There are two dimmers that can be used to control 277v Advance Mark X (model numbers starting with VEZ) ballasts.
They are both Nova T style dimmers:
- NTFTU-5A-277-xx (single pole)
- NTFTU-103P-277-xx-CPW0196 (3-way)
Both of these dimmers do require a neutral wire connection. See installation instruction attachments.