Reserves of Strength

[Anlashok]

Dragonlance Campaign setting p.86

Prerequisites
Iron Will, Caster Level 1

Benefit: When you cast a spell, you can decide to increase your caster level with the spell by 1, 2 or 3, but you are stunned for an equal number of rounds immediately. Your increased caster level affects all level based variables of the spell, including range, area of effect, spell penetration, and the difficulty of dispelling the spell. You can exceed the normal level-fixed limits of a spell with this feat, so a 9th-level wizard could use Reserves of Strength to cast fireball as a 12th-level wizard and deal 12d6 fire damage.
If you are not subject to stunning effects, you instead suffer 1d6, 3d6 or 5d6 points of damage when you call upon your Reserves of Strength.

Bolded section is mine for emphasis, otherwise it's the exact wording.

In another thread there seemed to be some disagreement over whether or not the increased limit applied to your total CL or only the increase provided by Reserves. i.e. a level 15 wizard does what the 9th level wizard in the example does. Does he do 18d6 or 13d6?

[Biffoniacus_Furiou]

It's entirely up to your DM, because the wording of the feat is ambiguous enough that it could be interpreted either way.

"You can exceed the normal level-fixed limits of a spell with this feat" could indicate that only the bonus granted by this feat exceeds the level-based limit. It could also indicate that you can exceed the normal level-fixed limits of a spell any time you're using this feat.

The most balanced ruling would be the former, that only the bonus granted by the feat can exceed the level-fixed limits of a spell. Otherwise you get a build that deals 150d8+15,000 damage to a single target (http://www.giantitp.com/forums/showthread.php?246510-Theurgic-Specalist-and-Necromancy-why-has-nobody-caught-onto-this&p=13398974&viewfull=1#post13398974).

[sleepyphoenixx]

The most balanced ruling would be the former, that only the bonus granted by the feat can exceed the level-fixed limits of a spell. Otherwise you get a build that deals 150d8+15,000 damage to a single target (http://www.giantitp.com/forums/showthread.php?246510-Theurgic-Specalist-and-Necromancy-why-has-nobody-caught-onto-this&p=13398974&viewfull=1#post13398974).

That build makes a number of assumptions and uses things that have nothing to do with Reserves of Strength. It's also far from the only way to get enough damage to one-hit KO epic wyrm dragons.
That's just a natural consequence of the abundance of source material, most of which has never been tested for overpowered synergies.

On its own neither interpretation is terribly gamebreaking. The feat doesn't specify, the example doesn't help, so you'll have to ask your DM.
Since even the low-powered interpretation is pretty good i'd go with that unless you're playing a really high power game, but that's a personal preference and not an official rule.

[Grod_The_Giant]

On its own neither interpretation is terribly gamebreaking. The feat doesn't specify, the example doesn't help, so you'll have to ask your DM.
Pretty much this. For what it's worth, I don't think it's a big deal even if it does uncap spells. Blasting for d6/level is weak enough as it is.

[Biffoniacus_Furiou]

The second interpretation is better than a +4 epic metamagic feat (http://www.d20srd.org/srd/epic/feats.htm#enhanceSpell), for comparison.

[sleepyphoenixx]

The second interpretation is better than a +4 epic metamagic feat (http://www.d20srd.org/srd/epic/feats.htm#enhanceSpell), for comparison.

That doesn't really mean much when most epic feats are crap that's not worth taking. There's quite a few feats that are better than most epic feats.

Sure, if you (ab)use Master Spellthief, Circle Magic or other heavy CL boosts Reserves of Strength is very strong.
If your games involve that level of power more damage will hardly be your biggest balance problem though.