Strain Reliefs

Strain Relief

The purpose of a strain relief is to protect the connection between portable cords and the equipment they service. The contact point between cables and equipment terminals is a weak mechanical point in the circuit, and it is also the area subject to the most stress from being repeatedly plugged and unplugged (for portable equipment). People also use the terms cable glands, cord grips, and cable fittings interchangeably to refer to strain relief devices.

Strain reliefs divert pushing, pulling, or twisting forces away from a terminal joint. For example, if you stretch a rubber band until it breaks, the break always occurs somewhere between your hands no matter how you stretch it. The same principle applies to strain reliefs. Just replace the rubber band with a portable cord and pretend one of your hands is a terminal joint. The bottom line is that strain gets diverted away from the mechanically weakest point to a more mechanically robust point, such as another portion along the length of the SOOW cable.

Strain Relief is Required for Flexible Cable

NEC article 400.10 states that flexible cable MUST be connected to devices such that tension on the cable does not transmit to the joints or terminals.

Exception: Listed single-pole devices (where there is only one common contact at the connection point) designed to handle such strain and tension at their joints or terminals are permitted for use with flexible cable (e.g. lug connections for car batteries).

Some sort of strain relief is required for flexible cord to protect the terminal connections.

Selecting a Strain Relief

The content we provide is meant to inform you and help support the proper selection and use of strain relief for flexible cables. As always, we recommend you consult a licensed and competent electrician to help you with the sizing and selection of parts for your particular application.>


You will need to know several diameters in order to find the right strain relief fitting for your cable.

Mounting Hole Diameter

This is the first specification to look for or ask about if you run cable through panels or any type of equipment enclosure where the edges of the mounting hole may cut the cable insulation.

You will need to find strain relief with appropriate mounting hole diameter ratings; otherwise, the strain relief will either not fit or fall right out of the mounting hole.

“Mounting hole” is another name for clearance holes, which are cut into equipment enclosures to allow the passage of cables. The NEC requires a protective fitting for cables passing through “holes in covers, outlet boxes, or similar enclosures” where sharp edges or rough surfaces (on thicker enclosures) could damage the cable insulation (article 400.14).

Nominal Outer Diameter (Nom. O.D.) of Your Cable

This is the outer diameter that includes the cable’s jacket insulation. You will need to know this specification to avoid buying strain reliefs for which your cable will be too large or too small.

Nom. O.D.’s that a Strain Relief Will Accept

Strain relief devices will specify one nom. O.D. or a range of O.D.'s. If the strain relief specifies a range of acceptable cable diameters, then choose a strain relief where the nom. O.D. of your SOOW cable is in the upper end of the range.

Construction Material

The strain relief itself needs to be able to withstand corrosion and damage in the operating environment. Thus, whatever the material of the strain relief (e.g. plastic, nylon, nickel-plated brass, stainless steel, or some other material), choose your strain relief device so that it will last the life of the application.

Thread Type

This depends on the thread type of the clearance hole in your equipment. This specification is probably the easiest to select, since it really depends on what part of the world you plan on using your equipment.

NPT (National Pipe Thread)

North American thread standard.

PG (Panzer Gewinde)

European thread standard first developed in Germany.


International standard used around the world.

Thread Adapters

Such adapters can be used to convert between NPT, PG, and metric.

Dome or Flex Nut Fittings

These are the most commonly used types of strain relief but not the only ones available. Some strain reliefs do not require nuts or a thread type at all. For example, for suspended cables, you may want a drop support cord grip.

Dome Style

The name comes from the dome shaped nut screwed onto the end where the cable enters the cord grip. Dome style strain reliefs protect cables from cuts and abrasion from the mounting hole, meeting the requirement of NEC 400.14.

Flex Style

Flex nut fittings also protect cables entering equipment panels but provide extra protection against kinking by restricting how much the cable can bend at the point where it enters the equipment enclosure.

Straight or 90° Angle (Snap Elbow) Body

If space restrictions require you to have cable entering equipment enclosures at a 90° angle, then you will need strain reliefs that allow for a 90° bend while still protecting the cable from kinking. Snap elbow cord grips allow you to easily insert the cable straight into the strain relief and snap into a 90° angle after the cable is pulled all the way through.

Both dome and flex nut fittings are available for straight and 90° body styles.

Single or Multi-hole Strain Reliefs

Multi-hole strain reliefs are most applicable to SOOW cable, which has at least two conductors, because these strain reliefs are designed to hold multiple conductors. These may come with either a dome or flex nut.

Protection Type

This specification refers to the degree of protection offered by the seal formed by the strain relief with the mounting hole. The mounting hole is cut into the enclosure, so clearly, your choice of strain relief or cable gland depends on how good of a seal you want between the strain relief, the cable, and the mounting hole. This is assuming you have first selected strain reliefs with the correct diameters (i.e. mounting hole diameter and nom. O.D.'s).