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 Power Factor Correction)
- 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 |
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]
where
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.
