# Power and Distribution Transformers Sizing Calculations – Part Three

we indicate that the contents of our articles for Power and Distribution Transformers sizing calculations will include the following points:

•  Glossary of Sizing Power and Distribution Transformers,
• Power and distribution transformer components,
• Power and distribution transformer classification: construction and application,
• Three-phase power and distribution transformer connections,
• Power and Distribution Transformers sizing calculations.

The following points were explained before ( or will be explained) in our course “

• Power and distribution transformer components,
• Power and distribution transformer classification: construction and application,
• Three-phase power and distribution transformer connections,

So, we will not go through these points here, we will focus only on the following two points:

• Glossary of Sizing Power and Distribution Transformers,
• Power and Distribution Transformers sizing calculations.

And we already explained the Glossary of Sizing Power and Distribution Transformers in Article “Power and Distribution Transformers sizing calculations – part One” .

Also, in ArticlePower and Distribution Transformers Sizing Calculations – Part Two” , we indicate that Our study for the Power and Distribution Transformers sizing calculations will include the explanations of the following points:

• Resources used to calculate basic ratings of power and distribution transformers,
• Selection Factors,
• Calculations procedures,
• Special cases.

And we explained in this article the Resources used to calculate basic ratings of power and distribution transformers, today we will explain Selection Factors for Power and Distribution Transformers sizing calculations.

Note: I’d like from all of you to review our course “EP-3: Electrical Procurement – Transformers Courseto be more familiar with the contents of our new articles about the Power and Distribution Transformers sizing calculations.

 Selection Factors  for Power and Distribution Transformers sizing calculations

Before going through the equations and calculations for sizing power and distribution transformers, we must know the factors which must be considered before making these calculations. Noting that without fulfilling these factors, your calculations will not be so accurate. These factors we called them the selection factors which are:

1. Voltage class,
3. Ambient Temperature,
4. Standard Sizes,
5. Cooling Classes,
6. Temperature Rise,
7. Altitude.

First: Voltage class:

There are three classes of voltage: low, medium, and high voltage.

1- Low Voltage, Three-Wire:

ANSI C84.1 lists four voltages in this class

 Low Voltage, Three-Wire Volt Application 120/240 Single-phase, three-wire, nominal system is primarily used for residential areas and light industrial loads. 240 Three-phase, three-wire, nominal system is primarily used for residential areas and light industrial loads. 480 Three-phase, Three-wire, nominal system is primarily used for supplying motor loads. 600 Three-wire, nominal system is primarily used for supplying motor loads.

2- Low Voltage, Four-Wire, 3 phase:

ANSI Standard C84.1 lists three voltages in this class:

 Low Voltage, Four-Wire, 3 phase Volt Application 208Y/120 It is typically used in commercial or very light industrial applications. 240/120 It is typically used in commercial or very light industrial applications. 480Y/277 It is used in most industrial applications and very large commercial applications (e.g., large office complexes, commissaries, etc.).

3- Medium Voltage, Three-Wire, 3 phase:

ANSI Standard C84.1 lists 9 nominal voltages in this class:

 Medium Voltage, Three-Wire, 3 phase Kilo-Volt Application 2.4 Voltages in this class are used to distribute large blocks of power and as a utilization voltage for large motors (isolated neutral). 4.16 4.8 6.9 13.8 23 34.5 46 69

4-High Voltage, Three phase, Three-Wire:

ANSI Standard C84.1 lists four voltages in the class:

 Low Voltage, Four-Wire, 3 phase Kilo-Volt Application 115 Nominal system voltages within this class to transmit large blocks of power over long distances. 138 161 230

It means that the minimum self-cooled kVA rating of each OA/FA transformer shall be equal to the maximum normal operating load plus projected future load.

Transformers may be operated under emergency conditions at ratings above normal load ratings. However, there will be some sacrifice of life expectancy. ANSI/IEEE Standard C57.91 and C57.92 provide methods for determining the life expectancy of power transformers when they are operated at loads above their listed ratings.

It means that for self-cooled transformers (OA or AA) only, a 10% load growth factor should be added to the calculated load (normal maximum operating load plus projected future load).

It is a known load that will be added in the future. Projected future load should not be confused with the 10% load growth factor that was discussed in the immediate previous paragraph.

Third: Ambient Temperature

• The temperature rise ratings of transformers are all based on an ambient temperature of 300C averaged over a 24-hour period, and the temperature not to exceed 400C at any time.

• If the transformer is operated at rated load and at temperatures greater than an average ambient temperature of 300C, some decrease in life expectancy will occur.

• To avoid this decrease in life, ANSI/IEEE Standard C57 requires that the transformer be de-rated as follows:

 Transformer type Actual Ambient temp. De-rating factor liquid-filled power transformers the average ambient temperature exceeds 300C 1.5% for each 10C over 300C 1500C dry-type transformers the average ambient temperature exceeds 300C 0.57% for each 10C over 300C 2200C dry-type transformers the average ambient temperature exceeds 300C 0.34% for each 10C over 300C

Fourth: Standard Sizes
After an initial calculation to determine the kVA load requirements, the next standard (ANSI C57) size transformer is selected for a particular application. The complete lists of standard size liquid-filled and dry-type single-phase and three-phase transformers are as follows:

 Single-Phase Transformers Three-Phase Transformers kVA kVA kVA kVA kVA kVA 3 167 5000 15 1000 15,000 5 250 6667 30 1500 20,000 10 333 8333 45 2000 25,000 15 500 10,000 75 2500 30,000 25 833 12,500 112.5 3750 37,500 37.5 1250 16,667 150 5000 50,000 50 1667 20,000 225 7500 60,000 75 2500 25,000 300 10,000 75,000 100 3333 33,333 500 12,000 100,000 750

Table-1: Standard Transformer kVA Ratings (Liquid-Filled)

 Single-Phase Transformers Three-Phase Transformers kVA kVA kVA kVA kVA kVA 1 167 5000 15 300 3750 3 250 6667 30 500 5000 5 333 8333 45 750 7500 10 500 10,000 75 1000 10,000 15 833 12,500 112.5 1500 12,000 25 1250 16,667 150 2000 15,000 37.5 1667 20,000 225 2500 20,000 50 2500 25,000 75 3333 33,333 100

Table-7: Standard Transformer kVA Ratings (Dry-Type)

Fifth: Cooling Classes

Table- lists the cooling classes for liquid-filled transformers and Figure 55 lists the cooling classes for dry-type transformers.
 Class Code Method Of Cooling OA Liquid-immersed, self-cooled OA/FA Liquid-immersed, self-cooled/forced-air-cooled OA/FA/FA Liquid-immersed, self-cooled/forced-air-cooled/forced-aircooled OA/FA/FOA Liquid-immersed, self-cooled/forced-air-forced-liquid-cooled
Table- : Cooling Classes for Liquid-Filled Transformers

 Class Code Method Of Cooling AA Ventilated, self-cooled AFA Ventilated, forced-air-cooled AA/FA Ventilated, self-cooled/forced-air-cooled ANV Non-ventilated, self-cooled GA Sealed, self-cooled

Table-: Cooling Classes for Dry-Type Transformers

Sixth: Temperature Rise

The rated kVA of a transformer is the kVA output that can be delivered for a specified time, at rated secondary voltage and rated frequency, without exceeding the specified temperature rise under prescribed conditions.

1- For liquid-filled power transformers:

• In liquid-filled power transformers, the specified time is continuous operation. The rated secondary voltage and frequency are as stated on the transformer nameplate.
• The specified temperature rise is the average rise of temperature in the windings, which is either 550C or 650C and is stated on the nameplate.
• The corresponding hottest spot winding temperatures are 650C and 800C and are not stated on the nameplate.
• Sixty-five degrees average rise is the “preferred” rating specified for modern transformer designs.
• Some transformers have a dual temperature rise rating of 550/650C, with a corresponding dual kVA rating specified on the nameplate.
• Prescribed conditions are an ambient temperature not to exceed 300C averaged over a 24 hour period and not to exceed 400C at any time.
• The kVA ratings of outdoor transformers account for the warming effects of full sunlight during daytime hours.

2- For dry type transformers:

There are 5 classes of insulation:

• Class 1300C with a 600C average rise,
• Class 1500C with an 800C average rise,
• Class 1850C with a 1150C rise,
• Class 2000C with a 1300C average rise,
• Class 2200C with a 1500C average rise.

The kVA ratings for dry type transformers have the same ambient temperature basis as for liquid-filled transformers.

Seventh: Altitude

To allow for reduced cooling at higher elevations de-rate the transformer nameplate kVA by 0.3% for each 330 feet over 3300 feet above sea level.

In the next article, we will focus on the other points in Power and Distribution Transformers sizing calculations which are:

• Calculations procedures,
• Special cases.

 Back To