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  1. #31
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    Quote Originally Posted by dionysius View Post
    I can clearly see MOPD is 40A for this 460V 3 phase compressor.

    Next I see Minimum Circuit Ampacity is 31.9A

    Two or more people told me that this is the load. I disagree.

    The load is Fan Motor FLA(1.0*2) + Compressor Motor RLA (7.3+10.3) =19.6A

    Who is correct????Attachment 20729

    I want to thank all for their valuable viewpoints.

    Let us get back on topic.

    This is all about a load schedule.

    First, these are super-smart units. Highly efficient variable speed motors. I have 11 units in a line up. If one unit drops out, redundancy management software causes failed unit's load to be picked up by the remaining ones.

    I say one 400A 3 phase panel will serve this monolithic family of outdoor units for one building.

    Why????

    Using the MCA approach:
    11*31.9*1.25 = 438.62 (Intuition alone says too high.....overshoot 400A by a measly 38.62A but does overshoot)

    Use the more reasonable approach:
    11*19.6*1.25 = 269.5A

    Does anyone think that a 400A service is undersized for this job and consequently force a 600A panel solution with increased expense in feeder and panel???? Please remember also that these are brand new latest technology split system units where the motors are never stopping/starting but are always running in medium to slow speed range under closed loop feedback control.

    Again keep your thoughts coming. This is the best site bar none.
    Last edited by dionysius; 07-10-18 at 02:03 AM.

  2. #32
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    Quote Originally Posted by dionysius View Post
    I want to thank all for their valuable viewpoints.

    Let us get back on topic.

    This is all about a load schedule.

    First, these are super-smart units. Highly efficient variable speed motors. I have 11 units in a line up. If one unit drops out, redundancy management software causes failed unit's load to be picked up by the remaining ones.

    I say one 400A 3 phase panel will serve this monolithic family of outdoor units for one building.

    Why????

    Using the MCA approach:
    11*31.9*1.25 = 438.62 (Intuition alone says too high.....overshoot 400A by a measly 38.62A but does overshoot)

    Use the more reasonable approach:
    11*19.6*1.25 = 269.5A

    Does anyone think that a 400A service is undersized for this job and consequently force a 600A panel solution with increased expense in feeder and panel???? Please remember also that these are brand new latest technology split system units where the motors are never stopping/starting but are always running in medium to slow speed range under closed loop feedback control.

    Again keep your thoughts coming. This is the best site bar none.
    The 31.9 already has the 1.25 added to it. I can not see any scenario where 400 amp service would be undersized. The compressors are DC inverter powered and will never reach overload amps all at one time. The odds of just 2 compressors drawing overload amps at the same time is astronomical.

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    Quote Originally Posted by kwired View Post
    I see how you arrived at that. I don't know if I agree with that being how it is determined.

    What doesn't make sense is if you have 156 or 170% motor overload protection, didn't you essentially increase the RLA that can be seen without tripping anything whether the windings can take that extra heat or not?

    Kind of like taking a 100 watt incandescent lamp but increasing voltage from 120 to 150 to get more light out of it. Will work but if you don't do something to manage the extra heat - it won't last as long as it would at rated input.
    Generally compressor manufacturers design their compressors at 1.44. This is a higher amp draw than the UL required 1.56 condenser manufacturers use. This gives the compressors a cushion to operate at "worst case" normal operation.


    Heat inside these compressors is removed by the refrigerant flowing inside. Heat will trip the overloads at around 302 degrees.

  4. #34
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    Quote Originally Posted by topgone View Post
    I guess that's how HVAC compressors are made-->the compressor motor is lubricated by oil and the lube oil removes heat at the same time.
    Quote Originally Posted by Mgraw View Post
    Generally compressor manufacturers design their compressors at 1.44. This is a higher amp draw than the UL required 1.56 condenser manufacturers use. This gives the compressors a cushion to operate at "worst case" normal operation.


    Heat inside these compressors is removed by the refrigerant flowing inside. Heat will trip the overloads at around 302 degrees.
    But what is the RLA based on? What I am getting at is if you have a motor design that naturally gets more cooling then usual, is the stated FLA/RLA the usual amount or is it the new rating you get because of improved cooling?

    I see crop drying fan motors all the time that are oddball HP rated or are in a small frame package for the HP rating - this because they are "Air over" rated. When determining conductor size or overcurrent devices for them you use the nameplate amp rating not 156% of it. The mentioned 156% rating is what an internal overload is allowed to be set at - and is information for the manufacturer more so then field installers. Field replacement would be where someone other then manufacturer is possibly selecting the device.
    I live for today, I'm just a day behind.

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    Quote Originally Posted by dionysius View Post
    I want to thank all for their valuable viewpoints.

    Let us get back on topic.

    This is all about a load schedule.

    First, these are super-smart units. Highly efficient variable speed motors. I have 11 units in a line up. If one unit drops out, redundancy management software causes failed unit's load to be picked up by the remaining ones.

    I say one 400A 3 phase panel will serve this monolithic family of outdoor units for one building.

    Why????

    Using the MCA approach:
    11*31.9*1.25 = 438.62 (Intuition alone says too high.....overshoot 400A by a measly 38.62A but does overshoot)

    Use the more reasonable approach:
    11*19.6*1.25 = 269.5A

    Does anyone think that a 400A service is undersized for this job and consequently force a 600A panel solution with increased expense in feeder and panel???? Please remember also that these are brand new latest technology split system units where the motors are never stopping/starting but are always running in medium to slow speed range under closed loop feedback control.

    Again keep your thoughts coming. This is the best site bar none.
    I have installed such systems, not a Daiken but Trane/American Standard. Place I have one is a school, we have 9 compressor units there, each with two compressors in one unit. They are not all identical, the size of units varies a little. This setup has one master controller, but is essentially three separate systems in the refrigeration circuits.

    Most of the time the compressor units are running at a low speed. They do increase on summer days when cooling demand increases, but still will vary speed at times as demand is constantly changing as each room will switch the refrigerant flow rates as needed to maintain individual setpoint. Same for heating season - but the colder it gets outside the faster the compressors typically run, but at same time there is less heat available to move in this condition so they don't reach full load rating even when running at full speed.
    I live for today, I'm just a day behind.

  6. #36
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    Quote Originally Posted by kwired View Post
    But what is the RLA based on? What I am getting at is if you have a motor design that naturally gets more cooling then usual, is the stated FLA/RLA the usual amount or is it the new rating you get because of improved cooling?

    I see crop drying fan motors all the time that are oddball HP rated or are in a small frame package for the HP rating - this because they are "Air over" rated. When determining conductor size or overcurrent devices for them you use the nameplate amp rating not 156% of it. The mentioned 156% rating is what an internal overload is allowed to be set at - and is information for the manufacturer more so then field installers. Field replacement would be where someone other then manufacturer is possibly selecting the device.
    RLA is just a number for everybody but the unit manufacturer. When a manufacturers design a condensing unit they are required to do specific test and calculations to determine "worst case" normal operation amp draw. They have to do things like restrict air flow to the coil, calculate amp draw on a compressor with worn bearing etc. This gives them a "worst case" amp draw. This "worst case" amp draw can not be higher than the RLA of the compressor.

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    Quote Originally Posted by Mgraw View Post
    RLA is just a number for everybody but the unit manufacturer. When a manufacturers design a condensing unit they are required to do specific test and calculations to determine "worst case" normal operation amp draw. They have to do things like restrict air flow to the coil, calculate amp draw on a compressor with worn bearing etc. This gives them a "worst case" amp draw. This "worst case" amp draw can not be higher than the RLA of the compressor.
    Kind of my point here, RLA is what the unit is designed for (worst case) so why would one add an additional 56 or 70% to it when selecting overload protection? That would just allow it to overload even more and cause damage in the process. No point in considering it for MCA either - the thing isn't rated to carry that kind of current. 125% of largest load is reasonable enough cushion for minor overloading and is there for sake of conductors not so much for the equipment supplied.
    I live for today, I'm just a day behind.

  8. #38
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    Quote Originally Posted by kwired View Post
    Kind of my point here, RLA is what the unit is designed for (worst case) so why would one add an additional 56 or 70% to it when selecting overload protection? That would just allow it to overload even more and cause damage in the process. No point in considering it for MCA either - the thing isn't rated to carry that kind of current. 125% of largest load is reasonable enough cushion for minor overloading and is there for sake of conductors not so much for the equipment supplied.
    IMHO The standards that compressors are manufactured to and the standards units are manufactured to have not kept up with technology. When the standards were changed to require RLA instead of FLA 40+ years ago most units had a compressor, a fan motor, and a contactor. Unit manufacturers would push these compressors to the limit. The change to RLA helped consumers and compressor manufacturers by prolonging compressor life.

    Some manufacturers of inverter units now give a recommended fuse size which is lower than the MOCP. This is to get around the required MOCP calculation.

  9. #39
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    Quote Originally Posted by Mgraw View Post
    IMHO The standards that compressors are manufactured to and the standards units are manufactured to have not kept up with technology. When the standards were changed to require RLA instead of FLA 40+ years ago most units had a compressor, a fan motor, and a contactor. Unit manufacturers would push these compressors to the limit. The change to RLA helped consumers and compressor manufacturers by prolonging compressor life.

    Some manufacturers of inverter units now give a recommended fuse size which is lower than the MOCP. This is to get around the required MOCP calculation.
    I still not seeing whatever it is you are getting at.

    If a compressor is rated for 18 amps then it is rated for 18 amps. If you put the same windings in some other unit that doesn't dissipate heat the same way then that assembly may have a lower amp rating even though it still has the same set of windings in it.

    This is no different than having a 7.5 hp totally enclosed motor for general purpose duty, but then taking same components, putting them in a open ventilated enclosure and labeling it as "Air over" and giving it a 10 or even 12 hp rating. It has a new FLA when used in the second application. You get a different overall rating out of same base components - but it is a result of managing heat dissipation and a different overall design even though some components are still the same.

    A compressor is going to be designed to move a certain amount of refrigerant at a certain pressure and rate. If you find a way to get more work out of same components without creating excessive heat, then you have a new rating with that particular assembly. You don't just say it can do 156% of it's rating.

    Bottom line is the compressors are not rated in HP, and not even rated in FLA or RLA, they are rated in BTU. HP, FLA or RLA is whatever it is when the unit is producing it's rated BTU output, and for RLA also factors in running at rated voltage. Properly designed/functioning refrigeration system will not exceed this level. As the electrician we don't care so much about the refrigeration system, but see that MCA on the nameplate and know that a conductor sized to that ampacity will be sufficient. If manufacturer wants to play games on arriving at that MCA I guess that is up to them. Most units I have ever paid attention to though it usually comes up really close to being 125% of largest compressor (RLA) plus all other loads.

    IDK but from my observations maybe possible that years ago when refrigerant was cheap I think you may have seen more overcharging of systems, which likely resulted in higher operating pressures and more load on the compressors - also less efficiency. That could have resulted in more systems being loaded to the limit rather than at a peak efficiency which seems to be the way they are more like today. They are much more careful today about testing systems for leaks, in the past they would just give it some extra refrigerant so it will last longer until it needed more. The R12 and R22 systems of the past also ran at lower overall pressures then the refrigerants used today, you can't get away with overcharging or improper sizing of components as easily on these new systems as you did on the old systems from my understanding.
    I live for today, I'm just a day behind.

  10. #40
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    My point is back in the 'old days' it was common to charge a unit to FLA. It was also common to see a lot of compressors with burned windings. Units were super no efficiency and tripping overloads were common. Compressor manufacturers were seeing too many compressors failing in the first year. (their warranty period) Switching to RLA decreased the rating of the compressor but also prolonged the life of the compressor.


    While you are correct that some compressors are rated in btu's many are designed to use multiple refrigerants and operate at different pressures and btu's.

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