Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit

start kit sizing

does anyone know the formula on how to size a start kit for a compressor in the field? i use to have it written down,but have since misplaced it.i know that you take the back emf reading with the compressor running,and then you select a potential relay with a drop out voltage about 80% of that,but i do not remember how to size the start capacitor.when needed i have been using the factory (trane,carrier,etc.) factory start kit when they list one as an option,however the cost of these factory kits are normally4 to 5 times what it costs if you bought the pieces seperatly and wired them up on your own.any assistance would be much appricated.thank you


  • Techman
    Techman Member Posts: 2,144
    edited August 2012
    What formula?

    I'd like that formula also!Are you adding the relay/cap or replacing the relay/cap ? I highly recommend using Copeland caps/relays on Copeland comps and Tecumseh cap/relays on Tecumseh comps and so on .If a Carrier cond unit has a Cope comp in it then it also has a Carrier pot relay which can be replaced w/ a Cope relay. You then contact a Cope-Tecumseh -Bristol wholesaler to get the spec's for the relay/cap for that given comp.I have done what you said w/ the measured BackEMF and put in a pot relay that comes close, but, only TEMPORARILY.There is Hot Pickup Voltage,Cold Pickup Voltage, Dropout Voltage and Continous Coil Voltage to be figured into that formula There is a "one size fits all"pot relay on the market w/ an adjustable Continuous Coil voltage screw but that is not my favorite. Now as far as the Start Cap goes, that is a total crap shoot.What I do is  I put on a 135-156mfd -330vac cap and start the comp w/ my ampmeter on. I rapid start/restart the comp and see how my cap/relay does, then I go up/down one cap size and retest.And then I go up/down another size and retest until I like how the comp starts. Then I'm first in line @ the supply house to get the proper stuff.And I've been dead-on w/my cap sizing only about 50%( ok ok maybe 40%) of the time, which is about 10-12 times in 40 some odd years. 
  • zepfan
    zepfan Member Posts: 309
    adding start kit

    i am talking about installing a start kit to a unit that has never had it,but needs it due to low voltage,long line set, txv installed w/ a recip. compressor.i had the formula from a company i worked with about 20 years ago.i am going to call my boss at the time (who is still there) and see if he still has the formula.i'll check back after i have talked to him.thanks.
  • Techman
    Techman Member Posts: 2,144
    edited August 2012

    Hay zepfan, you got me thinking so I checked and  came up w/ a few things. I knew of a  formula for figuring out an unmarked  run capacitor for mfd's,but not for the voltage of the cap. I did not realize it applied to start caps also until just now ( HUMPH) ! The most important voltage # of the pot relay is the "pick-up voltage" ,( somewhere around 66%-75% of back-emf)it has to less than the next most important voltage #  of the pot relay which is "continous coil voltage" which has to be higher than the measured back-emf. Mars makes a "6 pack" of pot relays ( plus 4 more) and their literature has those #'s in it. It is a lot cheaper to get just the relay and cap then a "kit" from Carrier ,ect.
  • paul_79
    paul_79 Member Posts: 91
    start kit

    when i started out about 23 years ago there was a sheet with a formula ,( which had been around for a few year) i will have to look through my old papers and get  back to you.
This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!