I hope Jim doesn't mind me posting this here:
I wrote this in 2005:

I’d advise treading carefully when condemning off-brand breakers. Sometimes, it’s not a violation.

If a breaker manufacturer pays for it, Underwriters Laboratories will test and “classify” one manufacturer’s breakers for use in another manufacturer’s panel. These breakers are known – not surprisingly – as “classified breakers.” From both a safety and a code perspective, these breakers are perfectly acceptable to use in the panels for which they’re listed. When you buy them, the breakers come with a long list of panels that are acceptable to put them in.

Check out the UL guide for these breakers at http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/showpage.html?name=DIXF.GuideInfo&ccnshorttitle=Ci rcuit Breakers, Molded-case, Classified for Use in Specified Equipment&objid=1074081964&cfgid=1073741824&versio n=versionless&parent_id=1073

Cutler Hammer, for instance, makes some particular classified breakers that UL says are acceptable to put in GE, Siemens, Murray, Thomas & Betts, Square D (both Homeline and QO) and Crouse Hinds panels. (Note to Chris: Not all Cutler Hammer breakers are classified as compatible with Square D, only some of them.) Check them out at http://www.eatonelectrical.com/unsecure/cms1/SA00304001E.PDF

As for warranties, every panel manufacturer, it seems, has similar language warning of the dire consequences that will result if another manufacturer’s breaker is used in their panel. The most curious of these is Cutler Hammer. (We make breakers that are classified for use in practically every other panel out there, but if you use someone else’s breaker in our panel, our warranty is void!) Talk about cheeky. The thing about all of this warranty voiding is that it may not be legal. The Magnuson-Moss Warranty Acthttp://www.ftc.gov/bcp/conline/pubs/buspubs/warranty.htm#Magnuson-Moss specifically prohibits a manufacturer from requiring the use of their own replacement parts when safe and effective alternatives are available from other manufacturers. I suppose that, to know for sure, this will someday have to be tested in court. I’m not holding my breath.

- Jim Katen, Oregon