30 gallon is, in my opinion, too small. Your sump as to be able to hold the water that will flow in if there is a power break and also has to be a able to hold enought water so that you dont have to worry too much about evaporation. I wouldn't go under a 50 gal sump for a tank your size, and it would give you a lot more room for your heater, reactor, etc.
Sort of have to disagree with this and needing a 50gal sump on a 150gal tank. It all comes down to how and where you put the overflow drains on how much water the sump "has to hold". Put them 3 " under the water, and yes, you will need the normal sump capacity plus probably 30+ gal for that 3" of water to drain out, but put them in a corner overflow with 3/4" teeth just breaking the surface, and only 3/4" of water would drain down in case of a power failure.
My last tank was a 125gal with about a 22gal acrylic sump plus bioballs, foam, etc, so less room than that and there was no way to flood it. Two corner overflows with teeth and standoff tubes. Even if the water drained through the bottom bulkheads where the standoff tubes sat (which is would slowly) only the contents of the overflow boxes would drain, which was a few gallons in each. It's easy to figure out and mark your safe line on a sump to figure out how much to fill to hold the sump water, and whatever overflows without being able to drain the tank.
Now if you're just using a DIY overflow with a tube and no siphon break, you can drain a lot of water. Corner siliconed in acrylic overflows, no, you can only drain up to the teeth level from the main tank and that's it, plus the contents of the overflow boxes themselves.