If you don't have conditional rules, you'll have to set your planes
Alternately, in this case you can set the copper to copper rule for the
+24V net to 30 mils (Design Rules -> Net -> pick desired net(s) ->
Clearance -> copper/copper in the matrix. Note, this will set the
copper clearance for +24V to all copper, not just the +3.3V plane. You
would need conditional rules for that,
We have a similar issue. IPC spacing in the US is different than similar agency spacing in Japan. We need 20mil spacing for 15V and 24V, but only require 7 mil for everything else (in the SELV group). In this case Olav's approach is best because it increases the spacing to all copper entities, not just planes and doesn't require conditional spacing. The Split/Mixed gap value defaults to 6 mils. This sets the gap between the native unflooded planes. When the planes are flooded, the spacing for the plane netname is used to back the plane edges away from everything. In the split/mixed gap made using the default 6 mil gap, the native edges are 3 mils each away from the center. A 10 mil copper spacing rule, when flooded, will back the flooded copper edge 7 mils away from the center because the spacing is to the NATIVE copper edge. If you increase the 24V copper spacing to 30 mils but left the default at 10, you would expect the 3v plane edge to be 7 mils from center and the 24v plane to be 27 mils from center, but both planes back 27 mils from center. That being said, though, if you manually seperate the native plane edges 15 mils each from center, a pour will not change these edges because the 30 mil spacing is already satisfied. Instead of a 54 mil gap you only have a 30 mil gap. Make sure to type SPO before editing the plane edges to make sure you are working on the native plane or a reflood will undo your edits.