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  1. #14
    Join Date
    Aug 2004
    North Richland Hills, Texas
    60A max, MCA of 38 means that your 40 amp service is "ok".
    I prefere to go with the maximum rated size whenever I can though.
    HVAC equipment is hard on breakers due to the high inrush current of the compressor. The 40A breaker is designed to handle that, but the breaker is much more likely to fail a few years down the road than a 60A breaker would be.
    If more government is the answer, then it's a really stupid question.

  2. #15
    Join Date
    Jan 2002

    Site -Rules~~~BOO~BOO

    No, it means that you should install a 60 amp (2)pole breaker at the main panel,then have 40 amp fuss disconnect at the condenser. That is if you want to do it according to the manufacture...

    This will protect the compressor from over amp, and you will be able to find shorted problems at the condenser before over (amp) occurs.
    'Life begins with the journey each day'

  3. #16
    RSmith> "hot out" is relative, design is db\wb two definite parameters, in my area those are usually 95\73. We actually reach those about twice a year for a matter of hours not days. The problem with the unit running all day and never satisfying can be due to a lot of things including undersized but more than likely the problem will be something else with airflow being the most likely,apparently the two service techs came to the same conclusion that the unit was to small. If they had said that they were seeing a higher than normal suction pressure and that the compressor has lost capacity then it would be more beleivable that the new unit is going to do fine, but they can not possibly be accurate with their diagnosis if the load calc was right also. Jan did not specify which type of days this happens on so I'm assuming that it is a regular occurence on warmer days, not just the two days a year that it runs at design conditions. There is more than likely an RH problem then too because theoretically an overloaded system will have a high DA and not be able to do very much dehum.

    On another note from what Jan wrote it sounds as though the Trane guys load calc was done based upon existing ducts, and not necessarily the heat load on the house. Why was he measuring the ducts to determine the unit size. Don't you do the load calc first then see if the ducts are compatible ? Did he do the calc right then and there or did he do it and then come back to measure ? just curious of the method.

  4. #17
    Join Date
    Oct 2004
    Oh no I wasn't referring to you or your guy. I was just making a statement in general. Sorry for the mix up my good man.
    Saddle Up!

  5. #18
    Fat eddy: This is a normal occurance on very hot days above say >92 degrees. Thermostat set on 75. Have been here 18 years and did not have a problem until after losing the shade trees. Would only have the problem before if outside temp got above 96-98 degrees which did not occur but a couple of times during the summer.

    The current system seems to do a good job DH and there is always a constant trickle of water out of the condensate drain.

    I will have to take back what I said about the Trane guy measuring the ducts. I do remember him mesuring the size of the main supply ducts leaving the furnace but not any branch sizes or lenghts of runs.

    He did not do the calculation right then but came back a few days later and showed me the figures but did not leave them with me to study.

    I had one guy come out but have not heard from him yet. He asked me everything for his calculation. He mesured the outside of the house then the inside rooms, measured windows and asked if they were low E glass, which side of house faced west, what R value inslation in walls and ceiling, what size and type of exterior doors, height of vaulted ceiling, how high walk out basement was below gound. Hope to hear from him.

    It's alot of money and I'm very thankful for everyones help with this and don't want to throw in a new system if it's not going to correct the problem.

  6. #19
    -80guru: Sorry, I didn't mean to jump on you. I'm just a little stressed over this.

  7. #20
    Join Date
    Apr 2004
    I suppose ther is alot of guessing but until your guy does a capcity check of your current system you can guess all you want as to why the reason is. If you currenly have a 4 ton system and you are only getting 38,000 btuhs of total cooling out of it, then the system has a problem, if you are getting a close to nominal 48,000 then your system may indeed be undersized.

    Manual J8, unfortunately doesnt account for trees, it does for whatever reason care if your bug screens are inside or outside of your windows, go figure.

  8. #21
    Join Date
    Feb 2005
    small island in the Pacific Ocean
    question, (still learning) if max is 60 and MCA is 38, can you not multiply 38 * 125% = 47.5 and go with a 50 amp breaker if such is available (i don't think they are)

  9. #22
    Join Date
    May 2001
    Kansas City
    Originally posted by phosgene
    question, (still learning) if max is 60 and MCA is 38, can you not multiply 38 * 125% = 47.5 and go with a 50 amp breaker if such is available (i don't think they are)
    You can use a 40 50 or 60 Amp breaker, whatever holds without tripping. Your wiring just has to be rated for at least 38 amps. Your wire needs to be at least #8 copper.

    Fas Eddy; If the load calc says 4.6 tons you put in a 4 ton unless the old 4 ton compressor has lost capacity and then you would put in the 5 ton?

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