I have a 500A CB feeding a 300A contactor and from the contactor to the 250A softstart. Can the tap rule be applied to the conductors on the load side of the 500A breaker to the contactor and from the contactor to the softstart? The distance from the breaker to the contactor is less than 10ft and from the contactor to the softstart is less than 10ft. Or do I have to use the OCPD rating to size the conductors? I will be using a DLO conductor.
If this is your ENTIRE circuit (other than maybe a CPT), then it is a motor branch circuit. In that capacity, the CB is only providing the Short Circuit protection, over current is handled by the Over Load Relay, or in the case of a digital soft starter, the OL protection built-in to the soft starter. So your conductor rules are determined by the applicable parts of article 430.
But 2 other issues:
1) You say "soft start", not "soft start
er" and there is a significant difference. A soft start
er will have OL protection included, either internal digital OL protection, or an external OLR as part of the starter. There are, however, soft
starts (no "er") that are intended for use BEHIND another electro-mechanical start
er. They do NOT always have OL protection and it is sometimes not clear. Be careful.
2) DLO cable, while nice for being extra flexible, has a major flaw for something like this. The ONLY approved termination method is crimp lugs as far as I am aware. The standard mechanical lugs that will come on your breaker, contactor and soft starter will NOT be rated for use in terminating DLO cable.