HSA - 415 Compensation

[ErisaGooroo]

From the W-2 instructions, I learned that an employer's contribution (including an employee's contributions through a cafeteria plan) to an employee's HSA is not subject to federal income tax withholding or social security, Medicare, or railroad retirement taxes (or FUTA tax) but are these amounts included in the definition of 415 comp? 

Plan sponsor contributes an employer contribution to the HSA on behalf of the employee but does not want to include this employer contribution in the definition of plan compensation (SH and ER). 

Total Comp is defined as W-2 Comp and Plan Comp does not currently exclude deferrals / cafeteria.  To exclude the employer contribution to the HSA from Plan Comp, is an amendment to the plan necessary? 

Greatly appreciate your input.

QNPG