The VA-FC series volume control with relay could use ABS mounting box (86x86x50mm, WxDxH), while the transformer will be placed outside of this box, and the wire of transformer needs to be connected well with the volume control. The metal mounting box(82.5x82.5x80mm, WxDxH) is used with VA-FC series volume control which is deep enough to place transformer, so metal mounting box is always recommended.