_s_get_parent_cat.php

/**
 * Get the top level parent category, even on sub-categories  
 * @return string
 */
function _s_get_parent_cat() {

	$term = get_queried_object();

	$children = get_terms( $term->taxonomy, array(
			'parent'    => $term->term_id,
			'hide_empty' => false
	) );

	$ancestor_ids = get_ancestors( $term->term_id, $term->taxonomy );
	

	if ( $children ) { // cat has children
		return $term->term_id;
	} else { // cat has no children
		$parent_id = $ancestor_ids[0];
		return $parent_id;
	}
}