D&D 5E When a Lightning Bolt spell met the floor ...

[Horwath]

I would go with your DMs wording on this.

Imagine now a Fireball spell.

Sphere 20ft radius. In mid air it is 10667 cu ft.

What if it is detonated on the ground? Is it then 25ft radius?
What if it is detonated in the corner of the room? 40ft radius then?
Or in a narrow tunnel? 100ft in both direction?
Or at the bottom of a 10by10 deep pit? 1000ft up?

 

[Enevhar Aldarion]

I would go with your DMs wording on this.

Imagine now a Fireball spell.

Sphere 20ft radius. In mid air it is 10667 cu ft.

What if it is detonated on the ground? Is it then 25ft radius?
What if it is detonated in the corner of the room? 40ft radius then?
Or in a narrow tunnel? 100ft in both direction?
Or at the bottom of a 10by10 deep pit? 1000ft up?

My group still applies older edition logic to things like this and lightning bolt, 5E be damned if it claims that bolts do not bounce, etc. A fireball in a room too small in volume to contain it? It will flow out whatever openings there are in the room til the full volume is satisfied. A 100-foot lightning bolt spell hits a wall at 40 feet? It will redirect in another direction until the full length is fulfilled. The same for the lightning attack in the OP. It travels 20 feet and you are aiming downward at a target's legs who is within 5 feet of you? It will still travel that remaining 15 feet. It may or may not bounce, but it will at the minimum travel along the ground for the remaining distance. The one addition I would make for lightning attacks like these is what is the ground made of that the target is standing on. It is possible it could absorb it, rather than reflecting it.

 

[Blue]

My argument against targeting the pall'y legs would be that you target a square with an AoE. So in this case, you're aiming at the square (cube really) below the pall'y

That is demonstrably not true. As a matter of fact, unless you are using a variant where you play on a grid, there are no "squares". So it can't even be possibly true in the base game.

It's a line - I can shoot in in any direction I like. I can fire ti at 20 degrees to the left - which if I was on a grid would start in the sam square as if firing 20 degrees to the right, but the next squares definitely wouldn't be the same.

Same, I can fire it straight up, or any other angle along that axis as well.

 

[TallIan]

That is demonstrably not true...

The OP did say they play with minis on a grid.