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I'm told that a 1% transformer costs more than a 5% transformer of the same power rating. I understand that the 5% transformer has higher output impedance, which seems to imply more windings and thinner wire. Is this the only construction difference between a 1% and 5% transformer of the same power rating? Or are there more considerations?

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  • \$\begingroup\$ I don't think I've ever seen a % rating in transformer specs. Where did you see these terms used? \$\endgroup\$ – Peter Bennett Jan 16 '14 at 17:01
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    \$\begingroup\$ I have some reference books open but I'm a bit too tired to make sense of them right now. So, I recommend that you look up the following texts: ABB's Transformer Handbook, 2e 2004, available from various places on the internet; and the J&P Transformer Book, 12e. Both books contain extensive discussion of transformers, including theory and design considerations as well as practical advice for procurement and maintenance. \$\endgroup\$ – Li-aung Yip Jan 16 '14 at 19:46
  • \$\begingroup\$ Also, since you live in America, you can get the ABB text from ABB's publisher ("OrderLiterature.com") for $10 + shipping. No such luck here in Australia, I would have to pay $10 + $130 shipping. Maybe you can order two copies and send one to me? :) \$\endgroup\$ – Li-aung Yip Jan 16 '14 at 19:49
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    \$\begingroup\$ @Andyaka: "percent" in context of power transformers means "percent per-unit impedance of the transformer on its rated MVA base", i.e. "Z = 5% on 500 KVA". It's our funny way of specifying transformer impedance in a way that simplifies power engineering calculations. See electronics.stackexchange.com/questions/96646/… . \$\endgroup\$ – Li-aung Yip Jan 17 '14 at 10:21
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From ABB's Transformer Handbook, 3e:

3.9 Short Circuit Impedance

Users have sometimes particular requirements regarding the short-circuit impedance. Such requirements may be determined by:

  • parallel operation with existing units
  • limitation of voltage drop
  • limitation of short circuit currents

The transformer designer can meet the requirements in different ways:

  • The size of the core cross-section. A large cross-section gives a low impedance, and vice versa.
  • A tall transformer gives low impedance and vice versa.

For each transformer there is, however, a smaller range which gives the optimum transformer from an economic point of view, that is the lowest sum of the manufacturing costs and the capital value of the losses.

The 'short circuit impedance' mentioned above is the transformer's percent impedance. The above quote says that the transformer's impedance can be varied by changing the construction of the core.

Note that a transformers' impedance is mostly inductive "leakage reactance", i.e. magnetic impedance. Therefore, the difference between a 1% transformer and a 5% transformer is mostly to do with the design of the transformer's magnetic core. The 1% transformer would require much more iron core than the 5% transformer, and would be physically larger to match, which explains the higher cost.

From J&P Transformer Book, 12e:

In Chapter 1 it was explained that the leakage reactance of a transformer arises from the fact that all the flux produced by one winding does not link the other winding. As would be expected, then, the magnitude of this leakage flux is a function of the geometry and construction of the transformer....

Since reactance is a result of leakage flux, low reactance must be obtained by minimising leakage flux and doing this requires as large a core as possible. Conversely, if high reactance can be tolerated, a smaller core can be provided.


The conductor resistance (i.e. copper winding resistance) is typically small, 1/10th of the total impedance or less. The guideline given in AS3851 is that power transformers of less than 10 MVA may be considered to have X/R = 10.

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For any transformer we have some basic equations

$$ V = N \frac{d\phi}{dt} = N \cdot A \frac{dB}{dt}$$

Here ϕ is flux, B is flux density, N is turns and A area.

For a given size core you can work out the number of turns required on the primary to prevent saturation. All magnetic materials will saturate if you apply too many volt-seconds to the primary. Note flux density B is limited by the material used.

As you have realised a transformer with a bigger impedance percentage will have either more turns or thinner wire. But there is only a limited space to put these windings which limits the VA rating of the transformer.

Considering two transformers of the same input and output voltage and VA rating the one with the lower impedance rating typically will have fewer turns of thicker wire but a larger core in order to avoid saturation with those fewer turns.

The need for a larger core is usually the key factor pushing the price up.

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  • \$\begingroup\$ Which one of the parameters you're talking about have to do with the percentage mentioned in the question? \$\endgroup\$ – Scott Seidman Jan 16 '14 at 20:33
  • \$\begingroup\$ The parameter mentioned in the question is in effect winding resistance by having more turns of finer wire there will be more resistance and hence a higher impedance percentage. So in effect they are all important but a core with bigger A will need less turns and so these turns can be wound with thicker wire. \$\endgroup\$ – Warren Hill Jan 16 '14 at 21:26

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