If you are simply trying to echo the excerpt in a specific part of a theme, you can simply use the following code within the loop:
<?php
$excerpt = get_the_excerpt();
echo substr( $excerpt, 0, 15 ) . '…';
?>
If you want to filter the excerpt globally (in archives, blog pages, etcetera) to match any post part of one or more given categories, you can place the following code in your theme’s functions.php file:
<?php
function wpse_225203_category_based_trim_excerpt( $excerpt ) {
//Return if there is not a except to work with
if ( '' === $excerpt ) :
return $excerpt;
endif;
//Get the post's categories
$categories = get_the_category();
//Get the post's categories ID
$categories_id = wp_list_pluck( $categories, 'term_id' );
//Set the post's categories ID we want to match
$categories_to_trim_ids = array( 1, 3 ); //Modify to match your ids
//Match the wanted categories ids and the post's categories ids
if ( ! array_intersect( $categories_to_trim_ids, $categories_id ) ) :
return $excerpt;
endif;
//$excerpt = wp_trim_words( $excerpt, 15 ); //Trim the excerpt to the first 15 words
$excerpt = substr( $excerpt, 0, 15 ) . '…'; //Trim the excerpt to the first 15 characters
return $excerpt;
}
add_filter( 'get_the_excerpt', 'wpse_225203_category_based_trim_excerpt' );
?>