Start Here: The Data-Sheet Rule
Before we split into scenarios, memorize this: the part number on the old contactor is a starting point, not a verdict. I've replaced 'identical' contactors where the old label was so faded we ordered by memory. That memory was wrong.
Search for the exact catalog number instead. If you need to find a Siemens 42AF35AJ contactor data sheet, don't just type '42AF35AJ contactor' into a search engine and grab the first PDF. Siemens's support portal (support.industry.siemens.com) has a product download section. The data sheet will tell you the rated operational current (Ie), the utilization category (AC-3/DC-1), the rated control supply voltage for the coil, and whether you need a separate surge suppressor.
Here's something vendors won't tell you: when you call a distributor and ask them to check the specs, they open the same public data sheet. So you might as well open it first.
Scenario 1: You Need a Drop-In Replacement
If you're swapping a failed contactor in a live panel, you need to match more than the frame size. The coil voltage has to match the control circuit—not the old contactor's coil, but the voltage actually present at the terminal.
I only believed 'check the coil voltage' after ignoring it. We ordered a 120V AC coil replacement, clicked it onto the rail, wired it, and it sat there dead. The feed was 24V DC. The old contactor had a rectifier module no one noticed. That mistake cost us a $350 after-hours service call. It could have been avoided with a multimeter.
How to test power supply with multimeter? It's not a secret. Set the meter to V AC (or V DC if the control circuit is DC), put the probes on the two coil terminals, and read the voltage while the circuit is energized. Compare that number to the data sheet's 'rated control supply voltage'—not the range, the nominal rating. If the measured voltage is outside the coil operating range, don't install the contactor.
This is where the data-sheet example matters. When you check the Siemens 42AF35AJ contactor data sheet, look for two values in the coil section. One is the rated control supply voltage (e.g., 120V AC). The other is the operating range (e.g., 85-110% of rated voltage). They are not the same.
Scenario 2: You're Switching a DC Load
Here's where people get burned: AC and DC are not interchangeable in a contactor. An AC contactor depends on the current crossing zero to help extinguish the arc. A DC circuit has no zero crossing. So a DC load contactor needs larger air gaps and an arc-chute design to break the DC arc. If you use a standard AC contactor for a DC motor or DC power supply output, contacts can burn well below the rated current.
Look for DC utilization categories in the data sheet. Siemens doesn't market every SIRIUS contactor as suitable for DC loads; only certain types list DC-1, DC-3, etc. Match those to your load.
Also don't confuse 'DC-controlled coil' with 'DC contactor.' A SIRIUS contactor with a 24V DC coil is still an AC load contactor if the main poles are rated for AC. It just has DC control. That's a separate spec. When someone says 'dc contactor,' they might mean a DC load contactor, a DC coil version, or a contactor for solar/DC applications. Ask which one.
Scenario 3: You Need a Siemens Auxiliary Contactor
If your control system needs to know whether the main contactor closed—for a PLC input, a status light, or an interlock—you need auxiliary contacts. Sometimes that's a front-mounted auxiliary switch block. Sometimes it's a mechanically linked additional contactor. In the Siemens world, a 'Siemens auxiliary contactor' usually refers to the add-on switch block (1NO+1NC or 2NO+2NC) that snaps onto a SIRIUS contactor, or the side-mounted auxiliary block for older 3TF/3 series frames.
The mistake I see in purchase orders: ordering the main contactor and no aux block. Then the electrician is on a ladder trying to tap a signal from a line contactor. Adding an aux block in the original order is cheaper than sending a cart back and paying for a second trip.
If you already have a 42AF35AJ or a 3RT contactor and need aux contacts, search the same data sheet or the base contactor's selection guide. The block has its own catalog number. Match the snap-in attachment and number of contacts. NO/NC configuration can often be changed by rotating the block, but the contact set itself has to be the right type.
Scenario 4: Sizing the 100 Amp Circuit Breaker
Now a pet peeve. A contactor is not a circuit breaker. The contactor only switches the load; it does not protect it. So when someone asks for a '100 amp contactor' and then says 'so give me a circuit breaker 100 amp,' stop. The breaker size should come from the motor FLA, the branch-circuit protection type, and the combination's short-circuit current rating—not from the contactor's rated operational current.
Here's my gut-versus-data moment. The spreadsheet said choose a 100A breaker because the contactor nameplate said 100A. My gut said that felt too high for the motor we were managing. It turned out the contactor's 100A rating was for AC-1 (resistive load). In AC-3 duty, the same contactor was rated 80A. The motor FLA was 52A, so a 100A breaker was borderline depending on the breaker curve.
We had to look at the entire line-start starter combination, not just the contactor. The contactor and breaker are separate safety functions. This is why a complete order often includes a 3-pole contactor, an overload relay, and the feeder breaker sized according to the short-circuit current rating and conductor ampacity.
How to Tell Which Scenario You're In
Quick decision path:
- Replacing a failed unit in an existing panel? Start in Scenario 1. Verify the exact catalog number and coil voltage before ordering.
- Switching a DC motor or DC load? Scenario 2. Look for DC utilization categories and arc-suppression features.
- Need a feedback signal or interlock? Scenario 3. Add an auxiliary contactor block to the order.
- Sizing the upstream breaker? Scenario 4. Use FLA and conductor ampacity, not the contactor frame size.
All four at once? Start with the data sheet for every part number. I know that's not a universal answer. But it's the real workflow.
Bottom Line
What was best practice in 2020—order by series and hope the replacement's coil voltage is fine—hasn't aged well. The fundamentals haven't changed: you still need the right contactor for the load, the right coil supply, and the right protection. But the execution has. Data sheets are online. Searchable catalogs exist. And a $30 multimeter can prevent a $350 callback.
If you're staring at a dead contactor now, start with the exact part number. If that part number is Siemens 42AF35AJ, open the data sheet, check coil voltage, check utilization category, then decide which scenario you're in. The contactor is the last piece, not the first.
