What the 42 A rating means for fit
The A40-30-10-84 is an Eaton A-Line Series IEC contactor on the A40 frame, rated for 42 A motor current (AC-3 duty) and 60 A general-purpose resistive load (AC-1). That 42 A figure governs the motor it can switch — sized for a 15–20 kW induction motor at 400 V, depending on duty cycle. The 120 V AC coil pulls in reliably at nominal line voltage; dropout voltage sits around 85 V, so a saggy control circuit won't hold it closed. Three power poles handle the motor feed; the built-in 1 N.O. auxiliary contact block gives you one spare signal for a PLC input or run-status lamp without adding a side-mount stack.
The 120 V AC coil draws roughly 7–8 VA sealed, so a 50 VA control transformer can drive several of these plus a few pilot lights. Spring-cage power terminals accept up to 16 mm² stranded — common for the motor circuit at this current level. The auxiliary contact is wired separately from the coil terminals; no shared suppression diode to worry about on AC coils.