The ESL 500N series 2&4 wire conventional photoelectric smoke detectors bring together trusted technology and a full line of needed features to meet the demands of every type of application. The 500N offers sounders in 2&4 wire applications, specifically to meet residential code requirements. The 521NCSXT has the added benefit of low current draw which makes it compatible with a wide variety of control panels. The ESL 500N series also offers choices for optional auxiliary relays and isolated heat sensors, not available before in the ESL 500 series. ESL is proud to offer proven technology like CleanMe®, dust compensation, field replaceable optical chambers, multi-criteria heat algorithms and self-diagnostics which are trusted by installers around the globe. Features and Benefits: Wide compatibility Built-in sounder Patented CleanMe® feature Built-in dust compensation Field-replacable optical chamber Listed: CSFM, cULus, MEA NOTE: Your alarm panel must support 2-wire operation to use the 2-wire version. Both types require circuit supervision with a relay and/or resistor. 4-wire version requires a EOLR on each circuit, see below.