I have 100 amp service panel that feeds a sub panel with a 50 amp breaker. In the sub panel, there are currently five 15 amp breakers that feed three rooms worth of lights and two outdoor outlets used to run 2 Christmas light controllers. All lights are LED and there is a celling fan on one of the breakers. I need to add a 30 amp breaker for an electric in-wall oven. Can I put this breaker in my sub panel, or should/must I make room in the main panel for this breaker?
Assuming you've got 6 gauge wire feeding the sub panel, you should be fine. As long as you aren't pulling more than 50 amps through that sub panel at once, you don't need to worry about over loading the panel. That's what the 50 amp breaker is there for.
There are two separate issues here: safety and usability.
From a safety point of view, as long as all the breakers are sized properly for the wires to which they are attached, and in good working order, you'll be fine. You have a 50A breaker on the input, so as long as the wires feeding that can handle 50A and the panel itself can handle 50A (gosh I'd hope so...) it's safe. As long as the wires attached to your proposed 30A breaker can handle 30A, it's safe. The breakers won't let you melt anything or light anything on fire just by plugging in too much stuff.
Now, from a usability point of view, you have one circuit that might be drawing 30A at any given time. Do you have more than one 15/20A circuit in the panel as well? If you do, and you try to use them all to capacity at the same time, you'll trip the feed breaker. If this happens regularly, it's both annoying and potentially wears out your breaker. If the 30A line is for something that's going to be used consistently, or which starts up on its own, you may want to consider upgrading the feed line to handle more than 50A. If the 30A is for something that you'll only be using after deliberately turning off everything else, then it's probably not worth the expense right at the moment, but you might still regret it later when your use habits change and you start tripping the feed regularly and have to tear into the panel again to upgrade it.
it sounds like your 100A service is full and your sub would be over loaded with an oven of that size on bkr count , it could be done if the load was measured and low enough not to be a problem (with all the led & christmas lights on ect),however I would think about a larger service.I stand by my original statement with a 100A main a 30A oven what about waterheater, furnace blower, refridgerators, washer and dryer. these all add to the load that was not discussed, a 100A service panel is very small by todays standards and they have already added a 50A sub, I am a licensed electrician and I would not be pushing a small pannel that far knock me down more i really dont care I am trying to keep someone from burning thier house down.