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. A16):

• 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
Fig. A16 – Standard apparent powers for MV/LV transformers and related nominal output currents

Apparent power kVA

In (A)
237 V[a] 410 V[a]
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
1. ^ 1 2 transformer no-load voltage

The nominal full-load current In on the LV side of a 3-phase transformer is given by:

$\displaystyle{ \mbox{In}=\frac{\mbox{Pa}\times10^3}{\mbox{U}\sqrt3} }$

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:

$\displaystyle{ \mbox{In}=\frac{\mbox{Pa}\times10^3}{\mbox{V} } }$

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.