipment to the circuit.
The units have internal thermal overloads if the compressor starts to fail and draws more than the rating the thermals will cut out and the circuit will not be overloaded.
Bob
[ November 25, 2004, 12:48 PM: Message edited by: iwire ]
thunder15j
Member
- Location
- Cali
Re: disconnect sizing
Bob, thanks for the info regarding minimum circuit ampacity. I've assumed that MCA included the 25% added for continuos duty rating but never could get a response from the AC contractors.
The MCA is a number that can be used for sizing out non-fusible disconnects, but when fuses are used, one might have to jump up in disconnect size to accomodate fuses large enough to take the starting load. We've been on jobs where the mechanical contr. asks for fused disconnects, but we try to avoid fuses because of costs of sometimes larger disconnects and nuisance service calls on fuse failure, especially on rooftops.
Bob, thanks for the info regarding minimum circuit ampacity. I've assumed that MCA included the 25% added for continuos duty rating but never could get a response from the AC contractors.
The MCA is a number that can be used for sizing out non-fusible disconnects, but when fuses are used, one might have to jump up in disconnect size to accomodate fuses large enough to take the starting load. We've been on jobs where the mechanical contr. asks for fused disconnects, but we try to avoid fuses because of costs of sometimes larger disconnects
and nuisance service calls on fuse failure, especially on rooftops.Re: disconnect sizing
Thank you Bob for the info and more clear now but i think it would be a code violation if i put a forty amp breaker for #10 thhn ,it does not protect my wire very well does it ? secondly i dont care who,how or why but I myself would never dream of putting a fuseless disconect on a unit whether on the roof or anywhere else because on such a long run back to a breaker arent we increasing impedence and increasing the ampacity in case of a fault so far away from the breaker?Isnt that why we put fused disconnects close to the unit ? and arent all breakers these days HACR rated so there is no nuisance tripping? curious to here your response,not saying im right, still learning, thanks
Thank you Bob for the info and more clear now but i think it would be a code violation if i put a forty amp breaker for #10 thhn ,it does not protect my wire very well does it ? secondly i dont care who,how or why but I myself would never dream of putting a fuseless disconect on a unit whether on the roof or anywhere else because on such a long run back to a breaker arent we increasing impedence and increasing the ampacity in case of a fault so far away from the breaker?Isnt that why we put fused disconnects close to the unit ? and arent all breakers these days HACR rated so there is no nuisance tripping? curious to here your response,not saying im right, still learning, thanks
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- Massachusetts
Re: disconnect sizing
Take a look at 2002 NEC 240.4(G)
but many people use non fused disconnects for this every day.
Yes the impedance increases but remember the breaker back at the panel is only there for short circuits and ground faults, it will take a heck of a long run before impedance becomes an issue under short circuit or ground fault conditions.
Please check out the following thread, read BPHGravity's and Charlie's posts carefully, BPHGravity provides some more code references to this subject.
feeder to heating and air package unit
Bob
[ November 26, 2004, 02:57 PM: Message edited by: iwire ]
No it is not a code violation, that I am 100% sure of.
Take a look at 2002 NEC 240.4(G)
I can not post table 240.4(G) but if you look at it one of the sections in the table is Air-conditioning and refrigeration equipment circuit conductors 440, Parts III, VI.
Yes and no, it protects the wire against short circuits and ground faults, it does not protect the wire against overload, it is not required to for this application. The thermal overloads in the HVAC unit itself protect the conductors against overloads.
You are free to make that choice
but many people use non fused disconnects for this every day.Yes the impedance increases but remember the breaker back at the panel is only there for short circuits and ground faults, it will take a heck of a long run before impedance becomes an issue under short circuit or ground fault conditions.
Yes I think you are right but regardless of that if the units tag only says maximum fuse size you must use a fuse, an HACR breaker will not meet code for a unit only marked with max fuse size.
Please check out the following thread, read BPHGravity's and Charlie's posts carefully, BPHGravity provides some more code references to this subject.
feeder to heating and air package unit
Bob
[ November 26, 2004, 02:57 PM: Message edited by: iwire ]
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