After a bit of testing, it seems like to me, that the TileEntityRedstonePowerCable#renderTileEntityAt is passing the players position OR the coordinates (0,0,0), cause in the IItemRenderer class (which i will be implementing once this is working) is passing the coordinates (0,0,0)
TileEntityRenderer.instance.renderTileEntityAt(new TileEntityRedstonePowerCable(), 0D, 0D, 0D, 0F);
but i don't know howto put the right coordinates there...