As in the title I’ve just unpacked some r501 wheels and on the instructions it says a max cacity of 30t. I’ve tried a 32t cassette on it and it doesn’t appear to be causing a problem , so just wondering why they would state that

Possibly something to do with over torquing on the freehub.

I think the largest road sprocket in the Shimano range is 30t and they like to maintain their standards.
Don’t blame me if you break it.

because thats the biggest cassette they sell for road bikes. When the biggest road cassette they sold was 27/28 their rear mechs were only rated to 27/28, as soon as they introduced a bigger cassette the limits changed, and I assume they did the same with their wheels. Shimano have always had a rep for being conservative with limits, if you google it theres forum threads/posts for years with people discussing running bigger cassettes than rear mechs were rated for with no problems whatsoever (I did a lot of such googling when I was setting up bikes for the Ronde Van Calderdale which is famous for its 20% gradient climbs!).

because thats the biggest cassette they sell for road bikes

No, it’s not.

maybe t stands for Ton?

Put 32 ton on and see what happens.

No, it’s not.

Oh yes it was……… (when those wheels were designed).

As above – probably the limit of what is/was available at the time.
I’m currently running a 30t Ultegra with a 105 rear mech that is only rated up to 27t but it works fine.

Oh yes it was……… (when those wheels were designed).

Fine with that caveat I guess as they’re not the newest of wheels 🙂

Claris, Sora, Tiagra, 105 & Ultegra all have mechs rated to work with a 32t cassette these days and there are 8,9,10 & 11spd Shimano cassettes available with a 32t large sprocket that you will find specced on various road bikes.