Have to replace a 55uf MFD 440 Volt VAC (model 97F9042). Is it OK to, at least temporarily, replace it with a 45uf 440V one (model ITAN PRO TRCF45)?

(Some specific details of my situation. It's not clear that the capacitor is the only issue. It's just obvious that it's broken. I could order one from Amazon and it'll be here in two weeks. I could have a local HVAC company come and fix it and it'll cost me $300. So if the above solution can last me 2 weeks without further damage to the system, I'll be happy to go with it while I wait for the part from Amazon.)

  • It might help to specify the make/model of the piece of equipment this is going in to.
    – FreeMan
    Jun 30, 2020 at 18:10
  • capacitor tolerance is typically 20% anyway (the most of any common components), so a brand new one marked 55 could actually be only 45 anyway, and vice versa. In short: don't stress about it, but do replace. If you can measure it, it would take the unknowns out.
    – dandavis
    Jun 30, 2020 at 21:06

2 Answers 2


Your terminology is a bit off A 55uf 440v cap Provides a phase shift for the motor so you will not be under powering the cap but the motor.

Will it work with a 45uf? Probably but start may be a bit harder. I change caps quite regularly on single phase systems , I try to stay within 10% of the original, if I don’t have one that is within 10% would I use one that is almost 20% under? Yes I have and it worked until I could get the proper size 3 days later.

I did monitor the start & run currents the system I believe was a 5 ton it was drawing close to max on startup ~35 amps and was taking 2-3 seconds prior to drawing less so I checked it it worked did not draw two much but did not leave them without cooling on a hot day(s).

Having the wrong size will cause the motor to take longer to start drawing more power so you don’t want to do this with a system that short cycles 5 starts per hour no problem from what I have measured but it should be replaced with the proper diced cap soon. The extra draw is not only hard on your motor but on the contactor an electronic switch inside the unit that uses a low voltage to turn on a higher voltage /high current device like the compressor motor or fan.

So it is possible but you should verify the system can get to speed (it usually takes longer) not all systems have the same sizes so without the motor size it is a possibility where I have done this.


It might depend on the type of motor and if you don't know, it's risky to experiment. Under sizing the capacitor on a Capacitor Start / Induction Run (CSIR) motor that has a centrifugal switch runs a risk of not allowing your motor to get to the speed necessary for the switch to change state, leaving the capacitor in the circuit and possibly damaging it (again) and/or damaging the motor windings. If it IS a CSIR type motor, the problem may have been the centrifugal switch to begin with, which is WHY your capacitor was damaged, and if so, putting in the new capacitor without fixing that first will result in the same thing.

If it is a Permanent Split Capacitor (PSC) type motor where there is no centrifugal switch, it might work, but as a general rule if less capacitance WOULD have worked, they would have used that. It also may stall or take longer to get to full speed, resulting in possibly causing an overload condition. If you are lucky when that happens, the protection circuit will prevent damage to the motor, but that too is a risk.

If you can find one fast with MORE capacitance, that would be better. Higher caps are often sold as a "Hard Start Kit" for HVAC compressors, you could look around for one of those.

  • You obviously know a lot about motors, but I thought a motor with a centrifugal switch was termed a "repulsion start, induction run" motor and that they had the highest starting torque of all. Always trying to learn more here! Thanks for any response. Jun 30, 2020 at 21:26
  • No, a repulsion start induction run motor is a completely different beast, predating the wide availability of capacitors. It uses a centrifugal moving arm that stars the rotation as a repulsion motor, i.e. with brushes and a commutator, then once it gets to speed, the brushes move away and the motor continues on as an induction motor. Capacitor Start motors replaced that system in around the 1930s. The centrifugal switch is closed when it starts, putting power through the capacitor to an auxiliary winding that is phase shifted from the main one, which give is relative rotation to start off.
    – JRaef
    Jul 1, 2020 at 4:13
  • This video is of an earlier version where instead of the centrifugal operated arm, it was manual, which allows you to see how it functioned. youtube.com/watch?v=_KdaipeodLo
    – JRaef
    Jul 1, 2020 at 4:15

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