The loading on an element such as a transformer or line is always based on the current.

For a transformer, the loading for each side is calculated from the actual current and the rated current for that side.

For example, for the HV side of a transformer, the loading is:

loading_hv = | I(HV) | / Irated(HV) * 100

where Irated (HV) is rated current of a transformer, given by:

Irated (HV) = Srated (HV) / (sqrt(3) * Un(HV))

and Srated is the rated power from the transformer Type.

Note that the parameter c:loading shows the maximum loading, e.g. for a three-winding transformer it is defined as:

c:loading = max { c:loading_h, c:loading_m, c:loading_l}

If the transformer has a thermal rating object associated with it, the voltage used for Un is the rated voltage of the busbar, but if there is no thermal rating object then the voltage used is the rated voltage of the transformer. So it is possible so see slightly different values in these two cases (for the same rating) if the rated voltages are not the same.

For a line, the calculation of loading also depends upon whether a thermal rating object has been assigned to the line (parameter pRating).

If there is no thermal rating, this is the calculation:

The line (current) rating is taken from the line type (Inom), taking into account the derating factor fline and characteristics applied to the type, if any.

Irat = Inom * fline

where

Inom = InomAir (optional) or sline, if a line type is being used, or
Inom = sline from the conductor type, if a tower type is being used, or
Inom = dInom from the TypCabsys, if a cable system is being used

(For a line type where the parameter cohl_ is set to Cable, it is possible in the element basic data page for the Laying parameter, InAir, to be set to Ground or Air. In the latter case, the rated current InomAir will be used instead of sline.)

Then the loading is calculated at both ends by comparing the magnitude of the current with the rating, and the resultant loading is the maximum of the two values:

c:loading = max {(100*I:bus1/Irat), (100*I:bus2/Irat)}

If a thermal rating object (pRating) is assigned to the line, then the rated current at each end of the line is calculated:

Irat = I/(ContRating /(Sqrt(3)*Un))

where

ContRating is the Continuous Rating in MVA taken from the thermal rating object, and

Un is the nominal voltage of the busbar at that end

Then the loading is calculated at both ends by comparing the magnitude of the current with the respective rating, and the resultant loading is the maximum of the two values:

c:loading = max { (100*I:bus1/Irat1), (100*I:bus2/Irat2) }

Normally, Un is the same at both ends, and so Irat1=Irat2.