How to Select a Wire Ferrule?

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The industrial control panel is like a car.

In both cases nearly anyone can gather the required parts and wrench together a working model. In both cases, there are hard-learned lessons implemented as regulations to consider if we are to safely operate in the presence of other people.

We will be tempted to select a ferrule based only on the metal length and inside diameter as suggested in Figure 1. That would be a mistake as we should first consider the regulations, specifically Underwriters Laboratories (UL) standard 508A. This standard is applicable to industrial control panels designed for general industrial use. The standard has a significant focus on high power electrical distribution with distinctions between feeder and branch circuit. The standard extends to seemingly mundane decisions such as wire termination using wire ferrules.

The standards are important especially if you work in a UL 508A certified panel shop. An incorrectly selected component could cause an inspector to reject the panel with costly and time-consuming rework. Just like an uncertified car on the open highway, there are questions of safety, reliability, with serious liability consideration for an uncertified control panel. Mistakes could hurt people or result in fire.

Figure 1: Measuring ferrule metal length and inside diameter of a ferrule.

Code applicability to ferrules

The first and perhaps most important consideration when selecting a ferrule is the crimp tool. Understand that the wire, ferrule, and crimp tool operate as a unit. For compliance, all three must be selected in accordance with manufacturer’s specifications. From a practical perspective, the ferrule and crimp tool operate as a matched set for a particular wire gauge. This is essential to ensure mechanical integrity of the crimp and a low resistance connection.

For further information about UL 508A you could purchase the standard from the source, become a Manufacturer’s Technical Representative (MTR), or search for freely available resources such as:

Tech Tip: UL recognized components may be determined using DigiKey tools, the datasheets for specific components, or by using the authoritative UL Product iQ (ulprospector.com). With respect to wire ferrules, the UL Product iQ lists 166 results roughly associated with independent manufacturers. Each for each manufacturer, we find lists of bare, covered, and covered twin ferrules. Be sure to search the additional information for all UL 508A associated components. For example, at the time of this writing, there are 2661 results for keyword circuit breaker.

Ferrule to block connections

The next step is to determine the things to which your ferruled wire will connect. For example, consider the Schneider Electric TM241CEC24U PLC.

Like nearly all such devices, it includes instruction for wiring the device. In this example we locate the data in the Modicom M241 User Guide. As shown in Figure 2, we may use stranded copper wire or ferrules. A single wire is allowed for each connection point unless a twin ferrule is used. The diagram in Figure 2 tells us to strip the wire ends to 9 mm. Similar information can be located for devices such as terminal blocks and switches.

Figure 2: Schneider Electric Modicom PLC rules for spring terminal block connections.

Figure 2: Schneider Electric Modicom PLC rules for spring terminal block connections.

#Selecting a Particular Ferrule

At this point we will assume the desired wire gauge is known along with the required insulating rating. Here the voltage rating determines the thickness of the insulation. To simplify this discussion, let’s assume the wire is known beforehand as the wire selection process is outside the scope of this article.

Let’s summarize what we know:

  • the ferrule and the crimp tool are linked together as a single unit under the rule that we must follow the manufacturer’s guidelines.

  • the length of the exposed metal is determined by the things to which the ferrule will be connected.

  • the wire gauge and insulation thickness have been determined.

At this point, we can select components while continually looking back to the UL Product iQ. Using the DigiKey parametric search engine, we can look for insulated ferrules with a 10 mm pin length suitable for 20 AWG wire. There are 22 results in the search provided by 8 unique manufacturers as shown in Figure 3.

Figure 3: DigiKey Search engine results for the 20 AWG wire ferrule.

Moving on, let’s select the Weidmüller ferrule as an example. The chosen 9025870000 ferrule is listed in the UL Product iQ as shown in Figure 4.

Figure 4: The representative Weidmüller ferrule is listed in the UL Product iQ database.

At this point we can verify that the insulation is small enough to fit within the orange collar. The final step is to locate a suitable crimp tool. This crimp tool selection process is complicated by Weidmüller’s recommendation to match the crimp style to the connection point. Crimp tools are available for trapezoidal, hexagonal, square, and WM profiles as described in an informative white paper. Figure 5 is an informative section of that white paper. DigiKey offers many of these specialized Weidmüller crimp tools.

Figure 5: Crimp types as described in a Weidmüller white paper.

Figure 5: Crimp types as described in a Weidmüller white paper.

Conclusion

There are many things to consider when selecting wire ferrules especially if you work within a UL 508A certified panel shop. As shown in this article, the ferrule selection is embedded into many decisions – the ferrule must live with the larger construct and certification of the industrial panel. As complex as this article has been, please know that there are deeper and nuanced factors that enter into the control panel design.

Best Wishes,

APDahlen

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