What is the Molex MX1.25 wire-to-wire connector, and how do I specify a replacement?

“I need PicoBlade 1.25mm terminals.”

If that is the whole specification, you have roughly a one-in-two chance of receiving a reel you cannot crimp. Not because the supplier got it wrong — because the PicoBlade 1.25mm wire-to-wire system uses two different families of crimp terminal, one for each half of the pair, and they are not interchangeable.

That single fact is the one that costs the most when it is missed, and it is the reason a wire-to-wire PicoBlade enquiry needs one extra piece of information before anything else can be quoted.

I work on connector and harness programmes at KONNRA, which builds a documented cross-reference to this system (the KR1251). Here is what actually matters about the part.

What “Molex MX1.25 wire-to-wire” actually means

It is the trade shorthand for the Molex PicoBlade 1.25mm-pitch wire-to-wire connector system — the inline half of the PicoBlade family. Molex’s own document for it is Product Specification 510211009-PS, the tin-plated specification.

• 1.25mm pitch, single row, inline — two cable halves mate to each other with no board involved
• 2 to 15 circuits
• 51021 receptacle housing mates with a 51047 plug housing — two different housing series
• 125V AC (RMS)/DC
• 2.0A at AWG #26 · 1.5A at AWG #28 · 1.0A at AWG #30 · 1.0A at AWG #32
• AWG #26 to #32, insulation φ0.5mm to φ1.04mm
• −40°C to +105°C, including terminal temperature rise
• 30 mating cycles in the tin-plated version
• Friction lock with a mating lock, polarised, two-point contact design
• UL File E29179
• Not a panel-mount part, and not for hot-mating

The thing nobody tells you: the two sides use different terminals

Four terminal part numbers, two sides:

• Receptacle side — 50079 series for AWG #26–28, 50058 series for AWG #28–32
• Plug side — 50125 series for AWG #26–28, 50133 series for AWG #28–32

The receptacle terminals are the same parts the wire-to-board PicoBlade uses. The plug terminals are unique to the inline system and exist for no other purpose. A complete inline harness therefore needs both families, and “PicoBlade 1.25mm terminals” does not identify which one you need.

The correct way to specify it is two steps. First pick the pair that matches your wire gauge — then order both the receptacle and the plug version of that pair:

• Heavier wire (AWG #26–28) → 50079 receptacle + 50125 plug
• Finer wire (AWG #28–32) → 50058 receptacle + 50133 plug

AWG #28 is the overlap, and it is where harnesses go wrong. Both the 50079 and the 50058 are specified for AWG #28, and the same is true on the plug side. So at AWG #28 the conductor gauge does not tell you which pair to order. Choose by what the rest of your harness uses: if any circuit runs at AWG #26 or #27, you are in the heavier family; if any circuit runs at AWG #29 or finer, you are in the lighter one.

One simplification worth noting: insulation diameter is a single window across all four terminals — φ0.5mm to φ1.04mm. That is simpler than the wire-to-board side, where the two terminal families had different insulation maxima.

The current rating is four numbers, not one

Molex publishes the wire-to-wire rated current per wire gauge, not as a family figure:

• AWG #26 → 2.0A
• AWG #28 → 1.5A
• AWG #30 → 1.0A
• AWG #32 → 1.0A

Maximum voltage is 125V AC (RMS)/DC.

Molex also publishes a reference derating table by gauge and circuit count, with columns at 2, 6 and 10 circuits:

• AWG #26 — 2.5A at 2 circuits, 2.0A at 6 and at 10
• AWG #28 — 2.0A at 2 circuits, 1.5A at 6 and at 10
• AWG #30 — 1.5A at 2 circuits, 1.0A at 6 and at 10
• AWG #32 — 1.3A at 2 circuits, 1.0A at 6 and at 10

Note Molex’s stated conditions, because they matter: those derating values are marked for reference only, they are based on not exceeding a 30°C temperature rise, the rise is measured in the barrel area of the crimp terminal, the data assumes all circuits powered, and PCB trace design can greatly affect the result. Molex also states that the sum of the current used on several circuits must not exceed the maximum allowable current, on top of the per-circuit figure.

One caution about those columns: the wire-to-wire table uses 2 / 6 / 10 circuits, while the wire-to-board table uses 2 / 8 / 15. The two tables are not interchangeable. Do not carry a wire-to-board derating figure onto an inline design, or the reverse.

Wire-to-wire versus wire-to-board at the same pitch

The two halves of the PicoBlade family share a pitch, a contact system and a housing standard — and they do not share a current rating. Molex publishes them as separate specifications, and KONNRA cross-references the inline half as KR1251 and the board half as KR1250.

• AWG #26 — wire-to-wire 2.0A, wire-to-board 1.0A
• AWG #28 — 1.5A versus 1.0A
• AWG #30 — 1.0A on both
• AWG #32 — 1.0A versus 0.8A

Two consequences. First, the inline version is the more capable one at the heavy end — twice the rating at AWG #26 — which is the opposite of the usual assumption that a cable joint is the weak link. Second, your harness takes the lowest rating in the chain. If a run goes from a board header, through a cable, to an inline joint and on to another board, the wire-to-board legs are the bottleneck at 1.0A, and a 2.0A inline joint in the middle buys you nothing. Work the limit out from the weakest component, not the strongest.

Cross-referencing to the KONNRA KR1251 — and the honest gaps

The KR1251 is documented as KONNRA’s equivalent to the PicoBlade 1.25mm wire-to-wire system. Where the documents agree is worth stating, because on a cross-reference it is not a given: the pitch, the 125V rating, the two-point contact design, the friction lock with a mating lock, and the 250V AC (RMS) one-minute dielectric withstanding voltage all line up. So does the 100MΩ minimum insulation resistance, and the 20mΩ maximum contact resistance — with the crimped portion separately specified at 5mΩ max on both sides.

Where they differ, in the order the differences bite:

• Current rating: four numbers versus one. Molex rates 2.0A / 1.5A / 1.0A / 1.0A by gauge. KONNRA documents a flat 1A — which equals the AWG #30 and #32 rating, understates AWG #28 by a third, and understates AWG #26 by half. The critical gauge is AWG #26: a 2-amp inline design can look like over-specification against a flat 1A figure when in fact it sits inside the original part’s rating. State your gauge and current and have the figure confirmed.
• The published wire range does not reach AWG #26. Molex covers AWG #26 to #32; the published KR1251 range is AWG #28 to #32. AWG #26 is exactly where this system’s advantage sits (2.0A against the wire-to-board half’s 1.0A), so if your joint uses #26, say so explicitly.
• The insulation window is one-sided on the KONNRA side. Molex specifies φ0.5mm to φ1.04mm — a two-sided window. KONNRA documents 1.0mm maximum with no minimum. Insulation below 0.5mm is outside Molex’s specification, and the KR1251 documentation gives no lower bound at all.
• Durability is published by Molex and not by KONNRA. Molex specifies 30 cycles for tin plating — a low figure for an inline joint a technician may separate to route a cable. Molex’s own force table shows withdrawal force on a 2-circuit connector falling from 2.8N at first mating to 1.8N by the 30th cycle.
• Plating. The document reviewed is the tin-plated specification, and Molex publishes a separate gold-plated specification for the same system. KONNRA documents tin over nickel — so the finishes line up on the tin route, but if your design assumed gold, confirm which specification applies.
• The component count is the biggest structural gap. Molex’s inline system is built from two housing series (51021 receptacle, 51047 plug) and four terminal part numbers (50079, 50058, 50125, 50133). KONNRA documents one housing and one terminal for KR1251. A wire-to-wire design has to say which half it is buying — the receptacle side, the plug side, or a matched set.
• Housing material differs between the two halves. Molex lists the 51047 plug housing as polyester (UL94V-0) and the 51021 receptacle housing as polyamide — and notes that the receptacle’s water absorption can change the insertion and withdrawal force and weaken the click feeling of the lock when mating, without damaging function. KONNRA documents a combined PA66 / PBT / UL94 / Phosphor Bronze list for the series without splitting the halves. If your specification names a material for a specific half, confirm which half it applies to.
• Circuit range. Molex’s wire-to-wire rating, derating and force tables run to 10 circuits — narrower than the wire-to-board tables, which run to 15. KONNRA documents KR1251 as 2 to 16 positions. Confirm the count you intend to order.
• Standards. Molex lists UL File E29179. KONNRA’s product documentation states RoHS compliance, with UL certification asserted at brand level. If your qualification requires UL recognised status for this part specifically, ask for the file reference.

Three design cautions worth carrying into your review

Molex publishes nineteen numbered usage notes in the wire-to-wire specification. They are not marketing — they are the known failure modes. Three that matter most on an inline joint:

1. Do not mate or unmate under load. Molex states the product is not designed for mating and unmating under an active electrical circuit, and that doing so may cause a spark and product damage. An inline joint is exactly the kind a technician is tempted to open live — write the service procedure so that it is not.
2. Tie the cable at least 50mm from the connector edge, with the tying force applied evenly across all wires. Molex defines that distance as a free length of wire, so the terminals can float freely in their pockets. On an inline joint both ends can be pulled, so you need a tie at each end.
3. Fit a new housing after extracting a terminal. Extracting a crimp terminal with a jig deforms the housing lance, and Molex states this “can reduce the terminal retention force enormously” after re-insertion.

What to send for a cross-reference check

1. Which half of the pair you need — the 51021 receptacle side, the 51047 plug side, or a matched set. This is the question that most often makes the difference between a correct and an incorrect quote.
2. The original part number if you have it — housing 51021**00 or 51047**00, receptacle terminal 500798**00 or 500588**00, plug terminal 501258**00 or 501338**00
3. Wire gauge, and specifically whether it is AWG #26 — the gauge that is inside Molex’s range and outside the published KR1251 range
4. Insulation outside diameter
5. Circuit count
6. Plating finish required, including whether you assumed gold
7. Connector only, or a finished harness — and if a harness, the length and far-end termination
8. Your load current, so the AWG #26 question and the derating position are settled rather than assumed
9. Application and annual volume
10. A drawing or photo if the part number is unreadable

With those, four things get confirmed against Molex’s own documentation: the side of the pair, the terminal family that matches your wire gauge, the housing series for the half you need, and the plating you require. Those four are where an inline PicoBlade cross-reference most often goes wrong.

One quick check if you are holding the part: measure the pitch. 1.25mm points at PicoBlade, PanelMate, Hirose DF13/DF14 or JST GH — 1.20mm at Pico-EZmate or JST ACH, 1.00mm at Pico-Clasp or JST SH. Then look at the contact geometry: the two-point contact design is the PicoBlade signature. And read the part number — 51021 is the receptacle housing, 51047 the plug housing, 50058 / 50079 the receptacle terminals, 50125 / 50133 the plug terminals.

Full technical write-up: Molex MX1.25 Wire-to-Wire Connector Guide

Disclosure: I work with KONNRA, which manufactures the KR1251 cross-reference. The Molex PicoBlade figures above are taken from Molex’s published Product Specification 510211009-PS (wire-to-wire, tin plating); the KONNRA figures are from our published KR1251 documentation. Where the two disagree, I have flagged the difference rather than averaged it — and where a parameter is not stated in either document, I have said so instead of filling the gap.

https://konnra.com/molex-mx1-25-wire-to-wire-connector-complete-guide/
Dongguan Konnra Electronics Co., Ltd