# Jam ratio in duct

Jamming is the wedging of three or more cables when pulled into a conduit. This usually occurs because of crossovers when the cables twist or are pulled around bends. The jam ratio is the ratio of the conduit inner diameter $Dd_i$ and the cable outside diameter $D_e$.

In calculating jamming probabilities, a 5% factor was used to account for the oval cross-section of conduit bends.

The cable diameters should be measured, since actual diameters may vary from the published nominal values.

Symbol
$JR_{p}$
Unit
%
Formulae
$\frac{1.05 Dd_{i}}{D_{e}}$
Related
$D_{e}$
Choices
Probability for jamminglower rangeupper range
very small$JR_p$ < 2.4$JR_p$ ≥ 3.2
small2.4 ≤ $JR_p$ < 2.53.0 ≤ $JR_p$ < 3.2
moderate2.5 ≤ $JR_p$ < 2.62.9 ≤ $JR_p$ < 3.0
significant2.6 ≤ $JR_p$$JR_p$ < 2.9