What SEU cable size is required for 100 amp?
Depends if it's your entire service...
Normally the answer is &quot;#1 aluminum&quot;.
However, if the 100A is your entire service, a &quot;discount&quot; kicks in: 310.15(B)(7) says you only need wire large enough for 83% of your service ampacity (83A obviously) so that allows use of #2 aluminum wire.
And you are better off using aluminum because the service drop itself will be aluminum, so why create a dissimilar metal problem?
... and if you want a &quot;Heavy-up&quot; later.
Honestly I'm a bit surprised that any power company is offering 100A service these days.  Usually, they want you to take 200A service.  And you may want to take it later.
And you know what: the difference in wire size cost outlay will not be large.  So it may make sense to spend the extra coin today so your wires are ready for 200A later.
For 200A service, you will need 4/0 aluminum (or 2/0 copper).    That's assuming again that it's your whole service and 310.15(B)(7) applies.
If the 200A is less than your entire service and you can't use the 'discount' then full-boat ampacity requires 250 kcmil aluminum or 3/0 copper.
Just to be clear: you never do this hot
To do any work on the service entrance cabling (weatherhead to meter) the power company must remove their service drop wire.
One option that can reduce downtime is to install a new, parallel weatherhead and meter pan right next to the existing one.  Have the new meter pan be a meter-main (required anyway in NEC 2020) which then allows you to de-energize everything downline with the flick of a switch.  Then, the power company is making 1 visit, and you can quickly punch over the.  However, some power companies will want to see that the new work has passed inspection.  Once the service drop is moved to the new weatherhead, the old weatherhead is stone dead, and you can easily de-energize the meter-main obviously.