my central air run capacitor went out it was 440 volts 20 microfarads can i use a capacitor with 450v 16mf?

Question

anyways bottom line will a 450volt 16 micro farad work
it takes a 440 volt 20 micro farad capacitor
its hot and i want to know if this will work till tommarrow

Answer 1

AC Motor Capacitors -

“In general, you may make substitutions for oil-filled run capacitors on permanent split capacitor (PSC) motors by going to the next [higher] standard or stock microfarad rating. For example, a 10 microfarad capacitor may be substituted for a 7½ microfarad capacitor.” Even in an emergency situation you should never substitute a capacitor with a lower capacitance value or voltage rating for the proper capacitor otherwise you run the very real risk of permanent motor damage.

“Another possibility when substituting capacitors is to wire them together in parallel to add microfarad ratings. For example, you may wire a 5 microfarad capacitor in PARALLEL with a 7½ microfarad capacitor to achieve the equivalent capacitance of a 12½ microfarad capacitor. You also may use the same procedures when making substitutions for the start capacitor on a motor.” Be sure you maintain the correct voltage rating for all capacitors when combining caps.

To avoid damage or personal injury you should always use the exact replacement capacitor whenever possible.

A little extra advice: never discharge a capacitor using a screwdriver or other dead short method, instead use a 500 Ohm, 15W or higher power resistor or a 100-200 watt light bulb to slowly and safely drain the capacitor's residual charge. Using a light bulb thread the bulb into either a pigtail socket like the type used with temporary light strings or use a handlamp (aka drop light) with a (homemade) pigtail cord end or plug the male plug into a (homemade) pigtail cord fitted with a standard 15A parallel-blade female plug. On the pigtail end use either a pair of heavy-duty, insulated alligator clips or 0.250-inch female quick disconnects to connect the lamp assembly to the capacitor.

P.S.
When you purchase the proper replacement caps buy an extra start and run cap for just such emergency situations. You may also want to consider purchasing a replacement relay and overload as well; these parts may be available as a kit.
 

Answer 2

the purpose of the capacitor is to change the phase of the ac so the motor will start.

the voltage is not an issue
the capacitance is 20% off. That may be within the tolerance of the cap. Of course if the new one is 20% low now u have about 13MF which makes matters worse.

it will probably work, but I think I would replace it ASAP with the right capacitance.

The lower capacitance may cause it to overload, especially if it tries to restart quickly after a stop. (peak load)