"Not cheap, but excellent for bringing T&E tidily into knockouts. As they're designed to keep the water and dust out, they're very tight, but if you use a dab of silicone grease on the outside of the grommet and on the outside of the cable, both slide in really easily.
I keep a small tube of silicone grease in every toolbox for just this sort of thing, as it's non-reactive and helps all sorts of things slip in just fine!"
"These simple grommets are a God send and no need for DumDum! Great for use in knockouts in say a garage/workshop (or other dusty areas) and outside (where cable enters bottom of the switch/outlet). I've got a 2.5mm and 2 x 1.5mm thro' these grommets (all three) without having to warm them up - just used a scewdriver to open the hole up a bit - and that was in temperatures of around 4deg C. But they are pricey for what they are though."