According to section 220.61 of the National Electric Code (NEC), the demand factor for a feeder neutral with a load of 400 A would be applied to 100% of the load.
This means that the neutral conductor of a feeder supplying a continuous load is required to carry the full current of the load, and the demand factor cannot be applied to reduce the neutral current.
This is because in a continuous load situation, the neutral current is always equal to or greater than the phase current, and the neutral conductor must be sized accordingly to handle the full current of the load.
Therefore, the demand factor cannot be applied to a continuous load when sizing the neutral conductor of a feeder.
In summary, according to NEC section 220.61, the demand factor is not permitted to be applied to any part of the load for a feeder neutral, and the neutral conductor must be sized to carry the full current of the load.