What the 40 A rating means for fit
The S804N-C40: The 40 A continuous rating governs the conductor sizing and upstream protection coordination. In a distribution panel, this breaker protects a feeder or a large branch circuit; the thermal-magnetic trip curve (C-curve, per the S804N family) provides moderate inrush immunity — suitable for motor starters, lighting panels, and general-purpose loads that don't pull a high magnetizing surge. Verify the SCCR of the downstream equipment is at least as high as the breaker's interrupting rating to maintain the series rating.