As stated previously, you may need more than just the 415 reg, since the plan provisions are also important.
Attached are two separate Q&As from the Gray Book, 1999-30 and 2008-48, that reiterate the IRS position. (In case you are not familiar with the Gray Book, it is the result of informal discussions between actuaries and the IRS. It carries no official weight, but can be useful in identifying IRS viewpoint for situations and questions not directly - or not completely - addressed in regulatory guidance.)
GrayBook 1999.30 and 2008.48.pdf