The 500N series two- and four-wire conventional photoelectric smoke detectors bring together trusted technology and a full line of features to meet the demands of every type of application. The 500N offers sounder models in two- and four-wire applications, specifically to meet residential code requirements. The 521NCSXT has the additional benefit of low current draw which makes it compatible with a wide variety of control panels. The 500N series also offers choices for optional auxiliary relays and isolated heat sensors, not available before in the 500 series.