Molex Mini-Fit Jr. Connector: Complete Guide to Part Numbers, Specs, Crimping & High-Power Applications
If you have ever opened a desktop PC power supply, a server chassis, or an industrial motor driver, you have almost certainly touched a Molex Mini-Fit Jr. connector without knowing its name. The white, dual-row housing with its distinctive 4.20mm pitch and positive terminal latch is one of the most widely deployed power interconnects in the world — the 24-pin ATX connector in every desktop PC is a Mini-Fit Jr. 5557-24R.
This guide consolidates everything: the full electrical and mechanical specification, the part-number logic for housings and headers, compatible crimp terminals, tooling requirements, and the application contexts where Mini-Fit Jr. outperforms alternatives. Ready to source? Shop Molex Mini-Fit Jr. 4.2mm connectors at Keszoox.
What Is the Molex Mini-Fit Jr.?
The Mini-Fit Jr. is a wire-to-board and wire-to-wire power connector family manufactured by Molex (series 5557 / 5559). It was engineered specifically for applications demanding high current density in a compact dual-row footprint — a combination that made it the de-facto standard for ATX motherboard power connectors.
- 4.20mm pitch (center-to-center terminal spacing)
- Dual-row housing
- Positive latch on the receptacle that locks to the header
- Natural (white) or black nylon housing, UL 94V-0 rated
- Circuit counts from 2 to 24
Key Electrical & Mechanical Specifications
Electrical
| Parameter | Value |
|---|---|
| Maximum current per circuit | 13A (45750 terminals, AWG 16, header mated) |
| Voltage rating | 600V AC/DC (UL), 250V AC/DC (CSA) |
| Contact resistance | 10 mΩ initial |
| Insulation resistance | 1,000 MΩ min |
| Dielectric withstanding voltage | 1,500V AC (1 min) |
Mechanical
| Parameter | Value |
|---|---|
| Pitch | 4.20 mm |
| Number of rows | 2 |
| Max circuits (dual-row) | 24 |
| Housing material | Nylon (PA66), UL 94V-0 |
| Operating temperature | −40°C to +105°C |
| Mating cycles | 30 minimum |
| Polarized mating | Yes (keyed housing) |
Agency Approvals
| Agency | File Number |
|---|---|
| UL | E29179 |
| CSA | LR19980 |
Part Number System: 5557 vs 5559 vs 5566 vs 5569
| Series | Component Type | Orientation | Typical Use |
|---|---|---|---|
| 5557 | Receptacle Housing | Free-hanging (wire side) | Harness / cable assembly |
| 5559 | Plug Housing | Free-hanging (wire side) | Wire-to-wire |
| 5566 | Vertical PCB Header | Vertical through-hole | Board-mount, vertical entry |
| 5569 | Right-Angle PCB Header | Right-angle through-hole | Board-mount, horizontal entry |
Common Circuit Count Reference
| Circuits | Typical Application |
|---|---|
| 4 | CPU auxiliary power (P4) |
| 6 | PCIe 6-pin power |
| 8 | EPS12V / CPU 8-pin |
| 12 | PCIe 12-pin |
| 18 | Industrial / custom harness |
| 24 | ATX 24-pin main power |
Pro tip: The 24-pin ATX connector in every desktop PC is a Mini-Fit Jr. 5557-24R — billions of units shipped annually.
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Compatible Crimp Terminals: 5556, 45750 & 44476
| Terminal Series | Type | Wire Range | Max Current | Notes |
|---|---|---|---|---|
| 5556 | Mini-Fit Female Crimp | AWG 18–24 | ~9A | Standard, most common |
| 45750 | Mini-Fit Plus Female Crimp | AWG 16–20 | 13A | Higher current |
| 44476 | Mini-Fit HCS Female Crimp | AWG 18–22 | ~9A | High-cycle-life variant |
- For 9A or less (AWG 18–24): Use series 5556
- For 9–13A (AWG 16–20): Use series 45750 (Mini-Fit Plus)
- For high-cycle or harsh environments: Use series 44476 (HCS)
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Compliance & Environmental Certifications
| Standard | Status |
|---|---|
| EU RoHS (2015/863) | Compliant |
| China RoHS (SJ/T 11365-2006) | Compliant |
| REACH SVHC (D(2025)7771-DC, 04 Feb 2026) | Not Contained |
| Low-Halogen (IEC 61249-2-21) | Compliant |
| IPC 1752A Class C & D | Supported |
| IEC-62474 | Supported |
The Low-Halogen certification per IEC 61249-2-21 is particularly significant for server and telecom applications. The REACH SVHC clearance was updated February 2026.
Crimping Guide
Step 1 — Select the Correct Terminal: Match your wire gauge to the terminal series. 5556 for AWG 18–24 (up to 9A); 45750 for AWG 16–20 (up to 13A).
Step 2 — Strip the Wire: Strip 2.5–3.0mm of insulation. Do not nick conductor strands.
Step 3 — Crimp the Terminal: Use Molex crimp tool 638190200A. Crimp height for 5556 on AWG 20 = 1.52 ± 0.05mm; for 45750 on AWG 16 = 1.78 ± 0.05mm. Pull-force minimum: 40N (5556/AWG 20), 80N (45750/AWG 16).
Step 4 — Load Terminal into Housing: Insert from the rear until you hear a positive click.
Step 5 — Verify Polarity: Housings are keyed — only one orientation is possible.
Step 6 — Mate to Header: Press firmly until the latch clicks. To unmate, depress the latch tab — never pull by the wire.
Current Derating by Wire Gauge (Series 5556)
| Wire Gauge (AWG) | Max Current (A) |
|---|---|
| 18 | 9.0 |
| 20 | 7.0 |
| 22 | 5.0 |
| 24 | 3.5 |
The Complete Mating System
PCB Headers (Board Side)
| Series | Orientation | Notes |
|---|---|---|
| 5566 | Vertical | Standard through-hole, vertical wire exit |
| 5569 | Right-Angle | Through-hole, horizontal wire exit |
| 87427 | Dual Row Header | High-density board-mount |
| 42404 | BMI Right-Angle Dual Row | Board-mount interface variant |
| 42440 | BMI Vertical Dual Row | Board-mount interface variant |
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Real-World Applications
PC Power Supplies (ATX Standard)
- 24-pin ATX main connector — 5557-24R to 5566-24R vertical header
- 8-pin EPS12V (CPU power) — 5557-8R receptacle
- 6-pin PCIe power — 5557-6R receptacle
- 6+2 pin PCIe — two housings for GPU power flexibility
Industrial Power Distribution
- Motor driver power inputs (24V / 48V DC bus)
- PLC power distribution boards
- Industrial UPS internal wiring
- Robotics joint power harnesses
Telecommunications & Server Infrastructure
Low-Halogen certification and REACH compliance make Mini-Fit Jr. suitable for telecom equipment where halogen-free materials are mandated.
EV Charging & Automotive Auxiliary
Used in EV charging station internal wiring, BMS auxiliary circuits, and DC-DC converter power inputs. Note: not automotive-grade (no LV214 or USCAR certification).
DIY & Maker Community
Popular for high-current 3D printer power wiring, custom PC cable sleeving, and bench power supply harnesses.
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Mini-Fit Jr. vs Micro-Fit 3.0: Which to Choose?
| Parameter | Mini-Fit Jr. (5557) | Micro-Fit 3.0 (43645) |
|---|---|---|
| Pitch | 4.20 mm | 3.00 mm |
| Max current / circuit | 13A | 5A |
| Max circuits (dual-row) | 24 | 24 |
| Wire range | AWG 16–24 | AWG 20–30 |
| Footprint | Larger | Smaller / more compact |
| Typical application | Main power rails, ATX, industrial | Signal + low-power, embedded |
Decision rule: More than 5A per pin or wire coarser than AWG 20 — choose Mini-Fit Jr. Board space constrained and current below 5A — Micro-Fit 3.0 offers 30% smaller footprint.
➡️ Shop Mini-Fit Jr. (4.2mm) | Shop Micro-Fit 3.0 (3.0mm)
Frequently Asked Questions
Is the Molex Mini-Fit Jr. the same as the ATX power connector? Yes. The 24-pin ATX main power connector, 8-pin EPS12V, and 6-pin PCIe power connector are all Mini-Fit Jr. series 5557 receptacle housings mating to 5566 or 5569 PCB headers.
What is the difference between series 5557 and 5559? Series 5557 is the receptacle housing that mates to a PCB header (5566/5569). Series 5559 is the plug housing used for wire-to-wire connections. Both use the same 5556 or 45750 crimp terminals.
Can I use 5556 terminals to achieve 13A? No. The 13A rating requires 45750 (Mini-Fit Plus) terminals crimped to AWG 16 wire. The 5556 terminal is rated for approximately 9A maximum at AWG 18.
What wire gauge for a 10A circuit? Use 45750 terminals on AWG 18 wire for a reliable 10A circuit with adequate margin.
Is Molex Mini-Fit Jr. RoHS compliant? Yes — EU RoHS (2015/863), China RoHS (SJ/T 11365-2006), Low-Halogen per IEC 61249-2-21, and REACH SVHC Not Contained as of February 2026.
How do I release a terminal from the housing? Use a Molex extraction tool to depress the terminal latch through the front of the housing while pulling the wire from the rear. Never pull by the wire alone.
What is the mating cycle life? Minimum 30 mating cycles. For higher cycle requirements, use the 44476 HCS terminal series.
Is Mini-Fit Jr. the same as Mini-Fit BMI? No. Mini-Fit BMI (42404, 42440, 43810, 44068) uses the same 4.20mm pitch but has a different header PCB footprint for board-to-board or board-to-panel applications.
What is the difference between Mini-Fit Jr. and Micro-Fit 3.0? Mini-Fit Jr. is 4.20mm pitch, rated 13A per circuit, for high-current power applications. Micro-Fit 3.0 is 3.00mm pitch, rated 5A per circuit, for compact power and signal applications. Not interchangeable.
What crimp tool do I need for Mini-Fit Jr.? Molex crimp tool 638190200A is the recommended hand tool. For production volume, use the Molex 63819-0900 series with the appropriate die set for 5556 or 45750 terminals.
Summary
The Molex Mini-Fit Jr. is not a niche connector — it is infrastructure. From the ATX power supply in your workstation to the motor driver in an industrial robot, the 4.20mm dual-row housing with its 13A per circuit capacity has become the default answer to high-current, latching, polarized power interconnect requirements.
- Use series 5557 for cable-side receptacles, 5566/5569 for PCB headers
- Match terminals to current: 5556 for 9A or less, 45750 for up to 13A
- Fully RoHS, China RoHS, REACH, and Low-Halogen compliant
- Operating range −40°C to +105°C covers most industrial and consumer applications
- When current exceeds 5A per circuit, Mini-Fit Jr. is the correct choice over Micro-Fit 3.0
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Related guides: Molex Micro-Fit 3.0 Guide | Molex Connector Types Guide | Molex PicoBlade 1.25mm Guide | Wire-to-Board Connector Types Guide