From multiplayer to singleplayer, you can chose your top 7 powers you would chose for your Commander Shepard. You are allowed to pick powers that are from different classes as long as they exist. The only exception are character powers which isn't something that would pop up on your power wheel when activated. And on that note, 7 is the maximum amount of powers you can have on a power wheel (7 + medigel), hence why the limit is 7.
For the sake of practicality, [url=http://masseffect.wikia.com/wiki/Powers_(Mass_Effect_3)]here[/url] is a link to the list (you will have to open new links to look through the mulitplayer powers).
Me:
>Adrenalin Rush
>Overload
>Combat Drone
>Concussive Shot
>Incendiary Ammo
>Devastator Mode
>Fortification
Go!
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Sabotage Tactical Cloak Warp Ammo Stasis Warp Armour Piercing Ammo Sticky Grenade