Siemens Contactors: The Questions We Actually Get
I admin purchasing for a company of about 200 people across two locations—we do maintenance on industrial equipment, some light manufacturing support. I'm not an electrical engineer, but I process enough orders for Siemens contactors and relays that I've learned what people actually need to know. These are the real questions we get, not from the catalog, but from the people wiring them up and the people signing the POs.
1. What's the difference between a Siemens Sirius 3RT and a standard magnetic contactor?
This is our most common question. Technically, the Sirius 3RT is a magnetic contactor, but it's built to a specific standard that makes it the default for a lot of Siemens-based control panels.
Here's the practical difference I've seen: the 3RT series is designed for modularity. You snap on auxiliary contact blocks, overload relays, and surge suppressors without needing extra wiring space. It's a system. A standard “loose” magnetic contactor might be physically larger for the same current rating and require separate mounting for everything.
From a buying standpoint (unfortunately), a 3RT is usually easier to replace because the form factor is consistent. I've ordered 3RT contactors across different revision numbers—they swap in. A random “magnetic contactor” from a less common line? I've had to triple-check mounting footprints every time.
(This was true back in 2022 when we standardized our panel spares. I'm sure newer generations have tweaked dimensions, but the principle holds.)
2. Do I need a separate overload relay, or is it built into the contactor?
A contactor and an overload relay are separate devices, but they're meant to be used together. The contactor handles switching the load (opening and closing the circuit). The overload relay protects the motor from overheating due to overcurrent.
You do not get overload protection in a standard contactor. If you buy just a contactor, you are buying a heavy-duty switch. The classic question is: “Can I just use a breaker?” No, not really. A breaker protects the wire. An overload relay protects the motor. For a motor circuit, you need both—or a combination starter that integrates the contactor and relay into one housing.
I went back and forth on this myself when I started ordering for our shop. I thought a “contactor” was one unit. It's two pieces. That mistake cost us a re-order (ugh).
3. Where can I find the Siemens contactor catalogue PDF?
The short answer: the official Siemens Industry Mall (mall.industry.siemens.com). The longer, more useful answer: the PDF you want is usually called the SIRIUS System Manual or the IC 10 Product Catalog.
Googling “siemens contactor catalogue pdf” sometimes sends you to third-party sites with outdated .pdfs. I've found the most current version by searching for “SIRIUS 3RT catalog pdf” and filtering by the Siemens .com domain.
What I actually tell our engineers to do is use the Siemens Online Configurator. It's faster than paging through a 1,500-page catalog. But if you need the PDF for an offline spec, the catalog code is usually “IC10” followed by the year (e.g., IC10_2024_En.pdf). Sometimes these catalogs are 200+ MB, so be patient.
4. Can I use a 50 amp generator transfer switch with a Siemens contactor?
That's mixing two very different things, and I get why people ask. A 50 amp generator transfer switch is a safety device meant to isolate your home or business from the grid when a generator is running. A Siemens contactor is an electrically operated switch for motors and industrial loads.
You wouldn't replace one with the other. The question usually comes from someone looking at a manual transfer switch vs. an automatic one. If you want automatic transfer, you can use a contactor controlled by a transfer relay. But a simple “50 amp generator transfer switch” from a hardware store is a mechanical, interlocked device—it's not an application for a standard industrial contactor.
If you're building an automatic transfer panel, you'd use a contactor rated for the full load (including inrush). A typical 50A transfer switch for a home generator might be rated for 50A continuous. A Siemens 3RT contactor in the same amp range would be rated for AC-1 (resistive) or AC-3 (motor) loads—different curves.
What I mean is: use the right tool. A contactor is for remote switching; a transfer switch is for safety isolation. They're not interchangeable.
5. What about the electric relay switch vs. a contactor?
People use “electric relay switch” loosely, but there's a practical difference. A relay is usually for low-power or control circuits (signal switching). A contactor is for high-power loads (motor circuits).
The classic boundary: contactors usually have arc suppression built in because they're switching inductive loads (motors). Relays, especially small ones, can weld closed if you try to switch a motor with them.
When to use a relay: Switching a 24V DC sensor signal, turning on a pilot light, controlling a PLC input.
When to use a contactor: Switching a 3-phase motor, turning on a bank of heaters, controlling a large lighting load.
If you're searching for “electric relay switch,” you might actually need an auxiliary contact block for your Siemens contactor—that's a common mix-up. The auxiliary block (like a 3RT19) mounts on the contactor and provides the relay-like contacts for your PLC to read.
6. Power inverter vs. generator—does the contactor matter?
This comes up with our service guys who run remote equipment. A power inverter (battery-powered DC to AC) vs. a generator is a different power source, but they both connect to a load through some switching gear.
The issue is the waveform. Generators produce “dirty” power (more harmonics, frequency drift). Inverters produce a simulated sine wave (modified or pure). A contactor doesn't care about the power source—it's just a switch. But the overload relay or starter might.
I've seen people try to run a contactor coil off an inverter without checking the coil voltage tolerance. A Siemens 3RT contactor coil might be rated for 208-240V AC. Running it from a modified sine inverter can cause coil hum, overheating, or failure to latch. The contactor itself didn't fail—the power supply did.
Also, from a purchasing standpoint, a diesel generator for a 50A load is way more expensive than an inverter + battery bank (initial cost is lower for the inverter, but fuel vs. battery lifecycle is a different calculation entirely). The contactor cost is the same.
7. Any gotchas with the 3RT2 series vs. older 3TF series?
Yes, and this is the kind of thing that bites you when you're in a hurry. The 3RT2 is the current generation (Sirius). The 3TF is the older generation. They are not physically interchangeable in most cases.
The 3RT2 is smaller, more modular, and uses the accessory system (the auxiliary contacts, the surge suppressor, the overload relay). The 3TF is a larger, older form factor. If you're swapping a 3TF for a 3RT2, you'll need a new mounting plate and possibly different wiring terminals.
I had a situation in early 2024 where a vendor sent us a 3RT2 as a replacement for a 3TF (the catalog number was close). It didn't fit the panel. We had to cut new DIN rail and re-terminate everything. The vendor meant well—they saw “Siemens contactor” and shipped the new generation. But they didn't check the form factor.
The lesson: always check the frame size. The 3RT2 comes in sizes S00, S0, S2, S3, S6, S10, S12. The 3TF has different sizing. The electrical specs might match; the physical fit is a gamble.
