Nominal Length (NL) vs Installation Length: Drawer Runner Technical Terms

The nominal length (NL) of a drawer runner refers to the standardised dimension of the slide in its closed position and usually matches the depth of the drawer box. The installation length (or clear internal depth) is the actual space required inside the cabinet carcass for mounting, which often exceeds the nominal length by several millimetres depending on the runner type and mechanism.

What do nominal length and installation length mean exactly?

When planning cabinetry, distinguishing precisely between the length of the runner and the actual space required within the carcass is crucial.

The nominal length is the catalogue dimension of the fitting. When a joiner plans a runner, they base their measurements on this value, as it directly determines the depth of the wooden drawer to be built.

The installation length (often referred to as clear internal depth in construction drawings) includes the nominal length plus the structural clearance needed at the back of the carcass. You can think of the nominal length as the exact dimension of the drawer, whilst the installation length represents the entire 'parking space' in the carcass including a safety buffer for the rear mechanism. This buffer prevents moving components or hooks from scraping against the back panel.

Dimensions, standards, and typical values

In European furniture making, dimensions are largely based on the System 32 standard. The nominal lengths of runners are standardised and generally increase in increments of 50 mm.

Typical lengths for standard furniture range from 250 mm - 800 mm. For narrow bathroom units, 250 mm drawer runners are often used, whereas 500 mm drawer runners represent the most common size for kitchen base units.

The required installation length is calculated differently depending on the manufacturer and the type of slide:

• Standard tolerance: NL + 3 mm
• With special rear mounting hooks: NL + 12 mm to NL + 15 mm

Dependency on runner type and mechanism

The difference between nominal and installation length is largely determined by the type of slide and its integrated comfort features.

Runner types

Concealed undermount drawer runners require space at the rear for the locking hooks that engage into the wooden rear panel of the drawer. Here, the installation length must always be greater than the nominal length. For side-mounted roller drawer runners or ball bearing slides, the installation length often corresponds almost exactly to the nominal length, as no components project beyond the end of the slide.

Extension length

A full extension runner can be pulled out 100% from the carcass, whereas a single extension (partial extension) can only be opened to around 75%. Although this extension property influences the geometry of the telescopic elements when open, it does not alter the fundamental definition of the nominal length when closed. For maximum access to the entire storage space, a full extension drawer runner is usually chosen.

Convenience mechanisms

Additional mechanisms require special consideration regarding installation length. Soft-close and push-to-open are distinct systems:

• Soft-close drawer runners cushion the momentum of the drawer and pull it gently shut. The damper is usually seated flat within the runner profile.
• Push-to-open drawer runners (tip-on) actively push the handleless front open when pressed. For the mechanism to trigger, a trigger stroke of 2 mm - 3 mm is required at the front. This travel path must be accounted for when calculating the clear depth, as the drawer box might otherwise strike the back panel of the carcass before the mechanism can engage.

For heavy-duty constructions requiring heavy-duty drawer runners with a load capacity of, for example, 45 kg or more, the slide profiles are often wider and deeper. The manufacturer specifications for minimum installation depth must be strictly adhered to in order to prevent screws from pulling out.

Application & selection during carcass planning

The following basic rule applies for fitting: first, measure the clear internal depth of the carcass (internal dimension from the front edge to the back panel). Deduct the manufacturer-specific buffer from this measurement. The remaining value defines the maximum possible nominal length.

Example: If the clear internal depth is 515 mm and the runner requires an installation clearance of NL + 10 mm, the maximum nominal length of the chosen runner can be 500 mm.

Related terms

• Clear carcass depth: The exact internal dimension of the cabinet available for the installation length.
• Trigger stroke: The necessary travel clearance at the front for handleless opening mechanisms.
• System 32: Standardised hole spacing system in cabinet making with 32 mm intervals.