What size overload do I use with a star delta starter ?

In the majority of cases, the overload of a star delta starter is fitted below the delta contactor with wiring connections to one side of each of the motor windings.
The other side of the windings are connected to the line contactor. This means the motor load is split between 2 contactors but the overload only sees the current carried by one.
It is not a 50/50 split but has to be calculated by a special formula which, simply put, means the overload and each of the delta and line contactors only “see” 0.58 x the motor current.
This configuration of the overload is described as “inside the delta loop”.

The overload will be rated at 0.58 x Motor Full Load Current ( Inom ).

Example: A typical 415V 11KW motor has a full load current rating of 21A. for overloads mounted “in the delta loop” the rating will be 21 x 0.58A = 12.18A

Sometimes, usually, in Type 2 configuration installations, the overload (often in the form of a motor circuit breaker such as a GV2ME** or similar) will be
mounted upstream of the starter in the main supply cables.
In this case all the current drawn by the motor will pass through the overload so it needs to be rated at 100% of full load motor current, i.e. in above example, 21A.