Diversity factor
The term diversity factor, as defined in IEC standards, is identical to the factor of simultaneity (ks) used in this guide.In some English-speaking countries however (at the time of writing) diversity factor is the inverse of ks i.e. it is always ≥ 1.
Fig A14: An example in estimating the maximum predicted loading of an installation (the factor values used are for demonstration purposes only)
Choice of transformer rating
When an installation is to be supplied directly from a MV/LV transformer and the maximum apparent-power loading of the installation has been determined, a suitable rating for the transformer can be decided, taking into account the following considerations (see Fig. A15):
- The possibility of improving the power factor of the installation (see chapter L)
- Anticipated extensions to the installation
- Installation constraints (e.g. temperature)
- Standard transformer ratings
|
Apparent power kVA |
In (A) | |
| 237 V | 410 V | |
| 100 | 244 | 141 |
| 160 | 390 | 225 |
| 250 | 609 | 352 |
| 315 | 767 | 444 |
| 400 | 974 | 563 |
| 500 | 1218 | 704 |
| 630 | 1535 | 887 |
| 800 | 1939 | 1127 |
| 1000 | 2436 | 1408 |
| 1250 | 3045 | 1760 |
| 1600 | 3898 | 2253 |
| 2000 | 4872 | 2816 |
| 2500 | 6090 | 3520 |
| 3150 | 7673 | 4436 |
Fig. A15: Standard apparent powers for MV/LV transformers and related nominal output currents
The nominal full-load current In on the LV side of a 3-phase transformer is given by:
[math]\displaystyle{ \mbox{In}=\frac{\mbox{Pa}\times10^3}{\mbox{U}\sqrt3} }[/math]
- Pa = kVA rating of the transformer
- U = phase-to-phase voltage at no-load in volts (237 V or 410 V)
- In is in amperes.
For a single-phase transformer:
[math]\displaystyle{ \mbox{In}=\frac{\mbox{Pa}\times10^3}{\mbox{V}} }[/math]
where
- V = voltage between LV terminals at no-load (in volts)
Simplified equation for 400 V (3-phase load)
- In = kVA x 1.4
- The IEC standard for power transformers is IEC 60076.
