I’m a procurement manager at an 80-person electrical controls distributor. For seven years I’ve managed roughly $180,000 a year in component spend, mostly Siemens control gear. The most expensive mistakes I see are not brand-to-brand comparisons. They are the small ones buried inside a single Siemens contactor order.
The useful comparison is never only “which contactor has enough amps.” Four other decisions decide the total cost: 3TF vs SIRIUS 3RT, manual vs automatic reset, line-side vs load-side wiring around a VFD, and the coil test you do with a multimeter.
Comparison 1: Siemens 3TF vs SIRIUS 3RT contactors
When someone says “siemens contactor,” it can mean the older 3TF line or the current SIRIUS 3RT line. Legacy panels are full of 3TF contactors. If one is working and the machine is not being modified, I don’t force a change. But when the choice is new stock or a new panel, I compare total cost of ownership, and SIRIUS 3RT is the side I land on.
- Current documentation. 3RT datasheets, wiring diagrams and selection tools are easy to verify. 3TF documentation often means chasing an older scanned catalogue.
- Accessory continuity. SIRIUS has an integrated family: overload relays, auxiliary contacts, busbars and labelling systems. With 3TF, I have to cross-reference older accessories.
- Support efficiency. A 2025 purchase order should be placed on the current platform. The old habit of ordering exact 3TF replacements comes from an era when panel drawings were expected to last for decades with no changes.
One important caveat: don’t assume a SIRIUS 3RT is a drop-in replacement for every 3TF. Compare the datasheets for the exact sizes because mounting and wiring differ in some cases.
Per IEC 60947-4-1, contactor load current is tied to utilization category. A 40 A contactor for resistive AC-1 loads is not automatically a 40 A contactor for motor starting under AC-3. That is why the “compare the amps on the nameplate” shortcut causes orders to fail.
Comparison 2: Manual reset vs automatic reset
Most searches for “Siemens SIRIUS contactor reset” are about a starter that won’t restart. The tripped device is normally the SIRIUS overload relay, not the contactor body. In a motor starter, the 3RT contactor does the switching, and the overload relay provides the protective trip.
The reset mode matters more than most cost comparisons. Manual reset requires someone to press the reset button after a trip. Automatic reset lets the overload relay reset itself when the overload condition clears. Manual reset is my default for anything with people nearby. If a conveyor trips while someone is clearing a jam, an unexpected restart is worse than downtime. Automatic reset can work for remote, guarded equipment, but only after a proper risk review.
The oversimplified advice is “automatic reset saves a truck roll.” In our cost log, the saving disappears when a machine starts itself and causes damage. The selector is not a small detail. It determines whether the machine can restart without warning.
Check the front label before replacing anything. More than once, a “failed SIRIUS contactor” was just an overload relay sitting in a tripped state waiting for a reset.
Comparison 3: Line-side vs load-side wiring for a single phase vfd
Another frequent request is for a “siemens sirius contactor wiring diagram.” The typical application that generates this search is a small panel with a single phase vfd. The first wiring decision is where the contactor goes.
The diagram I usually recommend is simple: run line power through the contactor main poles to the VFD input. Use the VFD control terminals for start, stop and speed commands. Do not put a contactor between the VFD output and the motor unless the VFD documentation specifically permits it. A VFD output is not a clean mains sine wave, and interrupting it can cause overvoltage, arcing or drive damage.
If the contactor exists for safety isolation or emergency stop, line-side is correct. Size the contactor based on the VFD input current, not the motor nameplate alone. The control circuit then needs the right coil voltage across A1 and A2. In our 2023 purchase history, coil-voltage mismatch created the largest SIRIUS-related return category. It was not an engineering failure; it was a process failure.
The VFD manual is the final authority. A generic diagram might look close, but if the drive manufacturer tells you not to use an output contactor, follow that instruction. Siemens SIRIUS manuals and the drive manual should agree before you assemble anything.
Comparison 4: SIRIUS contactor coil vs 3 pin ignition coil testing
Multimeter testing is another place where a general answer does not transfer. Somewhere on the internet there is a question like “how to test 3 pin ignition coil with multimeter.” The common first step is measuring primary and secondary resistance between the pins. That can find an open winding, but a 3 pin ignition coil may contain internal electronics, so a resistance reading alone cannot prove the coil works under load.
Testing a SIRIUS contactor coil is similar in one way and different in another. Measure across A1-A2 with a multimeter and compare to the Siemens data for the coil code. Zero or open indicates a winding failure. But a normal reading does not guarantee the contactor will pull in. Voltage drop, control supply problems or mechanical wear can still stop it.
When one of our guys killed his truck battery during a bench test, the real search was “what amp battery charger do i need for my car.” For a normal 60-100 Ah lead-acid car battery, a 10 amp charger is usually enough; a 2-6 amp maintainer is fine overnight. More amps doesn’t always restore a battery faster because the battery limits acceptance. That is a useful example for contactor buying too: more rating is not the same as correct selection.
Before you pay for a replacement contactor, do the cheap checks first: confirm the coil voltage, measure the coil resistance, check the overload relay reset mode, and inspect wiring against the diagram. That sequence has saved us more money than any vendor discount.
What I order, and why
- New panel or new stock: SIRIUS 3RT, not legacy 3TF, unless you need the old spare to match an existing panel.
- Machine with operators around it: manual reset.
- Remote guarded pump station: automatic reset only after the risk review says it is safe.
- Single phase vfd: line-side contactor, and leave the drive output connected directly to the motor.
- Before installation: confirm the full Siemens order code and measure the A1-A2 coil resistance.
The final step is unglamorous but cheap. Early in this job I skipped it and paid for expedited freight on a contactor with the wrong coil voltage. If you treat a Siemens contactor as one line item, it looks easy. The details are the actual product.
