All molded case tandem breakers

[mark petty]

Came across this panel with all molded case tandem breakers; could there be any reason that this could be an issue

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[Roger Frazee]

Nope all is fine. What you have is a square d QO 30-40 single phase panel and looks like service equipment usage.
You have 30 bus stabs. If only full size single pole breakers are installed (15 on each hot leg) you would have 30 120 volt circuits, however if you opt to use tandem single poles in the spaces allowed on the panel wiring diagram you could install 40 120 volt circuits.

Just to add you will notice the electrician has installed tandems in all the allowed spaces and then installed a full size single pole 120 volt breaker on the bottom right of those tandems . Note that the spaces used for tandems in your panel pictured are the only spaces you are allowed to use a tandem breaker. You cannot install a tandem in any of the other spaces. This requirement varies depending on the panel and the only way to know for sure is to read the panel wiring diagram, some panels will allow tandems in all available spaces. There are a lot of reasons for this and I'll let the others here expand on why that has merit. In a nut shell you are only allowed so many circuits existing in a panel at the time of installation and the panel listing and specifications must comply with the code requirements in effect at that time.

[mark petty]

Roger, thanks for the quick reply, Mark