A job I'm on has a small 240v 20A - 2 space/4 circuit - subpanel located outside for some minor use on a farm. One 20A circuit will feed a small mobile trailer on the property. There was no existing fourth wire run from the main panel in the house so the subpanel needs to be grounded. Can I drive in grounds at the panel if there is no other grounded lines (tel, CATV...) from the house nearby? Do I need two 8' ground rods for such a small panel? Thanks.