Okay, this is a tricky situation. Safety is paramount when dealing with electrical work, especially when chandeliers are involved. Here's a breakdown of your options and considerations, prioritizing safety and code compliance.

Why Your Initial Idea (Screw and Fender Washer) is NOT a Good Idea



Not Code Compliant

Electrical codes (like the NEC in the US) specify how knockouts must be closed. A screw and fender washer are not an approved method for closing a knockout and won't pass inspection.

Not Secure

A screw and washer won't effectively seal the knockout against dust, moisture, or accidental contact with wiring inside the box.

Potential for Damage

You could damage the box or the beam by over-tightening the screw.

Potential Sharp Edges

It may leave sharp edges.

Safer and Code-Compliant Alternatives


The Ideal Solution: Remove the Chandelier and Use a Proper Knockout Closure Safest and Most Recommended

This is the right way to do it. You need to kill the power to the circuit, carefully remove the chandelier, and then properly close the knockout.

Options for Knockout Closures



Knockout Seal (KO Seal)

These are inexpensive plastic or metal plugs designed specifically for closing knockouts. You can get them at any hardware store. Simply push or snap them into the hole.

Knockout Closure with Screw

Some knockout closures come with a screw to tighten them down.

Procedure


Turn Off the Power

Absolutely the first step. Turn off the breaker that controls the chandelier. Double-check with a non-contact voltage tester to ensure the wires are dead.2.

Carefully Remove the Chandelier

Have a helper if it's heavy. Support the chandelier as you disconnect the wires. Use wire nuts to cap off the wires for safety.3.

Install the Knockout Closure

Clean the area around the knockout. Insert the appropriate closure and secure it according to the manufacturer's instructions.4.

Reinstall the Chandelier

Carefully reconnect the wires, making sure the wire nuts are secure. Reattach the chandelier to the box.5.

Turn the Power Back On

Test the chandelier to make sure it works.2.

The Less Ideal, but Perhaps Acceptable, "Don't Remove Chandelier" Method: Flexible Cable/Conduit AdapterThis method is only appropriate if it is impossible to remove the Chandelier and you need to run a low voltage cable from the knockout.

Principle

You're using a flexible conduit connector as a sort of plug.

Procedure


Turn Off the Power

Absolutely the first step. Turn off the breaker that controls the chandelier. Double-check with a non-contact voltage tester to ensure the wires are dead.2.

Choose the Right Connector

Select a

metallic flexible conduit (liquid-tight is preferred) connector that fits the 1/2" knockout.3.

Install the Connector

Remove the lock nut. Insert the connector through the knockout and tighten the nut on the inside of the box.4.

Apply Silicone Caulk

Apply a ring of silicone caulk around the outside of the connector where it meets the box. This will create a seal to help prevent dust and moisture from entering.5.

DO NOT run any wires through this, this is for filling in the hole ONLY.6. Turn the Power Back On

Test the chandelier to make sure it works.

Important Considerations



Sealing

The caulk is crucial for sealing.

Accessibility

This method could make it harder to access the inside of the box in the future if needed.

Important Safety Reminders



Turn Off the Power! Always turn off the power at the breaker before doing any electrical work. Test for Voltage

Use a non-contact voltage tester to confirm the power is off.

If Unsure, Hire an Electrician

Electrical work can be dangerous. If you're not comfortable or confident in your abilities, hire a qualified electrician. It's worth the cost to ensure the job is done safely and correctly.

Local Codes

Be aware of your local electrical codes. They may have specific requirements.

In summary, the best approach is to remove the chandelier (safely!), install a proper knockout seal, and then reinstall the chandelier. If removal is absolutely not possible, the flexible conduit adapter with sealant might be an option, but it's less ideal and may not be acceptable in all jurisdictions. Flag for review