Ok so i know how to read a capacitors mfd but my question is multi fold regarding a formula i came across. In one video the guy said to measure a capacitor under load take the amps of the wire leading to the compressor start multiply by 2650 and divide by the vac reading on the common to start of the cap and it will give you the actual capacitance of the cap. Another website used the exact same formula except for the number 2650, they used 2652 as the number and said that is the formula to check what cap a motor actually needs.

My questions.

1) Is there any truth to the second formula? Bearing in mind a capacitor has to cause a 90 deg phase shift, is a simple formula accurate to determine what size cap is needed by a motor if you dont have a reliable source to work from ie a manufacturers rating etc.

2) If the first formula is accurate then what would happen if the compressor was drawing more amperage than it should ie from an overcharge or similar factors affecting the compressor amp draw. Not that i wouldnt check these things but for the novice out there who would rely on this i could foresee this formula as a problem?

3) From personal experience ive come across techs using the +- 5% rule a bit too lenient. first cap being a 35uf, tech replaces it with a 40 because he doesnt have a 30 or 35 in the van and he is 50km away from a supply shop. Next guy does the same thing, only putting the next size up before you know it the compressor has failed due to this problem. Is there a formula to determine what a motor needs cap wise?