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Stranded wire is flexible and easy to route in a control panel, but it creates problems when you insert it bare into a terminal block. The individual strands spread apart under the clamp, some may break, and loose strands can touch adjacent terminals. A ferrule solves all of this by turning the stranded bundle into a solid, uniform conductor. LANZ Manufacturing supplies wire harnesses, cable assemblies, and terminal blocks that work together for reliable industrial connections. This guide covers what ferrules do, how to pick the right size, and how to crimp them correctly.
Ferrule Crimping 101: Why Stranded Wire Needs a Ferrule
What Happens When You Put Stranded Wire in a Terminal Bare
The Splaying Problem and Uneven Contact
When you insert bare stranded wire into a screw terminal and tighten the screw, the strands do not stay neatly bundled. They spread outward under the pressure plate. Some strands move to the side of the clamp instead of under it. The ones that remain under the plate carry all the current.
This uneven contact means the effective cross-section of the connection is smaller than the wire rating. A 2.5 square millimeter wire might make contact through only 1.5 square millimeters of copper. The reduced contact area increases resistance, which generates heat at the terminal point. Over time, this can damage the terminal block insulation and create a fire hazard in high-current circuits.
Loose Strands and Short-Circuit Risk
The strands that spread sideways present another danger. In a dense terminal strip where blocks sit side by side on a DIN rail, a stray strand can touch the adjacent terminal. This creates a short circuit between two circuits that should be isolated. The result is a blown fuse at best and equipment damage at worst.
Push-in terminals are even less forgiving. Bare stranded wire simply will not insert cleanly into a push-in port. The strands catch on the spring fingers, bend backward, and fail to make a proper connection. If your design uses push-in terminal blocks, ferrules are not optional for stranded wire. They are mandatory.
Application detail — Lanz Manufacturing
What a Ferrule Actually Does
Bundling Strands Into a Solid Conductor
A ferrule is a thin-walled metal tube, usually made of tin-plated copper, that slides over the stripped end of a stranded wire. When you crimp the ferrule with the correct tool, it compresses the strands into a solid, uniform cylindrical shape. The crimp is tight enough that the individual strands cannot move relative to each other.
The result is a wire end that behaves like a solid conductor. It inserts cleanly into any terminal type. The entire cross-section of the wire makes contact with the terminal clamp. There are no stray strands to manage. The connection has low, consistent resistance across the full rated current range of the wire.
Gas-Tight Connection and Vibration Protection
A properly crimped ferrule creates a gas-tight bond between the strands and the ferrule wall. Oxygen and moisture cannot reach the copper-to-copper contact points inside the crimp. This prevents oxidation that would otherwise increase resistance over years of service.
The ferrule also protects against vibration. Individual strands in a bare wire termination can work-harden and break under constant vibration. The ferrule keeps all strands locked together so the wire flexes as a single unit at the point where it exits the crimp. This dramatically extends the life of the connection in motor control centers, pump panels, and any equipment subject to mechanical vibration.
How to Pick the Right Ferrule Size
- 🎯Ferrules are sized by the cross-sectional area of the wire they accept, given in square millimeters. The ferrule package or barrel will show a marking like “1.5 mm²” or “2.5 mm²”. Match this number to the wire cross-section. A 1.5 mm² ferrule fits wire rated at 1.5 square millimeters.
- 🔧The ferrule length also matters. The stripped portion of the wire must reach the bottom of the ferrule barrel. The insulated collar, if present, should cover the un-stripped portion of the wire insulation. Most manufacturers publish a strip-length table that tells you how much insulation to remove for each ferrule size. Standard strip lengths range from 8 mm for small ferrules to 18 mm for larger ones.
- 📋Insulated ferrules have a plastic collar at the entry end. The collar provides strain relief by supporting the wire where it exits the ferrule. It also provides a visual indicator of proper insertion depth. The collar color follows the DIN 46228 standard, with each color mapping to a specific wire cross-section range.
- 🛠️Uninsulated ferrules are just the metal tube without the plastic collar. They are used in applications where space is tight or where the terminal block itself provides the insulation. Twin ferrules accept two wires side by side in a single ferrule. They are used for jumpering or for doubling up wires that terminate in the same terminal, such as daisy-chained power distribution in a control panel.
Crimping a Ferrule in Four Steps
Strip, Insert, Crimp, Inspect
Step one is stripping. Use a wire stripper set to the correct gauge. Strip the insulation to the length specified for your ferrule size. A clean strip with no nicked strands is critical. Nicks create weak points where strands can break during crimping or after installation.
Step two is insertion. Slide the ferrule onto the stripped wire end until the conductor bottoms out inside the barrel. For insulated ferrules, the wire insulation should enter the plastic collar but not extend into the metal barrel. You should see no bare copper between the ferrule collar and the wire insulation.
Step three is crimping. Place the ferrule into the correct cavity of your crimp tool. Squeeze the tool handles fully until the ratchet releases. The tool applies a precisely shaped deformation that compresses the ferrule barrel onto the wire strands. Using the wrong cavity or stopping mid-crimp both produce a weak connection.
Step four is inspection. Give the ferrule a gentle tug. It should not slide off. Look at the crimp profile. It should show a clean, symmetrical deformation with no cracks in the ferrule wall. The wire should be visible at the tip end of the ferrule, confirming that it bottomed out during insertion.
Common Crimping Mistakes and How to Avoid Them
The most common mistake is using the wrong crimp cavity. Each cavity is sized for a specific ferrule cross-section. Using a cavity that is too large leaves the ferrule loose. Using one that is too small can crack the ferrule wall or cut through individual strands.
Another frequent error is stripping too little or too much insulation. Too little means copper does not reach the bottom of the barrel. Too much leaves exposed copper between the ferrule and the insulation, which defeats the purpose of the insulated collar. Always use a strip-length gauge or mark your stripper to the correct length for the ferrule series you use.
Ferrules vs Tinning vs Bare Wire
Why Tinning Is Not a Substitute for Ferrules
Some technicians tin stranded wire with solder before inserting it into a terminal. The idea is that the solder holds the strands together. This practice is discouraged by most terminal block manufacturers and is explicitly warned against in UL 508A for industrial control panels.
Solder is soft and creeps under pressure. When a screw terminal clamps down on a tinned wire end, the solder slowly deforms over time. The connection loosens, resistance rises, and the joint fails. A properly crimped ferrule avoids this problem because the tin-plated copper ferrule is harder than solder and does not creep under normal clamping force.
When Bare Stranded Wire Is Acceptable
Bare stranded wire is acceptable in screw clamp terminals that are specifically rated for it. Many terminal blocks have a pressure plate design that captures strands without cutting them. The manufacturer datasheet will state whether the terminal accepts bare stranded wire and what the minimum and maximum cross-sections are.
Spring clamp terminals also accept bare stranded wire, but you must open the spring fully before insertion. If you try to force stranded wire into a partially open spring clamp, the strands will splay. Push-in terminals do not accept bare stranded wire at all. For push-in connections, ferrules are required for any stranded conductor.
LANZ Cable and Terminal Solutions
LANZ produces custom wire harnesses and cable assemblies with ferrules pre-crimped to your specification. Every ferrule is crimped on calibrated industrial tooling with a four-indent or hexagonal crimp profile that meets DIN 46228 standards. The harness arrives ready to install.
For panel builders who want to eliminate onsite crimping entirely, LANZ Wire Harness assemblies ship with ferrules, labels, and connectors installed. You unbox the harness, mount it in the panel, and land the connections.
Terminal Blocks That Work Best With Ferrules
LANZ push-in terminal blocks and spring clamp terminal blocks both accept ferruled stranded wire directly. The ferrule provides a rigid, uniform surface that the spring fingers can grip securely. The result is a gas-tight connection that stays reliable across thousands of thermal cycles.
For screw terminal blocks, ferrules offer the same benefits. The screw clamp bites into a solid cylindrical surface instead of splayed strands. Torque transfers evenly across the full wire cross-section. LANZ recommends ferrules for any stranded wire termination in panels that will operate for more than five years without rewiring.
Frequently Asked Questions
Q: What is the difference between a bootlace ferrule and an insulated ferrule?
There is no difference. Bootlace ferrule is the British English term for what North American manufacturers call an insulated ferrule. Both refer to a thin-walled metal tube with a plastic insulating collar that slides over stranded wire and is crimped in place.
Q: Do I need ferrules for screw terminals?
Ferrules are recommended but not always required for screw terminals. Many screw terminal blocks accept bare stranded wire if the pressure plate design captures all strands. Check the terminal block datasheet. For high-vibration environments or when using fine-stranded wire with many thin strands, ferrules provide a more reliable connection.
Q: What crimp tool do I need for ferrules?
Use a ratcheting crimp tool designed for ferrules. These tools have color-coded cavities that match DIN-standard ferrule sizes. The ratchet mechanism will not release until the full crimp cycle completes, which prevents under-crimping. Square-profile and hexagonal-profile tools are both acceptable. Avoid using pliers or generic crimp tools.
Q: Can I reuse a crimped ferrule?
No. Once a ferrule has been crimped, the barrel is permanently deformed. If you need to re-terminate a wire, cut off the old ferrule, re-strip the wire, and crimp a new ferrule. The wire must also be trimmed back because the previously crimped portion may have weakened strands.
Q: What happens if I use the wrong ferrule size?
Using a ferrule that is too large will not compress the strands fully. The wire may pull out and the connection will have high resistance. Using a ferrule that is too small will not fit over the wire, or it may cut strands during crimping. Always match the ferrule cross-section to the wire cross-section exactly.
Talk with Lanz Manufacturing about specifications, samples, and production support.
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