Hi, how do you figure out what size PVC conduit to use if your pulling 6/3 NMWU into it for an outdoor hottub? My guess is to use 1"

One inch is too small. It's a fight to get it into a short one inch sleeve. 1.25 inch is minimum but you need at least 1.5 inch if you have an LB.

I just pulled a run into 30 feet of 2 inch with 3 elbows and an LB. I did it alone, so it was probably oversized.

I am going to just pull in #6 T90 and transition to 6?3 NMD 90 inside the house. Easier by far!

Can you install T90 in an underground PVC run? We can't even install it outside in Saskatchewan.

My pvc run will not be underground. Also if it was I am sure you can because it also has THWN classification. Please correct me if I am wrong on this one.

The THWN is the saviour. but I think the Canadian designation is TWN75 as T90 is only ok for damp and dry locations. I presume the wire you have chosen is Dual rated? As for NMD in a raceway You are allowed %53 fill from table 8 for a single multi conductor cable Based on the measured area.

This page talks about the use of T-90.

Building Wire T-90

We can't run T-90 outside because of its minimum temperature rating.

Here in Ontario we are permitted to use T90 underground because it is dual rated. It is also rated TWN75.