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# How to choose transformer rating?

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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:

• 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) – 237V – – 410V – 100 244 141 160 390 225 250 609 352 315 767 444 400 974 563 500 1218 704 630 1535 887 800 1949 1127 1000 2436 1408 1250 3045 1760 1600 3898 2253 2000 4872 2816 2500 6090 3520 3150 7673 4436

Figure. 1 : Standard apparent powers for MV/LV transformers and related nominal output currents

## 3-phase transformer

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

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

## Single-phase transformer

For a single-phase transformer:

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.

SOURCE: Schneider Electric

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### Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

1. Ahmed Almayyahi
Jan 13, 2019

Hi,

For three-phase Formula – transformer rating, is the voltage a line or a phase voltage ? , my understanding it should be line voltage if you multiply by root square of 3
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

2. Duncan Lance
Oct 18, 2018

Wow, I had no idea that there was so much math involved when it comes to choosing a transformer for your electrical needs. Although, I suppose that it does make sense since there is so much power involved. After all, you wouldn’t want to accidentally overload your transformer or anything like that.

3. sirajuddin Hamdard
May 28, 2018

Hi Sir , hope you fine and healthy:
Rated Capacity of Tr (Amp)
i saw in an electrical artical the average of actual load(amp) is =(R+S+T/3)
and Rated capacity (amp) is =(Total KVA/0.692) what is (0.692)
I confused with such formulas , please clear how much is correct that and where we use these Formula.

4. krishan
Oct 12, 2016

Thank you for the excellent consise articles you publish so frequently. I am currently employed as an project electrical consulting engineer so most of your topics are relevant to the everyday information we deal with. I would really appreciate it if you could forward me any nice literature pertaining to good building services design, power sub stations and residential distribution applications.

Looking forward to your next article.

Best regards,

5. krishan
Oct 12, 2016

how to estimate electrical load in new constriction building plz help me

6. Khaldoun
Aug 24, 2015

Dear Sir
I have an air compressor powered by 120 v and I am going to use the compressor in my country where main power used in my country is 220 v , Would you please advise me what transformer rate can be used.
for more technical specification of compressor I will supply you ASAP.

Regards

7. Ebukanepa
Jan 26, 2015

Thank you for the excellent consise articles you publish so frequently. I am currently employed as an project electrical consulting engineer so most of your topics are relevant to the everyday information we deal with. I would really appreciate it if you could forward me any nice literature pertaining to good building services design, power sub stations and residential distribution applications.

Looking forward to your next article.

Best regards,
Ebukanepa
Lagos, Nigeria

8. ravicharan
Sep 11, 2013

NICE

9. shalendra
Oct 07, 2011

Hi Edvard

Thank you for the excellent consise articles you publish so frequently. I am currently employed as an electrical consulting engineer so most of your topics are relevant to the everyday information we deal with. I would really appreciate it if you could forward me any nice literature pertaining to good building services design, power sub stations and residential distribution applications.

Looking forward to your next article.

Kind Regards
Shalendra Singh
Durban, South Africa

• Vishnu
Feb 02, 2013