Another Play:
 

Fed only (Apparently there is no agreement on this play at the national, state or local levels):

B20 intercepts a pass at the B3 and his momentum carries him into the end zone. While trying to return it, he fumbles the ball in the endzone and the ball bounces out to the B2 where A21 kicks the ball over the endline. Enforcement spot?

Haven't dug into the football books in awhile, but I'll give it a shot:

Foul occurs during a running play.

B is in possession at the time of the foul.

Basic spot for a running play is where the related run ends (end zone, ie: Goal line) for a foul that occurs beyond the end of the run.

Result of the play is a touchback for B.

Enforce the illegal kicking foul on A from the goal line, resulting in 1-10 for B at B's 15, or decline the penalty and take the touchback, 1-10 for B at B's 20.

Agree with jTheUmp

The Momentum Exception is in affect up to the point when B fumbled and the ball came out of the EZ. This means B is responsible for taking the ball into his on EZ (not the pass) and his team is in possession when Team A fouls ... 10-4-7

I agree with both of you.

However our supervisor told us there is no agreement on this play all the way to the top, so we're to enforce it from the 20 yard line.

My contention is that regardless of momentum, B is responsible for putting the ball in his own endzone, therefore 10-4-7 is the correct application.

Do not bet on it:)

It seems to me that while B did take the ball into the endzone, they fumbled the ball out of the endzone. The ball was bouncing so it was a grounded loose ball at the time it was kicked. The kick applied a new force which then caused to ball to re-enter the endzone and over the endline. The result of the play is a touch back due to A applying a force to the ball, causing it to go over the endline. Assuming that the kick was intentional, then we also have a foul on A for an illegal kick. The choices given to B as to take the penalty, and have 1/10 at their 7, or decline the penalty and take the result of the play which is a touchback. Obviously they would decline the penalty and take the ball at the 20.

How did you come up with this? IK is a 15-yd penalty in FED. Plus, the basic spot for a loose ball foul after a fumble is the end of the related run, which in this case in in B's EZ.