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If the use is only for a few convenience outlets, the need for a sub-panel is questionable. Based on the 30 foot run, you can install a 20 amp branch circuit breaker in your existing main panel and run the branch feeder in #12 wire several ways, referring to table 300.5 Minimum Cover Requirements. A 12" burial depth is acceptable for type UF, with GFCI protection. If you'd like to use NM conduit, the burial depth is 18". With RM or IM conduit, the depth is 6". All of these depths are based upon no vehicular traffic over the burial run.
If you want to do more with this circuit than run a weedwhacker or power a bugzapper, then a sub-panel is in order, along with associated requirements. Note that any receptacles installed on the referenced branch feeder, if installed outside of the shed where defined as a wet location must have a protective cover as defined by 406.8(B)(1).
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There ain't nothin' more powerful than the odor of mendacity -Big Daddy
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