The only rule I'm aware of is "Ib<=In<=Iz", which means that the load current must be smaller or equal to the overcurrent protection device's nominal current and the latter must be smaller or equal to the wiring's maximum rated current (depending on cross-section but also factors such as ambient temperature and grouping as well as length, i.e. voltage drop). An additional design guideline states that appliances larger than 1.5 kW should be supplied by a dedicated circuit but this applies to the design of new installations, not to plug-in appliances in existing installations. There's nothing in the electrical regs to keep a homeowner from plugging a 3.5 kW electric oven into the place's only 16-amp circuit. Or into an ancient 10-amp circuit that also supplies all the kitchen sockets, as I saw in an older place a few months ago. The ceramic Diazed fuse holder showed considerably signs of heat damage! I only looked at the place as it was cleared out after the tenant had passed away so I'm fairly certain it'll all get rewired or may already have been.

Austria does have a rather arbitrary rule that is somewhat similar to the US 80% rule. In any other country that I'm aware of, the rated current of an RCD (e.g. 40 or 80 or 100 amps) equals the required overcurrent protection (to protect the RCD from thermal damage). In Austria, unless the manufacturer explicitly specifies the maximum overcurrent protection, the rated current is taken to be the "thermal limit current", i.e. tied to the OCPD's trip curve! Example: a class gG/gL fuse must blow within one hour at 1.6x its rated current. Therefore, according to the regs, an RCD must be able to withstand that overcurrent for one hour and a 40-amp RCD must be protected by a 25-amp fuse. Most manufacturers have two ranges of RCDs for the Austrian market, one regular, affordable, and one much more expensive "nameplate rated" series (we're talking almost three times the price!). Eaton calls this the X-series and that's become a synonym for RCDs that don't need de-rated OCPDs. A 40/4/0.03XG would be a time-delayed 40-amp four-pole RCD that can be connected to a 40-amp main fuse according to their nomenclature. Otherwise you could also fit a 63-amp regular G-type RCD but that's as expensive as the 40-amp XG. In domestic properties in Vienna that's rarely an issue because the standard supply is only 25 amps per phase but in the surrounding areas it's a considerable problem because the standard supply is 35 amps there. In some areas of Germany it's 63 amps (three-phase) even for the smalles studio apartment, which I find slightly ridiculous.