How To Calculate Rating Of Distribution Transformer

Hello everyone, in this article I will discuss how to calculate the rating of the distribution transformer or how to select a suitable transformer.

If you want an article on some other topics then comment us below in the comment box. You can also catch me @ Instagram – Chetan Shidling.

Also read – Electrical Machines Questions And Answers. and now I will discuss how to calculate rating of distribution transformer.

How To Calculate Rating Of Distribution Transformer

Let’s take an example

Q. Select a suitable transformer for an Industrial area. The load conditions are as follows :

• a. Available voltage = 11 KV
• b. Lighting load = 10 KW
• c. Demand factor = 0.7
• d. Required voltage = 440/230 volts
• e. Industrial load = 50 KW at 0.8 pf
• f. Diversity factor = 1.46

Solution :

Total connected load 

= Lighting load + Industrial load

= (10 × 1) ÷ pf + (50 ×1) ÷ pf

= 10 + 62.5

= 72.5

Maximum demand 

= Connected load × Demand factor

= 72.5 × 0.7

= 50.75 say 50 KVA

The required capacity of the transformer

= Maximum demand / Diversity factor

= 50 / 1.46

= 34.246 or say 34 KVA

Allowance for future expansion at 20 % of the required capacity

= 34 × 0.2

= 7 KVA

Hence capacity of the transformer = 34 + 7 = 41

(41 KVA, 11 KV/440 V 3 -phase is not available)

Market availability = 50 KVA

Therefore, a transformer of 50 KVA, 11 KV/440 V, 3-phase delta-star, 50 Hz is selected.

Q. A small workshop has the following load to find the transformer capacity required.

• a. A 1 HP, 400 V, 3-phase for drilling machine
• b. A 1/2 HP, 230 V, single phase for Grinding machine
• c. A 3 HP, 400 V, 3-phase motor for lathe machine.
• d. A 5 KVA, 230 V, welding transformer.
• e. A milling machine 5 HP, 400 V, 3-phase
• f. A power Backsaw, 3 HP, 400 V, 3-phase.
• g. Two power sockets
• h. Lamp loads – 25 points
• i. Ceiling fans – 15 nos
• j. Two numbers, high-pressure mercury vapor lamps of 500 W, 1-phase, 230 V, 50 Hz, supply.

Calculate the total connected load. If the demand factor is 0.6 and the diversity factor is 1 %.

Solution :

(a) All three-phase motor loads 12 Hp assuming Pf 0.8 and efficiency 85 %

Therefore, VA = (HP × 735.5) ÷ (efficiency × Pf)

                          = (12 × 735.5) ÷ (0.85 × 0.8) = 7853 VA

(b) Taking all single-phase loads :

Power sockets – 2 × 1000 = 2000 w

Lamps – 25 × 40 = 1000 w

Ceiling fans – 15 × 80 = 1200 w

Mercury vapour lamps – 500 × 2 = 1000 w

1/2 HP Grinder – (05 × 735.5) ÷ (0.8 × 0.6)

                            = 766 / 5966 (efficiency = 08 & Pf = 0.6)

(c) 5 KVA welding transformer – 5000 VA

The total load in VA 

= 7853 + 5966 + 5000

= 18819 VA

= 18.819 KVA

Actual demand

= connected load × demand factor

= 18.819 × 0.6

= 11.2914 KVA

The required capacity of the transformer

= Actual demand / diversity factor

= 11.2914 / 1.10

= 10 KVA

Allowance for future expansion at 25 % of the required capacity

= 10 × 1.25

= 12.83 KVA.

Market availability = 15 KVA

Hence, a transformer of 15 KVA, 11 KV/440 V, 3-phase delta-star, 50 Hz is selected.

I hope this may help you………

Tag: How To Calculate Rating Of Distribution Transformer.

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