Derating associated with busbars, enclosures and protective devices

[LM2]

Indications are that derating is a concept that is not well appreciated by all. Consider some real-world scenarios below, i.e. examples pertaining to power distribution at 400V three-phase AC:

1. Typical 500kVA miniature substation on local authority/utility network [miniature substation that could be exposed to ambient 35°C, as well as direct incident solar radiation], with secondary-winding rated current approximately 722 Amps and one may expect the LV compartment to be equipped with one 800 Amp main LV circuit breaker, e.g. 800 Amp molded-case circuit breaker (MCCB), like Compact NS800N;
2. Typical Low Voltage Power Switchgear and Control-gear Assembly supplied by 1000kVA transformer, with rated secondary-winding current 1443 Amps; oftentimes the incomer circuit breaker proposed at the enclosure is a 1600 Amp LV power circuit breaker [such as NW16 or MTZ16], where one would notionally set Ir at 0.9 and also adjust short-time protection delay, deployed in conjunction with 1600 Amp [nominally-rated] main LV busbars.
3. Low Voltage Power Switchgear and Control-gear Assembly supplied by 2MVA transformer, secondary-winding rated current 2886 Amps at 400V, paired with NW32 incomer breaker matched to main LV busbars with nominal 3200 Amp rating (a component selection associated with a so-called 2MVA energy-centre block deployed on many data centre projects in the region).

What seems to be common to the above scenarios is that when all transformers would supply rated current, this current is about 90% of the nominal ratings of the ACB and main LV busbars. This points to the designer's recognition that derating is to be expected with different environmental conditions, notwithstanding the responsibility to consider a rated diversity factor.

One might reasonably pose some questions: Why are the circuit breakers [and busbars] selected with nominal ratings that exceed the rated transformer secondary currents [?] Are there compelling reasons to select devices with ratings that provide room to manoeuvre?

See the Schneider Electric expert blog to discover more insights on variables impacting derating:

https://blog.se.com/power-management-metering-monitoring-power-quality/2018/05/23/iec-61439-rated-cu...

[Bhekumuzi_Hlebeya]

Spoiler

Is there a rule of thumb for choosing the derating factor when sizing the breaker

[LM2]

As a suggestion to provide some insight into this question, consider downloading a circuit breaker catalogue, such as the MasterPact or ComPact NS range catalogues that may be accessed on Schneider Electric website. As an example, see the catalogue at the link: https://www.se.com/ww/en/download/document/LVPED211021EN/. The catalogues typically provide guidance about derating associated with temperature, busbar connection orientation and installation recommendations, etc.