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
Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.
Profile: Edvard Csanyi
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
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.
Hi Sir , hope you fine and healthy:
Please kindly write the formula of (Average of actual load in(Amp))
Rated Capacity of Tr (Amp)
Percentage of Tr loading
Over loading(%).
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)
percentage of TR loading is =(Rated capacity(amp))/average of actual load
over loading(%) is= (percentage of loading-100)
I confused with such formulas , please clear how much is correct that and where we use these Formula.
I am waiting to your response or reply ,help me please.
Dear admin,
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,
how to estimate electrical load in new constriction building plz help me
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
Dear admin,
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
NICE
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
Shalendra , good technical guides on design are available for free download at schneider website.