I'm going to add a new 200 Amp meter face mount to the side of my house opposite of where the original service panel was located. (converting from overhead to underground service), then I'm going to use existing electrical panel (installed in-wall) as a Sub-panel.

I need to confirm wire gauge to run from the new meter to the old panel( 2/0 copper ?).
It would run in crawl space. Can I use conduit on walls only, where exposed and multi-wire cable (SER) under the floor?
What's the best way of connecting a PVC conduit to a In-wall(flush mount) sub-panel?
200 Amp Service
100 Amp Sub-panel
75' foot max. run

Elec,
Are you saying that you propose to install a new 200 ampere service to a new 200 ampere demand meter on the exterior and then run through the crawl space to the existing or old main service panel? Are you installing a main overcurrent device next to the new meter before you run the wire into the crawl space?
This would be required.

You mention a 100 ampere panel that will become a sub-panel... does that mean you are installing a new 200 ampere main panel? You have not mentioned that.

Can you provide a little more detail to enable someone to offer you a more complete recommendation?

Yes I am installing a new 200 ampere main panel combined with the meter.
Old panel is 100 Ampere, which I like to use as a sub-panel.

Elec,
Assuming you are in USA as I'm not familiar with Canadian code...
A 240/120V, 1P, 100 ampere feeder would be (3)-#1/0 AWG AL + #6 AWG AL ground, or
(3)-#2 AWG CU + #8 CU ground. (Conductors size based on 110.14(C)(1) and 310.16)
If this is residential you should use conduit where located on the exterior and where exposed and then maybe consider conduit in the crawl space as well.
You may need a conduit punch for connecting the new conduit into the existing panel if all the KO's are already used... terminating SER into a panel may be easier.
There is a lot you haven't covered here, are you confidant that you understand what you need to do to upgrade an electrical service?

Just to add:
1) NEVER use aluminum inside a residence.

2) Hope you are allowed by local regulations to re-wire. If not, and the residence burns down, you can be charged with a hefty fine, and jail time. (In the US it's up to a $10,000 fine, and 10 years in the pen) EVEN IF IT IS YOUR HOUSE.

3) Your having to ask what gauge of wiring alone, suggests to me that you should forgo what you are about to attempt.

There are MANY things that you need to know, besides the basic parameters of wiring gauge, use or not use conduit, and connecting to a Service Panel.

For example;
Did you know that AC is Alternating Current?
It alternates back and forth across ONE of the wires. It also vibrates the wire/s.

Vibration causes loose connections over time.
This is why, for one, after making a Ground, or Neutral, connection in the Service Panel, you are supposed to wiggle the wire at the connection.

THEN re-tighten once more.

Same with the connection of the Black power wire to the Circuit Breaker.
Same with the connections of the Main Power leads, ( 2 ), and Neutral lead, coming into the Service Panel.

There is to be NO more than 1 inch of insulation of the wires, coming into the Service Panel, exposed inside the Service Panel.

This is why an Electrician is the one deemed by local, and federal, as the one who is deemed qualified for installing a Service Panel, and new electrical construction in a residence.

Years of experience.

If you are h3ll-bent on continuing, and not use an Electrician, post back.
Get ready for a L-O-N-G read.

Regards,
joecoolvette

I need to note corrections to the last answer.

Aluminum wire is allowed in homes, especially feeders. Aluminum wire is not the problem, the connections at panels and devices such as receptacles can be a cause of problems. Connections/terminals shall be rated to handle aluminum wire, and the wire shall be treated properly.

I have been in the trade as a licensed electrician for 40 years, and never heard of tighten, wiggle, and tighten again. Each connection, terminal, wire lug, etc, has a label for torque settings, that requires a torque wrench or screwdriver be utilized to tighten the wire and connection screw to a specific value.

AC power does not cause loose connections.

Most, if not all, states in the US allow homeowners to do their own electrical work in their own home, providing the permit and inspection process is followed.

I have no idea what this statement means:

There is to be NO more than 1 inch of insulation of the wires, coming into the Service Panel, exposed inside the Service Panel.

Yes, granted Aluminum IS allowed in homes. Look at trailers, Ha!
However I wouldn't want it used in MY home, and wouldn't recommend it in other homes.

Corrosion is the biggest factor. Copper against Aluminum.

Vibrating AC does NOT cause loose connections? Okay, believe what you will.
I have been called on jobs where a licensed electrician has been, going to perform a job, and had to re-tighten several of their connections over the last 20 years.

Did they use a torque value when tightening? Give me a break.
(Tighten the screws down? It's almost break time, or time to go home.
Let's hurry )

I haven't had a connection come loose yet. (And I have checked homes I re-wired back in 2003 )

States as an entity MAY allow homeowners to perform their own work.
They DO want an inspector to sign off on the completed work, however.

There are a LOT of individual municipalities, that will NOT let the homeowner perform their own electrical work.
(Nor plumbing, nor tree removal, etc)

Better to check before beginning.

When installing a lead into a Service Panel, NO more than 1 Inch of insulation, shall be exposed inside the Service Panel.

Basic 101. Check your NEC code book.

Angered that I posted a strong statement to the poster?
I would rather the poster be angry with me, than the possibility of the home catching fire, and the possibility of losing life, or lives.

(Homes can be replaced, not people)

Glad you 'spoke' up tkrussel, however.
Shows me that possibly someone on this site is observing. ESPECIALLY in this area.