Wire Ferrules - When to Use, How to Crimp, Insulated vs Non-Insulated, and UL 508A Guidelines

A ferrule is a thin tin coated copper cylinder that is placed over the stripped end of a wire to help prevent strands of wire from not being properly inserted into an electrical terminal.  

In this video we go through UL 29.3.6 A guidelines of UL 508A to determine when you should use ferrules and how to properly crimp them.

UL 508A 29.3.4 of the Wiring Methods section covers the use of ferrules in a UL control panel. 

It states:

  • 29.3.4 A connection to a terminal of a component shall be made by:
    • a) Wire inserted directly into a pressure wire terminal of the component;
    • b) Quick-connect terminal of the component, where the mating part is provided with a dimple, depression, or spring-type connection such that a mechanical snap-action connection is made that does not rely solely upon friction between the two parts;
    • c) Crimped-on pressure terminal connector or closed-loop eyelet;
    • d) Solder terminal specified in 29.3.2;
    • e) Wire-binding screw specified in 29.3.3;
    • f) Open-type eyelet specified in 29.3.5; or
    • g) Wiring ferrule specified in 29.3.6.

G of UL 508A 29.3.4 refers us to 29.3.6 for the specifications of how ferrules are to be used in an industrial control panel stating:

  • 29.3.6 A wiring ferrule shall be:
    • a) Used with stranded copper wire(s) only;
    • b) Terminated in a connector rated for copper wire and rated for the number and size of wire(s) crimped to the ferrule;
    • c) Crimped with an appropriate tool as recommended by the ferrule manufacturer before terminating in a terminal of a component;
    • d) Sized in diameter appropriate for the number of wires and wire size(s) as recommended by the ferrule manufacturer; and 
    • e) Crimped to the wires such that the length of the uninsulated portion of the wires does not result in the reduction of electrical spacings when the ferrule is installed.

Should you use insulated or non-insulated terminals? There is a lot of debate over this but it comes down to a matter of preference.  Insulated terminals offer a larger guide for inserting the wires into the ferrules, are color coded so that it is easier to verify the proper size ferrule has been used, and may help assure minimum a bend radius is held in tight spots.  Non-insulated terminals work fine also as long as properly installed.

Ferrules can be used to make a stranded wire capable of being inserted into a solid wire only electrical connection?  This is a useful feature and one we hear a lot, however we couldn't find a specific reference in the UL 508A guidelines that allows for it.  If you know the correct reference, let us know.

Ferrules can be used to connect more than one wire to a terminal designed for only one wire?  According to 29.3.6.b a "Terminated in a connector rated for copper wire and rated for the number and size of wire(s) crimped to the ferrule".  This seems to insinuate that if as long as a ferrule is designed for more than one wire, then it is acceptable to have multiple wires in a ferrule.  Thee question then becomes, is this multiple wire ferrule connection now a reclassified as a single wire termination point?  We couldn't find a specific reference in the UL 508A guideline that states this.  If you know of this reference, let us know.

Where can I find a ferrule kit?  Here is a good ferrule kit that meets UL 508A guidelines.


Next Steps

Go to the UL 508A Control Panel Lesson series to select your next lesson.  There are also many other Lesson Series on PLC Programming and Industrial Automation.