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Old 07-11-2012, 04:39 PM   #1
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Dual capacitor mfd


What would happen if you install a 35-5 mfd capacitor and the unit requires a 40-5 mfd ? Or visa versa?

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Old 07-11-2012, 05:18 PM   #2
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Dual capacitor mfd


The rule is is you have to stay within 10% of the original rating you'll be ok....however, I see 5 over or under the rating of the original cap being used all the time.

Best bet is use what the original spec called for.

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Old 07-12-2012, 09:04 AM   #3
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Dual capacitor mfd


Either won't start or burn the unit.
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Old 07-12-2012, 05:49 PM   #4
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Dual capacitor mfd


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Originally Posted by clocert View Post
Either won't start or burn the unit.
that is incorrect.
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Old 07-12-2012, 07:19 PM   #5
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Dual capacitor mfd


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Originally Posted by hvac5646 View Post
that is incorrect.
Shorten the motor life?
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Old 07-12-2012, 09:32 PM   #6
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Dual capacitor mfd


I've seen several where the compressor label says 45mfd and the unit mfr. uses a 40mfd. They said testing showed a better SEER with the capacitor they used instead of the one on the compressor tag. As HVAC said use what's specified by the mfr.
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Old 07-12-2012, 10:49 PM   #7
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Dual capacitor mfd


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Originally Posted by Yoyizit View Post
Shorten the motor life?
If call for 40uf, you install one with only 20uf, it won't start.
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Old 07-12-2012, 11:34 PM   #8
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Dual capacitor mfd


Quote:
Originally Posted by clocert View Post
If call for 40uf, you install one with only 20uf, it won't start.
the op asked about going 5mfd over or under...not dropping from a 40mfd to a 20.
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Old 07-13-2012, 03:51 PM   #9
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Dual capacitor mfd


I could swear I've seen start run electrolytic capacitors with a -10%, +50% tolerance but I can't find them now.

Here's what I found with
http://www.google.com/search?client=...UTF-8&oe=UTF-8

+/-6%, 2 ea. cases
+/-5%, 3 ea. cases
270-324 MFD (~+/-10%), 1 case, Grainger
161-193 MFD (~+/-9%), 1 case, Grainger

and

"a start capacitor rated 10% over or 10% under the recommended MFD rating can be substituted"

"A 20 % tolerance is acceptable for start capacitor values less than 200 MFD."

In general, the farther you are from design center values the more likely you can expect problems. How relevant capacitor sizing is in light of all the ways motors can be abused is hard for me to say.

One link had you trying +100% and -50% capacitor values so we don't seem to need, nor can we get, nor is it financially feasible, to get +/-1% on these units.

Last edited by Yoyizit; 07-13-2012 at 04:05 PM.
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Old 07-13-2012, 04:10 PM   #10
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Dual capacitor mfd


(2652 x Amps) / Volts

Amps- at S Terminal
Volts - at Herm and C

Check if it will be close to 40MFD
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Old 07-13-2012, 07:31 PM   #11
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Dual capacitor mfd


Actually, it's 2653. 1,000,000 x 1/(2 x PI x 60), for 60 Hz.
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Old 07-13-2012, 10:04 PM   #12
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Dual capacitor mfd


My units take a 35/5 cap. Before the #1 unit cap was replaced it was testing right around 30, so 5 under. I didn't notice anything wrong with the unit (started and ran fine). However long term I would suspect you would damage the compressor.

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