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Hi, some electricians put a capacitor across the power supply to a light bulb to correct its power factor, can someone tell me how to calculate the capacitor value to place across a 110 volt bulb???

Incandescent light bulbs are resistive. Their power factor is perfect already. Perhaps you saw a capacitor in series with light bulb, to use it on a higher voltage. Or your electrician saw an electrician who knew what he/she was doing put a capacitor across a motor to correct its power factor (motors are inductive). Or perhaps the light bulb was an easier place to put the capacitor than the motor.

Florescent bulbs may (repeat may) have an inductive ballast. A small amount of capacitance won't hurt.

LED bulbs, are not resistive, but are modern enough that power factor was considered during design.

(just for info - Old linear power supplies are a capacitive load, and only draw current at AC peaks. Makes a really screwy power factor that is hard to correct. Certainly not with another capacitor.)

Or or, the capacitor was to throw the power factor off in a lame attempt to save a couple cents on the electric bill. (double "or" on purpose )

Dear Dale Y, i saw several youtube shorts featuring MOES smart switch for 110 volt bulbs with furnish capacitor across line 1 and neutral.

Nothing to do with power factor correction.

Why do smart wall switches need a capacitor?
Some smart switches come with a capacitor that is installed at the light socket end between the active line and neutral wire. This takes up the small power flow used by the smart switch and prevents it reaching the bulb, which solves the issue.

If your smart needs a capacitor it will come with one. Only qualified electrician should install it to prevent shock or fire hazard.

(there was no mention of smart switches in post 1.)

Cheers,

Hard to say exactly what that electrician was trying to achieve but as noted, it wasn't likley for PF correction unless he was misunderstanding how that works, that's mostly valid for inductive loads like motors. Are you sure it wasn't a surge protector instead?

Assuming the value of the cap was not very large, it may have been to reduce EMI. I have a few switches that briefly interrupt the HDMI signal on a long HDMI cable nearby in the wall and blanks the screen for a few seconds, a cap would probably help reduce or eliminate the EMI pop that causes it for example.

It may also be a hack to keep an older motion sensor switch alive when using a LED bulb (older models need some current flowing through the filament to power the switch but the LED is too high impedance). A properly sized cap creates enough AC leakage current to power the switch without all the heat a resistor would make. Usually that's a "no neutral wire" switch. Switches that require a neutral wire should not benefit from a cap unless it's for EMI perhaps.

Just a few guesses since not enough detail to know for sure. If you say what the switch was controlling and the value of the cap, I can get closer. I want to add that this is unusual, I can't say I have ever seen anyone actually do something like that in practice.

Outside of an industrial setting, power factor correction is usually an exercise in smoke and mirrors!
The utility company will hang capacitors at the distribution transformer for places like grocery stores with their highly inductive compressors and motors.
To those that may be in a fog about all this, just recall the old school saying...ELI the ICE man!(look it up!)

above is spam by dowdkwy
not the up arrow
completely invisable, something "... cat...", use view source

"...use v. s."
That's SOP anymore.
I could see the tiny dot on the screen.
Done.