Brian,
It can be quite difficult sometimes to determine this but usually you will see a few hints of mixing breakers.
1.) Notice the name of the breaker itself and the name of the enclosure, Usually the name of the inclosure will match the breaker name but as stated their are some exceptions.
2.) very rarly if ever will you see a compliant panel with 3 or more different brands of OCPD’s located within it…and is a good sign of mproper breakers for the enclosure.
Another issue which is kinda MOOT now since it is an old enclosure but notice this is a 12 circuit panel and it has some tandoms within it…this is also a sign in some panels of exceeding the OCPD rating of the enclosure.
Now some might not know this but Eaton now makes UL Listed breakers to fit MANY panels…see the PDF below
http://www.eatonelectrical.com/unsecure/cms1/SA00304001E.PDF
Man you have to say why didn’t others think of that years ago…
I can’t seem to find the chart I posted on here a few months ago…so I will go of my head…
If the breaker fits tight and does not appear too out of place and is not stopping the operation of the panel front or cover the chances are it will be hard for a HI to know the difference…probably best to give them the winning edge of the DOUBT war in regards to that.
However, here are some…
American Switch, Arrow Hart, Challenger(sylvania), GE ( full size only), General switch, Murray ( crouse-hinds), Seimens ( ITE,Gould ) and Westinghouse will FIT in the same load centers…BUT the load center NEEDS to say this and that it allows OTHERWISE on older enclosures it simply may not be something that IS allowed…so it makes it TOUGH for the HI.
Cutler Hammer CH and BR are kinda to their own…FPE is kind a given…but they do make replacements ( Challenger & American made some ) and GE Half Size breakers are only designed for those enclosures that allow them…technically all half sizers or tandoms SHOULD be in a panel that is designed for them…hense the 30/40 and 20/40 type panels you see in the market…
Square D has QO and Homeline…but as you see on the attachment Eaton ( Cutler Hammer ) not makes a UL approved breaker for most all of these…note however if you run into one of the new UL breakers made by EATON in a new consruction …they should have the added UL label from eaton placed in the panel to be truly allowed.
Marc may have some more insite into this…I will look for my chart but again this is very hard…you have to look for things that stand out because a prime example of improper use of a enclosure we see ALOT is the abuse of the tandom breaker in older panels that were never intended to function for these devices.
Hope this was helpful…