Good questions. The following are all my own interpretations -- anyone please correct me if you find errors in my thinking. These include using the Barrier Rules.
First off, the spell is "Wreck Door", not "Wreck (Door)". Wreck Door is simply a Powerbolt with a lower drain code and a restricted target. Anything Wreck Door can do, Powerbolt can do but with a tougher drain. Your magician might decide that wrecking doors will be so infrequent that he'd rather take Powerbolt and be able to do Doors and lots of other things too.
Secondly, doors and other inanimate objects don't resist spells. They are tough to affect, but if you have enough force and make the dice rolls you succeed.
Third, you can
not simply perform 4 separate attacks that do moderate damage in order to break through a door (a barrier). Consider a car stuck in the end of a hallway. Doing deadly damage will make the car non-functional, but it is still there and blocking the hallway. After 4 separate attacks that do moderate damage the door is no longer
functional, which might mean it no longer swings open or closed or locks and unlocks, but it may still block your passage.
I will contrast powerbolt on a Car, on a metal sculpture, and on a door.
---
The TN for Powerbolt on a Car is its Object Resistance of 8 plus its Body Rating plus half its Armor Rating. (SR3 p. 150)
The TN for Powerbolt on a fragile high-tech metal alloy sculpture is its Object Resistance of 8 (OR table p. 182).
The TN for Powerbolt on a high-tech metal alloy door with barrier rating 12 is it its OR of 8.
---
The Force to affect the car has to meet two rules:
Equal or exceed HALF of (OR + Body + Armor/2) (see Errata
http://www.srrpg.com/resources/errata_sr3.shtml).
Exceed the Armor rating (SR3 p. 150)
The minimum Force to affect the fragile high-tech sculpture is HALF of the OR (Sorcery Test p. 182).
The Force to affect a Door (Barrier), like the car, has two requirements.
First, it must be at least HALF the OR simply to affect the door (p. 182).
Second, you're not just trying to make it non-functioning, but actually remove enough of the door to get through. Therefore it must meet the power requirements listed below for barriers.
----
Affecting a barrier (including Doors) is covered on SR3 p. 125 under "Breaking Through". We're looking for enough power in our attack to get onto the Barrier Effect Table (p. 124) where a successful attack with:
power < 1/2 ADJUSTED barrier rating does no damage
power up to ADJUSTED barrier rating lowers real barrier rating by 1
power > ADJUSTED barrier rating blows a hole in barrier (see page for size) and lowers real rating by 1
Regardless of the holes in a door, a regular door will break open when its Barrier Rating is reduced to one-half the original. A security door's rating must be reduced to 0 before it will break open. ("Breaking Through" SR3 p. 125)
I'll contrast 5 types of attack to break through a door ("Breaking Through" SR3 p. 125):
Combat Axe: Adjusted Barrier Rating is twice its real barrier rating.
Firearms: Adjusted is twice real
Combat Spell: Adjusted is twice real
Elemental Manipulation Spell: Adjusted same as real
Explosives: Adjusted same as real
Therefore to break through a high-tech metal alloy ( OR 8 ) security door (barrier rating 12) needs a successful attack by a combat spell, such as Powerbolt or Wreck Door, with a force of 12 just to damage it enough to lower the barrier rating by 1. Seven successful attacks will lower the barrier rating of the door to 5 whereupon successive spells will blow 1/2 meter holes in it, but since it is a security door it won't simply break open till its rating gets down to zero.
Note that rating 12 is
tough. And deliberately so. Rating 12 covers structural materials (see p. 124) such as concrete block walls, and load-bearing interior walls. It also covers Security Doors made out of Heavy Material. Heavy Material is normally barrier rating 6, but Security Doors are twice the rating of their material. ("Barriers" p.124)
With an Elemental Manipulation Spell, you'll get by with half the force needed by a Combat Spell. Pick an elemental attack that will affect the materials in the doors you are interested in.
As far as I can see, using the barrier rules doesn't involve the
number of successes you obtain on your Sorcery Test, nor the Damage Level (L, M, S, D) you select. Your GM may make a house rule that additional successes or greater damage level have greater affect.
Magic does not easily destroy exterior walls and doors. I guess you see that.
Some people think it takes years to learn a Force 12 spell.