When using a triac coupler as a trigger device for the main triac, a load current directly flows through the triac coupler until the main triac turns on. When using zero-cross triac-output photocouplers, since the voltage when they are turned ON is kept low, current limiting resistor RT does not have to be inserted. When using non-zero-cross triac-output photocouplers, a large current flows. The current limiting resistor must therefore be defined as Peak ON-State Current or ON-State Current (Pulsed) (ITP or IONP), and designed not to exceed the value specified for the maximum rating (2A as an example). In general, Toshiba recommends that the current be controlled at about half of the rated current. For 100VAC power supplies, 100Ω is recommended, and for 200VAC power supplies, 200Ω is recommended.
How to design RT also described in Toshiba Application Note "Triac couplers-basic properties and application design", section 9.5.