Ready-Link® Rack (V4) Add-on. The add-on rack is designed to expand the base rack system allowing up to 16 units back to back on a free standing rack system. Designed to work with NPE, NPE-2, NPN, NCB-E, NCB-H, NFC, NFC-H, NHB, NHB-H, NFB and NFB-H series units for infield assembly. Approved for indoor and outdoor installations.