Per the Wild Magic sorcerer's Wild Magic Surge table (PHB, p. 104)
07-08: You cast fireball as a 3rd-level spell centered on yourself.
There's other questionable area spells in that list (grease is a 10 foot square, so your square and which three?), but I'll focus on spheres like fireball (PHB p. 241):
(…) Each creature in a 20-foot-radius sphere centered on the point must (…)
My problem is, the sources I found so far – including the other answers to similar questions here – talk about measuring spheres from grid square intersections, whereas this clearly talks about it being centered on the unlucky Wild Magic sorcerer. Plus, since it's a wild magic effect, it doesn't exactly have a caster that can pick a point of origin. Is there an accepted way to measure an effect like this?
Best Answer
This is a DM call.
As observed in another answer from Thomas Markov, there are two ways of reading this.
The first reading is literal: the Fireball is centered on yourself, hence the point of origin is the center of the square in which the characters is. The situation is depicted in the below figure, where the green circle is the Sorcerer, the red circles represent other creatures and the small circle the point of origin of Fireball.
The second reading applies the rules for using a tactical map from the DMG and the rules from XGTE:
In this case, there are 4 intersections around the square in which the Sorcerer is: then it has to be decided from which intersection the Fireball originates. The 4 situation are depicted in the figures below: pay attention that this choice affects in different ways the creatures around the Sorcerer.
North - east corner
South - east corner
South - west corner
North - west corner
It is up to the DM deciding who picks the point of origin: the player or themselves.