The curve obtained by arranging the load elements of a load curve in order of decreasing magnitudes is called load duration curve. No extra data or information is required for plotting this curve as it can be plotted with the same data as that for load curve.
The ordinates from the load curve arranged in order of deceasing magnitudes. The peak load is shown to the left while the decreasing loads are arranged to the right in the descending order. It is quite obvious that the area under load curve and load duration curve is same.
The area under load duration curve indicates the total number of units that are consumed in that time. The load factor of the station can be determined from this. It also suggests the division of load between various generating units (alternators) of the plant to operate them near their maximum efficiency points.
Fig. 1(a) shows the daily load curve. The correspondingly daily load duration curve is shown in the Fig. 1(b).

The load elements of 3 kW, 6 kW, 9 kw, 12 kW, 15 kW and 18 kW are arranged in decreasing order for the respective time duration on the load duration curve. The total load supplied can be divided into various sections (sections I to VI in this case). Let us say that there are 6 alternators corresponding ti these sections.
The alternator which supplies load of 3 kW will be in operation almost continuously with high load factor. When the load is increased to 6 kW, then the second alternator will be put in service which will have comparatively low load factor. Likewise the alternators will be put in operation with decreasing load factors.
The important points that can be observed from the load duration curve are summarized as given below
1.The data obtained from load duration curve is in more presentable from as it shows te number of hours for which the load was present on the station.
2. It gives number of units generated after finding the area under that curve.
3. The load duration curve cab be plotted for entire year by considering time period of 8760 hours (hours of a year 24 hours x 365 days = 8760 hours) on x-axis. Thus the variation and corresponding distribution of load cab be obtained.