Siemens 3RT2027-1BB40 — 32A AC-3 Contactor Selection Guide

By Abdullah Zahid
12 min read
Siemens 3RT2027-1BB40 SIRIUS S0 power contactor 32A 24V DC screw terminal for three-phase motor control

Siemens 3RT2027-1BB40 Power Contactor, AC-3e/AC-3, 32 A, 15 kW / 400 V, 3-Pole, 24 V DC, Auxiliary Contacts 1 NO + 1 NC, Screw Terminal — Specs, Ratings and Selection Guide

When a controls engineer is specifying a power contactor for a 15 kW three-phase motor starter and the control architecture runs on 24 V DC logic, the Siemens 3RT2027-1BB40 lands on the shortlist almost every time. This SIRIUS family S0-size contactor carries a 32 A AC-3 rating at 400 V, integrates mirror contact technology for ISO 13849 Type 3 safety compliance, and terminates via screw connections — making it a direct fit for PLC-driven direct-on-line motor starter panels across manufacturing, water treatment, HVAC, and material handling applications worldwide. If you are at the confirmation stage and just need to verify the part number before ordering, check current pricing and availability for the 3RT2027-1BB40 at LeadTime.ca — ships worldwide.

Who Should Buy the 3RT2027-1BB40 — and Who Shouldn't

The Siemens 3RT2027-1BB40 is the correct contactor when all five of the following conditions apply to your application:

  • Your control circuit operates at 24 V DC — not 24 V AC, 110 V AC, or 230 V AC
  • Your motor full-load current does not exceed 32 A under AC-3 (induction motor) duty
  • Your motor power is 15 kW or less at 400 V (or up to 18.5 kW at 690 V)
  • Your panel wiring scheme requires 1 NO + 1 NC auxiliary contacts for PLC feedback and interlocking
  • Your control cabinet uses screw terminal field wiring — push-in and crimp-only terminations are not compatible with this model

If your motor exceeds 32 A AC-3 or 15 kW at 400 V, this model is undersized for your load. The 3RT2028-1BB40, rated at 40 A and 18.5 kW at 400 V, is the correct next step up in the SIRIUS family. If your control system runs on an AC coil supply, the 3RT2027-1BB40-0UA0 NEMA variant is the appropriate alternative.

On this page:

What the 3RT2027-1BB40 Actually Does in a Motor Control System

The Siemens 3RT2027-1BB40 is an electromechanical power contactor — the automatic switching device that sits between a three-phase power supply and a motor or industrial load. Where a manual circuit breaker requires a person to open or close the circuit, the 3RT2027-1BB40 does this under remote command from a 24 V DC signal, typically the digital output of a PLC, a relay, or a sensor-driven logic circuit.

At its core, the device operates three main poles simultaneously when the 24 V DC coil is energized. Each main pole is rated at 32 A under AC-3 duty — the IEC category that governs the most demanding regular switching condition for squirrel-cage induction motors: full-voltage starting and switching off during normal run. The 1 NO + 1 NC auxiliary contacts provide the control system with a verified switching status signal, not just a command echo. This distinction matters in safety-critical applications where the PLC needs confirmation that the contactor has actually opened before permitting maintenance access.

The S0 compact form factor keeps panel real estate consumption low. At a weight of 0.639 kg, it mounts upright or horizontally on a standard DIN rail without requiring dedicated support brackets. The screw terminal design allows direct field wiring without specialised tooling.

Typical System Architecture: Where the 3RT2027-1BB40 Sits in the Signal Chain

In a standard DOL motor starter panel, the 3RT2027-1BB40 occupies the power switching position between the motor protective circuit breaker and the motor terminals — controlled entirely from the low-voltage logic side of the panel.

  • Three-phase supply (400 V or up to 690 V) enters through a motor protective circuit breaker or fused disconnect upstream of the contactor
  • The 3RT2027-1BB40 main terminals (L1, L2, L3 input; T1, T2, T3 output) carry the switched motor current at up to 32 A AC-3
  • A thermal overload relay is mounted downstream of the contactor output to protect the motor from sustained overcurrent — the contactor itself does not provide motor protection
  • The coil terminals (A1 positive, A2 negative) connect to a 24 V DC output from the PLC or control relay, completing the command circuit
  • Auxiliary contacts (13-14 NO feedback; 23-24 NC interlock) return status signals to PLC digital inputs, closing the confirmation loop

Industries and Applications Where the 3RT2027-1BB40 Performs

In water treatment and municipal utilities, the 3RT2027-1BB40 is a standard choice for centrifugal pump DOL starters. A level sensor triggers a PLC digital output, which energizes the 24 V DC coil to start a 15 kW three-phase pump. The 1 NC auxiliary contact confirms de-energization before maintenance personnel can access the pump housing — a direct application of the ISO 13849 Type 3 safety rating.

In soft-start motor acceleration setups, the 3RT2027-1BB40 frequently serves as a bypass contactor. The soft-start unit reduces inrush current during ramp-up, and once the motor reaches full speed, the soft-start unit signals the contactor to close, bypassing the soft-start electronics for normal run. This reduces thermal load on the soft-start module and extends its service life.

Conveyor belt drive control across manufacturing and material handling applications regularly uses this model. The 32 A rating covers standard 11 kW to 15 kW three-phase conveyor drive motors, the 24 V DC coil interfaces directly with PLC output cards without a control transformer, and the compact S0 footprint fits the densely-packed panels typical of conveyor line control cabinets.

Compressed air and industrial vacuum systems with 11-15 kW motor loads also fall within the 32 A AC-3 rating, making the 3RT2027-1BB40 a standard fit for pneumatic system motor switching in chemical processing and food and beverage plants where safety-certified switching is a compliance requirement.

Application Typical Deployment
Centrifugal pump DOL starter Level sensor → PLC digital output → 3RT2027-1BB40 coil → 15 kW pump motor; NC auxiliary confirms de-energization
Soft-start bypass contactor Soft-start unit ramps motor; 3RT2027-1BB40 closes on ramp-complete signal to bypass soft-start for run state
Conveyor belt drive control PLC 24 V DC output energizes contactor to start 11-15 kW three-phase conveyor drive motor
Compressed air motor switching Pressure switch or PLC triggers 3RT2027-1BB40 to start compressor motor in pneumatic system
Multi-motor interlock system Two contactors cross-interlocked via NC auxiliary contacts to prevent simultaneous motor operation

Purchase-Decision Specifications: 3RT2027-1BB40 Electrical and Physical Ratings

Parameter Value
Rated current (AC-3 at 400 V) 32 A
Operating power at 400 V 15 kW
Operating power at 690 V 18.5 kW
AC-1 rated current at 400 V 50 A
Maximum main circuit voltage 690 V
Coil voltage 24 V DC
Minimum coil pickup voltage 19.2 V DC
Auxiliary contacts 1 NO (10 A / 250 V AC / 28 V DC) + 1 NC (10 A / 250 V AC / 28 V DC)
Form factor / Weight Size S0 / 0.639 kg
Terminal type / Mounting Screw terminal / Upright or horizontal DIN rail

Full technical specifications are available on the product page at LeadTime.ca.

3RT2027-1BB40 vs. 3RT2028-1BB40: Which Current Rating Do You Actually Need?

Feature 3RT2027-1BB40 3RT2028-1BB40
AC-3 current rating 32 A 40 A
Operating power at 400 V 15 kW 18.5 kW
Coil voltage 24 V DC 24 V DC
Auxiliary contacts 1 NO + 1 NC 2 NO + 2 NC
Form factor Size S0 Size S0
Safety rating ISO 13849 Type 3 / IEC 61508 Type A ISO 13849 Type 3 / IEC 61508 Type A
Relative price Standard Approximately 10-15% higher

If your motor full-load current exceeds 32 A or your application requires four auxiliary contacts for more complex interlocking schemes, the 3RT2028-1BB40 is the correct upgrade. For 15 kW and below at 400 V, the 3RT2027-1BB40 is the right-sized, lower-cost choice — check current stock and pricing at LeadTime.ca.

Expert Verdict: Honest Assessment for Controls Engineers and Procurement Teams

The Siemens 3RT2027-1BB40 earns its place on the BOM for 15 kW and below three-phase motor applications where 24 V DC control is already present in the panel. The mirror contact design — where both the main contact and the auxiliary NC contact are mechanically linked so they cannot both show a closed state simultaneously — provides ISO 13849 Type 3 compliance without adding a separate safety relay module to the panel. That translates to real cost and space savings in machinery directive-compliant builds. The 200,000-cycle service life rating and 20-year design life make it a reliable long-term component, and the SIRIUS family's global distribution network means spare parts sourcing does not become a maintenance crisis years down the line.

Where this model has real limits: the 32 A AC-3 ceiling is firm. If a motor's full-load current on the nameplate is within a few amperes of 32 A, and the application involves frequent starts, elevated ambient temperature, or voltage sag conditions, the 3RT2028-1BB40 at 40 A is the safer specification choice. The 3RT2027-1BB40 also does not include integrated motor protection — a thermal overload relay is a mandatory downstream addition, not an option. Engineers who have specified this contactor without a thermal overload relay have created a protection gap that neither the contactor nor the upstream circuit breaker fully covers for sustained motor overcurrent conditions. Additionally, if your control cabinet runs 110 V AC or 230 V AC control circuits, this model's 24 V DC coil is simply the wrong variant — verify coil voltage before ordering.

From a procurement standpoint, the 3RT2027-1BB40 is a stable, non-obsolete SIRIUS family product with in-stock lead times of 1-2 business days at major distributors and factory lead times of 2-4 weeks if stock is exhausted. Volume pricing at 100+ units can reach below CAD 100 per unit. For standard panel builds and OEM production runs, this is a predictable, sourceable component. If this matches your system requirements, view current stock and pricing for the 3RT2027-1BB40 at LeadTime.ca.

For volume pricing, blanket order arrangements, or to confirm lead time before committing to a production schedule, contact the LeadTime.ca team directly — we ship worldwide.

Wiring and Installation Overview

  • Main circuit input terminals (1, 3, 5) accept the three-phase supply; output terminals (2, 4, 6) connect to motor leads — phase sequencing must be verified to prevent reverse motor rotation
  • Coil terminals A1 (positive) and A2 (negative) connect to the 24 V DC control circuit; minimum reliable pickup voltage is 19.2 V DC, so a regulated DC supply with tight output tolerance is required
  • Auxiliary contact terminals 13-14 (NO) connect to PLC status feedback inputs; terminals 23-24 (NC) connect to the interlock circuit — both rated at 10 A maximum at 250 V AC or 28 V DC
  • Screw terminals require correct wire gauge and torque — consult the official Siemens datasheet for terminal torque specifications before assembly; under-torqued terminals are a leading cause of contactor field failures
  • A thermal overload relay must be mounted and wired downstream of the contactor output — the 3RT2027-1BB40 provides no internal motor overcurrent protection

Safety Compliance and Mirror Contact Technology Explained

The 3RT2027-1BB40 carries an ISO 13849-1 Type 3 device classification, which means it is suitable for use in safety-related control functions without requiring supplemental external safety relay hardware for basic interlocking applications. The underlying mechanism is the mirror contact principle, defined in IEC 60947-4-1: the 1 NC auxiliary contact is mechanically forced open whenever the main contacts are closed, and forced closed whenever the main contacts are open. It is physically impossible for both the main contact and the NC auxiliary contact to be simultaneously closed, which allows a PLC safety input to use the NC contact signal as verified proof of motor de-energization rather than a software inference.

The IEC 61508-2 Type A classification additionally confirms that the device's failure modes are well-defined and documented — relevant for engineers who must demonstrate functional safety compliance under the Machinery Directive or PSSR regulations. CE marking confirms conformity to applicable European directives. For North American installations requiring NEMA-format products, the 3RT2027-1BB40-0UA0 variant is available.

What Engineers Get Wrong When Ordering This Contactor

The 3RT2027-1BB40 is a frequently specified part, and the most common ordering errors are also the most preventable. Before submitting a purchase order, verify every item on this checklist:

  1. Verify coil voltage is 24 V DC; do not confuse with 24 V AC or single-phase variants
  2. Confirm AC-3 (induction motor) duty matches application; AC-4 (plugging) requires different model
  3. Check main circuit voltage: this model rated to 690 V; ensure supply voltage does not exceed this
  4. Confirm 32 A AC-3 current rating sufficient for motor full-load current (FLA); do not undersize
  5. Verify motor power does not exceed 15 kW at 400 V or 18.5 kW at 690 V
  6. Ensure 1 NO + 1 NC auxiliary contact configuration matches interlock/feedback wiring scheme
  7. Confirm screw terminal compatibility; do not mix with push-in or crimp-only systems

If any item on this checklist surfaces a mismatch, do not proceed with this part number. Contact the LeadTime.ca team to confirm the correct SIRIUS contactor variant for your specific application before ordering.

Frequently Asked Questions

Can I use the 3RT2027-1BB40 with a 230 V single-phase supply?

No. The 3RT2027-1BB40 is rated for three-phase AC-3 duty across three main poles. Applying a single-phase supply would not energize all three poles equally and creates an overheating and overload risk. A single-phase contactor model is required for single-phase applications.

What happens if the coil pickup voltage drops below 19.2 V DC?

Below the 19.2 V DC minimum pickup threshold, the coil may fail to fully seat the main contacts, resulting in contactor chatter, incomplete switching, or failure to energize. A regulated 24 V DC power supply with a stable output is required — avoid unregulated or heavily loaded supplies where output can sag under transient conditions.

Is an external thermal overload relay required, or does this contactor include motor protection?

A separate thermal overload relay is mandatory. The 3RT2027-1BB40 is a contactor only — it switches the motor on and off but provides no protection against sustained motor overcurrent. Without a downstream thermal overload relay, the motor is unprotected against locked-rotor or overload conditions that the upstream circuit breaker will not respond to quickly enough to prevent motor winding damage.

Can I wire two 3RT2027-1BB40 contactors in parallel to handle 64 A?

No. Paralleling contactors creates current imbalance between the parallel paths due to slight differences in contact resistance and switching timing, resulting in uneven contact wear and premature failure of one unit. For loads exceeding 32 A AC-3, the correct approach is to select a higher-rated single contactor such as the 3RT2028-1BB40 at 40 A, or to use separate individual contactors for separate motors.

How do I confirm the auxiliary contacts are wired correctly for PLC feedback?

With the contactor de-energized, the NO contact terminals 13-14 should show open circuit and the NC contact terminals 23-24 should show continuity. When the coil is energized with 24 V DC at A1/A2, those states reverse. If the PLC input does not register the state change, check terminal seating and verify the PLC input threshold is compatible with 24 V DC logic levels.

What is the rated service life of the 3RT2027-1BB40?

The 3RT2027-1BB40 has a design life of 20 years and a rated mechanical and electrical operating cycle count of 200,000 cycles under specified conditions. Applications with very high start/stop frequency — such as multiple motor starts per hour — should account for accumulated cycle count in their maintenance planning and schedule proactive replacement before the cycle limit is approached.

Why Order the 3RT2027-1BB40 from LeadTime.ca

  • Global shipping on all orders — no geographic restrictions on sourcing
  • In-stock lead times of 1-2 business days for standard SIRIUS contactor inventory
  • Volume and OEM pricing available on request for panel builder and production run quantities
  • Direct access to hard-to-find SIRIUS family variants and accessories through established distributor channels
  • Technical team available to confirm part numbers and compatibility before you commit to an order

At-a-Glance Summary

  • Model: Siemens 3RT2027-1BB40 — SIRIUS family 3-pole power contactor, Size S0
  • Main current rating: 32 A AC-3 at 400 V; 50 A AC-1 at 400 V
  • Motor power: 15 kW at 400 V; 18.5 kW at 690 V
  • Coil voltage: 24 V DC; minimum pickup voltage 19.2 V DC; release voltage 4.8 V DC
  • Auxiliary contacts: 1 NO + 1 NC, each rated 10 A at 250 V AC or 28 V DC
  • Maximum main circuit voltage: 690 V
  • Safety compliance: ISO 13849-1 Type 3; IEC 61508-2 Type A; IEC 60947-4-1 mirror contact
  • Terminal type: Screw; mounting: upright or horizontal DIN rail
  • Weight: 0.639 kg; rated service life: 20 years / 200,000 operating cycles
  • Typical in-stock lead time: 1-2 business days; factory lead time if backorder: 2-4 weeks
  • Not for: AC coil systems, AC-4 plugging duty, motors above 32 A FLA, single-phase applications