Updated as of 15/10-2010, patch 4.0.1.
*** THIS INFORMATION IS OUTDATED AS OF 4.0.1!! ***
Armorpen for dummies
Getting tired of all the ArP threads? So am I, therefore I will write this short ArP guide.
Lets get started then shall we?
The optimal amount of ArP to reach is 100%, during proc or passive.
13.99 Armor penetration rating equals to 1%
Mace (15%): 209.85 rating
Battle Stance (10%): 139.90 rating
T9 Two piece bonus (6%): 83.94 rating.
Take into account that the ArP you recive from macespec, Battlestance and t9 2p does NOT show up in your character info
***How much ArP should I get if I have a Grim toll?***
787.6 Armor penetration rating will bring you to 100% during proc.
***How much ArP should I get if I have molnir' runestone?***
734.6 Armor penetration rating will bring you to 100% during proc.
***How much ArP should I get if I have Needle-Encrusted Scorpion?***
721 Armor penetration rating will bring you to 100% during proc.
***How much ArP should I get if I have Deathbringer's will?***
This trinket was changed as of 6/1 or 7/1 to not proc STR, CRIT and ARP but now STR, CRIT and HASTE!
This was a major buff for fury(assuming you are gearing for hard-cap arp), and a minor for arms(since haste isn't as good for them)
The best way to go with this trinket is arp-harcap, or as far as you can go.
When you reach these "soft-caps" stacking STR will generally be the best choice.
***If I got 30% arp, and a tank apply sunders. Does that mean I got 50% arp?***
NO! Armourdebuffs like Sunder armor, expose armor and faerie fire are calculated before your armor penetration rating is calculated.
Boss X has 10 000 armour, a prot warrior applies Sunder armour(20%) which brings X's armour to 8 000.
(now it's time for your debuffs)
You have 45% armor penetration, which reduces 8000 armour to 4400.
Final boss armour is 4400.
There you go! That wasn't hard.
***HEELP STR OR ARP?!?!***
That depends on your gearlevel, do you have a Grim toll, Molnir runestone or Needle-Encrusted Scorpion? If you do, it would be your best interest to gem for arp until you reach the soft cap(read above).
No Molnir runestone, grim toll or Needle-Encrusted Scorpion? Can you reach more then ~75% passive arp(*This is a rough estimate and depends a lot on your current gear! Consult your spreadsheet to be certain*) without dropping your hit and expertise far below caps? If you can, then you should.
Dont have a either GT, MJ or NES(Lulz, funny abbreviation) and can't get that high passive ArP? Stack strength.
OBS: These are rough way lines, for the most precise answer, head over to Elitist jerks -> warrior forum -> Landsoul's spreadsheet.
***Is it possible to reach the arp hardcap(1400 rating)***
Yes it is, it was possible before ICC came out, and is now a lil' bit easier to reach.
Crom-EU have put together two wowhead profiles with(HC-gear), or close to(non HC-gear), hardcap reached.
Non-hc ICC gear: http://www.wowhead.com/?profile=19983581
Some HC ICC gear: http://www.wowhead.com/?profile=19984585
And a profile from milenariogk, this one is based on only icc10 normal(first two wings) and toc25 normal!
Thanks to Crom and Milenariogk for putting these profiles together.
*** Is sunder amour useless when you have 100% arp? ***
Sunder does benefit you even if you have 100% Armor pen, like earlier said. ArP will never get your target to 0 armor.
If you look at the wowwiki link you will find a graph(not exactly a graph perhaps but I can't remember the correct term)you will see that if the level 83 target has 10k armor, then 99.9... arp will reduce it to ~1100, but if the target has 8k armor(Like it would after sunders) the target will have a minimum of 6 armor.
And like you probably know armor doesn't scale linear, at 100 armor, a 10 armor increase is worth a lot more then 10 armor at 25k.
Illustrated here: http://www.wowwiki.com/File:Armor.JPG
So those sunders do make a huge difference even if you are at 100% ArP
And as a sidenote; 100% armor penetration does not equal to zero armor on target. There's a reason why it says “Armor penetration reduces your opponents armor by up to **.**%”
More detailed answer regarding that topic: