If you’ve ever opened a panel, stared at a Siemens SIRIUS contactor, and thought, “Which coil voltage is this thing?”, you’re not alone. Drawings go missing, labels fade, and downtime doesn’t wait. This U.S.-focused guide shows you exactly how to decode the MLFB (e.g., 3RT1056-6..6), verify the voltage safely at A1/A2, and pick the correct coil code (AB/AF/AP/AV/AR 36)—so you can order the right part the first time. Clear steps, a quick cheat sheet, and an ordering checklist inside. Let’s turn guesswork into a confirmed part number in minutes.
Why this problem is so common
- The front label shows the base contactor (frame and current ratings) but not always the exact coil variant.
- SIRIUS 3RT1 coils are available in multiple voltages and are replaceable; the coil code appears in the full MLFB and on a small side label.
- U.S. control circuits vary: 24 VAC (HVAC), 24 VDC (PLC), 120 VAC (legacy), 208–240 VAC (via CPT), and occasionally 480 VAC.
Step-by-step (U.S. practice)
- Capture the base MLFB from the nameplate. Example:
3RT1056-6..6(S6 frame, 3-pole, AC-3 185 A @ 400 V). - Check the drawings/BOM for the coil note (e.g., “Control 120 VAC,” “24 VDC PLC,” or “CPT 230 VAC”).
- Trace the control source: look at the control transformer secondary (480→120 or 480→240), PLC/PSU (24 VDC), or HVAC board (24 VAC).
- Measure safely across A1–A2 with the circuit made safe and energized for test. Record the actual voltage and whether it’s AC or DC. Don’t guess by resistance.
- Match the coil code from the table below and complete the full part number.
- Optionally replace just the coil if the frame is healthy (S6 coils are withdrawable).
Coil code cheat sheet (Siemens SIRIUS 3RT1, size S6 example)
UC = Universal Coil (works with AC or DC within the rated range). Always confirm against the latest datasheet.
| Coil code | Full MLFB example | Voltage range (UC) | Typical U.S. use |
|---|---|---|---|
| AB36 | 3RT1056-6AB36 |
23–26 V UC | HVAC 24 VAC, PLC 24 VDC (check tolerance) |
| AF36 | 3RT1056-6AF36 |
110–127 V UC (~120 V) | Legacy 120 VAC control |
| AP36 | 3RT1056-6AP36 |
220–240 V UC | 208/230/240 VAC control via CPT |
| AV36 | 3RT1056-6AV36 |
380–420 V UC | European imports (rare in U.S.) |
| AR36 | 3RT1056-6AR36 |
440–480 V UC | 480 VAC control (uncommon—verify code) |
Tolerance tip: UC coils typically operate at about 0.8…1.1 × Uc. That’s why a 220–240 V coil will usually work on 208 V control (0.87×). Always verify on the current datasheet for your exact MLFB.
Worked example: Build the full Siemens part number
Your faceplate says 3RT1056-6..6. You measure 232 VAC at A1–A2. Choose coil code AP → final MLFB 3RT1056-6AP36.
Ordering checklist (U.S.)
- Complete MLFB (with coil code).
- Aux contacts needed? (S6 commonly includes 2NO+2NC; add blocks if required.)
- Overload relay sized to motor FLA (e.g., 3RB2 family for S6).
- Coil only vs complete contactor (withdrawable coil kits available).
Safety notes
- Follow LOTO and UL 508A practices. Verify absence of voltage before touching A1/A2.
- Do not over/under-voltage a coil “to test.” It shortens life and affects dropout timing.
Related topics
- IEC vs NEMA contactor ratings: HP vs kW
- How to size a control transformer for 120 VAC or 24 VAC control
Reference
-
Siemens Industry Mall product pages and RS datasheets confirming
…AP36= 220–240 V UC, aux 2NO+2NC, and S6 frame details. Industry Mall -
Datasheet note on UC coil operating range (~0.8–1.1×Uc). Siemens Industry Support
FAQ
Often yes, because UC coils typically allow 0.8…1.1 × Uc. But confirm against the datasheet for your exact MLFB and application.
On SIRIUS S6 frames the coil is withdrawable. If the contactor body is healthy, order the correct coil kit and reuse the frame.
On the control schematic/BOM, by reading the small side label on the coil itself, by safe measurement across A1–A2, or by checking the product page for the full MLFB.
