I think that if you only have 2 120v breakers in the sub, on opposite phases your max neutral load is 20a, 140% of that is less than 30 so you could use a #10
220.22 Feeder or Service Neutral Load.
The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net computed load between the neutral and any one ungrounded conductor, except that the load thus obtained shall be multiplied by 140 percent for 3-wire ...
