I want to know if a drywall ceiling support hanging a 20kg chandelier
No, not with just the drywall.
Fasten a bracket to the ceiling joists with some good lag bolts for that much weight.
The problem isn't today but 5 or 10 years from now. Will the light eventually weaken the drywall enough to break it?
Why take that chance with you and your family?
I know ceiling fans require a special box for that much weight. I would at a minimum add a cross brace to attach the box to similar to a ceiling fan for such a heavy fixture. sheetrock will not carry this much weight with a standard fixture box.
NO! Ceiling mounted drywall alone cannot support any significant weight.
There are several types of toggle bolts that have some degree of tension strength. For example, according to this chart from one of the toggle bolt manufacturers, a single toggle bolt can hold up to 50 lbs under a tension load.
However, as @WolfHarper points out in a comment, those Togglers are rated for clearspan. In mounting a ceiling fixture, they'd be bolting near the edge of a big hole made for the junction box. This drastically reduces the strength of the drywall (and consequently the toggles in the drywall) Further, how is the drywall attached to the joists? The drywall may begin to deteriorate or pull away from the joists, especially if it is nailed instead of screwed.
ALMOST NO EXPERIENCED DIYER OR PROFESSIONAL WOULD RECOMMEND THAT YOU DO THAT! AND VIRTUALLY EVERY ELECTRICAL INSPECTOR WOULD REJECT SUCH AN APPROACH!
The risk is huge. A falling ceiling fixture can be fatal. Ceilings are prone to flexing and shaking from traffic on the floor above, Drywall can be fragile stuff, easily compromised around mounting holes.
There is a fairly easy safe solution, a fan brace
These are retrofit devices. A hole the size of the junction box is cut into the ceiling drywall. The brace, which detaches from the box for mounting, is inserted into the hole and then expanded until it grips two of the framing members (joists) in the ceiling. The box is then remounted on the brace.
These are designed to support fixtures up to 150 lbs. and fans up to 70 lbs.
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