The 3RT1276-6NF36: This is a Siemens SIRIUS vacuum contactor in the S12 frame size — the vacuum bottle extinguishes the arc inside a sealed chamber, so the contact gap stays clean and the switching life holds up under high-cycle motor starting or furnace duty where an air-break contactor would pit the contacts after a few thousand operations. The physical envelope is 225 mm deep by 214 mm high by 160 mm wide — the depth is the dimension that usually bites you on a shallow backpanel or a retrofit into an existing enclosure where the gland plate or busbar trunking limits how far the contactor projects from the mounting surface. Coil voltage is 96 to 127 V AC/DC with screw-type terminals — the wide AC/DC tolerance means it holds in on a control transformer with a sagging secondary or a battery-backed DC bus, provided the initial pickup voltage stays above 0.8 × rated value and the full-scale overvoltage does not exceed 1.1 × rated. Main power cables land on terminals accepting 70 to 240 mm² stranded copper — that is a 4/0 AWG to 500 kcmil range, sized for the full rated current of the contactor without needing a separate lug adapter on the line or load side.
Switching frequency and duty-cycle limits
The contactor is rated for 750 operations per hour in AC-3 and AC-3e duty (squirrel-cage motor starting) and 700 ops/h in AC-1 (resistive load) — the vacuum bottle handles the arc without the thermal recovery delay that limits an air-break contactor, so the cycle rate is set by the magnet system heating, not the main contacts. In AC-2 (slip-ring motor starting) and AC-4 (plugging / inching) the rate drops to 250 ops/h — the longer arcing time during reversal heats the vacuum bottle envelope, and the manufacturer's rating assumes the contactor is not operated above this frequency without a duty-cycle derating.