why does table 1 in chapter 9 restrict the conduit fill to 31% for two conductors and 40% fill for more than 2 conductors?
Thanks
Because of the potential for jamming.
Consider a conductor that has an overall diameter half that of the conduit's inside diameter:
Conduit area: pi*D?/4
Conductor area: pi*(D/2)?/4 = pi*D?/16
Conductor area to conduit area percentage: (pi*D?/16)/(pi*D?/4)*100% = (1/16)/(1/4)*100% = 25%
Two such conductors would be 50% fill. Though they would fit in the conduit physically, you would not be able to pull them in through any substantial length of conduit.
Now take into consideration the conductors may twist while pulling them in, and a somewhat oval cross-sectional area through bends in the conduit. At 50%, you would not be able to get two conductors through without deforming or damaging them in some way.
Exactly how the 31% figure was arrived at, I do not know...
There is an FPN to this effect for three conductors for the same reason.