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.

$\frac{1.05 Dd_{i}}{D_{\mathrm{e}}}$
Probability for jamminglower rangeupper range
very smallJRp < 2.4JRp ≥ 3.2
small2.4 ≤ JRp < 2.53.0 ≤ JRp < 3.2
moderate2.5 ≤ JRp < 2.62.9 ≤ JRp < 3.0
significant2.6 ≤ JRpJRp < 2.9