# Thread: run cap 440v or 370v

1. Regular Guest
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my fellow tech said it makes a difference to compressor operation, which voltage run cap you use. i say that voltage stated on the run cap is it`s dielectric limits, and the microfarads in new, should be same as old, at usual 208v to 240v conditions.

2. Originally posted by somized
my fellow tech said it makes a difference to compressor operation, which voltage run cap you use.
Well I have to agree with that statement. I would rather use the correct mfd than volts in an emergency.

3. Use Same MFD
Use Equal to or higher than Voltage rating.
(i.e. its ok to replace a 370v with a 440v)

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You are right. The voltage is just a maximum operating voltage and has nothing to do with the function as a capacitor. Operating within its voltage limits, any capacitor with the same MFD value will perform the same.

paul

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The voltage rating on a capacitor is the voltage limit of the capacitor. Exceeding the capacitor's rated voltage may cause the dielectric to break down and the capacitor to short out.

That said, you need to realize that a motor is an inductive device. When you the supply voltage is applied to the motor run winding, the voltage across the start windings will be increased to a higher voltage value. A motor acts like a transformer with the run winding acting as the primary and the start winding acting as the secondary.

This is why the capacitor has a much higher voltage rating than the supply voltage.

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What problems would result in using a 35 mfd, when a unit had a 30 mfd ?

Just wondering, They always say stay within a certain percentage

7. sopositly you can go up or down 5% but i always put in the same size mfd but only use 440v caps
less to carrier on the truck
5mfd and 7.5 mfd 370v and 440 v on truck

8. Originally posted by tinknocker service tech
sopositly you can go up or down 5%
I always thought 20% but heck what do I know.

9. mr bill it may be 20% i was tought so long ago 5% and have always gone by that

10. I thought it was 10% over and 5% under.

11. Originally posted by tinknocker service tech
mr bill it may be 20% i was tought so long ago 5% and have always gone by that
No, your probably right for some reason that just came to mind.

12. Had a blown up cap today at 5pm. It was a 40/3 370V cap and I didn't have one on the truck. So I took 2 10uf, and 4 5uf, and linked them with a 3uf so I could get them a/c until I could get the right cap. It was ugly and I had to strap it under the electrical box but it works.

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I'll bet that looked like a time bomb strap to the undercarrage.

No one has said what will happen if the higher mfd is used and not changed.

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