Give Me A Quarter Inch, But do I need to? Cable Sheathing

Published by Don Hester on February 11, 2020February 11, 2020

We often think we know something then we find out we actually may not know what we think we know. Case in point is the 1/4 inch rule for nonmetallic sheathed cable sheathing into a enclosure, panel enclosure and boxes. 

Well when I started looking in the code what I found is not exactly what I thought… so let dive in.

First, Let us look at the metal box and plastic box requirements first. This is where we have had the 1/4 inch requirement that many recognize but it was only for nonmetallic boxes prior to the 2017 NEC. 

2014/2017 NEC– (I underlined and highlighted the language as reference)

314.17(C) Nonmetallic Boxes and Conduit Bodies.
Nonmetallic boxes and conduit bodies shall be suitable for the lowest temperature-rated conductor entering the box. Where nonmetallic boxes and conduit bodies are used with messenger-supported wiring, open wiring on insulators, or concealed knob-and-tube wiring, the conductors shall enter the box through individual holes. Where flexible tubing is used to enclose the conductors, the tubing shall extend from the last insulating support to not less than 6 mm (¼ in.) inside the box and beyond any cable clamp. Where nonmetallic-sheathed cable or multi-conductor Type UF cable is used, the sheath shall extend not less than 6 mm (¼ in.) inside the box and beyond any cable clamp. In all instances, all permitted wiring methods shall be secured to the boxes.

When installing nonmetallic-sheathed cables, it has been standard practice to make sure at least 1/4 inch of cable sheathing enters the box before stripping off the jacket to make terminations. This has always been a requirement for NM/Romex (nonmetallic-sheathed cables) entering plastic boxes. This requirement was not there for metal boxes until the 2017 NEC where similar code language was introduced for nonmetallic-sheathed cables entering metal boxes and conduit bodies.

2014 NEC Code-

314.17(B) Metal Boxes and Conduit Bodies. 

Where metal boxes or conduit bodies are installed with messenger-supported wiring, open wiring on insulators, or concealed knob-and-tube wiring, conductors shall enter through insulating bushings or, in dry locations, through flexible tubing extending from the last insulating support to not less than 6 mm (1∕4 in.) inside the box and beyond any cable clamps. Except as provided in 300.15(C), the wiring shall be firmly secured to the box or conduit body. Where raceway or cable is installed with metal boxes or conduit bodies, the raceway or cable shall be secured to such boxes and conduit bodies.

2017 NEC Code (Underlined and highlighted the new section)

314.17(B) Metal Boxes and Conduit Bodies.

Where metal boxes or conduit bodies are installed with messenger-supported wiring, open wiring on insulators, or concealed knob-and-tube wiring, conductors shall enter through insulating bushings or, in dry locations, through flexible tubing extending from the last insulating support to not less than 6 mm (1∕4 in.) inside the box and beyond any cable clamps. Where nonmetallic-sheathed cable or multi-conductor Type UF cable is used, the sheath shall extend not less than 6 mm (1∕4 in.) inside the box and beyond any cable clamp. Except as provided in 300.15(C), the wiring shall be firmly secured to the box or conduit body. Where raceway or cable is installed with metal boxes or conduit bodies, the raceway or cable shall be secured to such boxes and conduit bodies.

Now let look at the code section for cabinets, cutout boxes and meter socket enclosures.

2014/2017 NEC

312.5 Cabinets, Cutout Boxes, and Meter Socket Enclosures

312.5(C) Cables.
Where cable is used, each cable shall be secured to the cabinet, cutout box, or meter socket enclosure.

So there is no requirement here for the 1/4 inch sheath. Where we do get the 1/4 inch rule is for an exception, when cables are entering via a conduit/raceway in the top of a surface mounted panel. But this is an exception- (Underlined and highlighted)

312.5(C)

Exception: Cables with entirely nonmetallic sheaths shall be permitted to enter the top of a surface-mounted enclosure through one or more nonflexible raceways not less than 450 mm (18 in.) and not more than 3.0 m (10 ft) in length, provided all of the following conditions are met:

1. Each cable is fastened within 300 mm (12 in.), measured along the sheath, of the outer end of the raceway.
2. The raceway extends directly above the enclosure and does not penetrate a structural ceiling.
3. A fitting is provided on each end of the raceway to protect the cable(s) from abrasion and the fittings remain accessible after installation.
4. The raceway is sealed or plugged at the outer end using approved means so as to prevent access to the enclosure through the raceway.
5. The cable sheath is continuous through the raceway and extends into the enclosure beyond the fitting not less than 6 mm (¼ in.).
6. The raceway is fastened at its outer end and at other points in accordance with the applicable article.
7. Where installed as conduit or tubing, the cable fill does not exceed the amount that would be permitted for complete conduit or tubing systems by Table 1 of Chapter 9 of this Code and all applicable notes thereto.

So you can see that the code does not specify  this requirement unless you are using the exception. 

So this is where a product such the one pictured above actually would be okay since there is no requirement for the sheath to be a 1/4 inch into the panel enclosure. From what I can read there is no requirement for the sheath to enter into the box by a 1/4 inch in this case so these connectors may be perfectly fine. 

Again, we often believe we know something till… we don’t. I have always thought it was an requirement for the sheath to enter a 1/4 inch and it may be good practice but I cannot see where it is a requirement, so there you have it.


A video below on this NM connector product-

The correct application of Chapter 3 wiring methods is based on a group effort from Mike Holt and EC&M

The Four Pillars of NEC Chapter 3    <<<<<  Article  Link

If you find any errors or have additional information that would expand on any code, building standards or manufacturer requirements please let me know.

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