In the cast restoration, cusp protection is mandatory when:
Indications:

If the width of the lesion is 1/3 to 1/2 the intercuspal distance - cuspal protection is to be considered.
If the width of the lesion exceeds 1/2 the intercuspal distance - cuspal protection is mandatory.
If the length:width of the cusp is more than 1:1, but not exceeding 2:1 - cuspal protection is to be considered.
If the length:width of the cusp is more than 2:1 - cuspal protection is mandatory.
When there is a need to change the dimension, shape and interrelationship of the occluding tooth surfaces.

MDS Prep Bytes( Cast Restoration – Onlay)
The lesion depth is ⅓  to ½ the intercuspal distance
The lesion depth exceeds ½ the intercuspal distance
The lesion width is ⅓  to ½ the intercuspal distance
The lesion width  exceeds ½ the intercuspal distance