I would say that SER is not approved for underground use. Even in conduit. It may be debatable, especially now with the wording of 300.5 (D)(5), but I believe USE would have been be the proper method. See 338.2
Type SE cable is suitable for use underground but type USE is unsuitable for use inside buildings becuase the jacket is not fire retardent. The U in the type designation is to indicate that the cable is not suitable for use in buildings.
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Tom Horne
