Technical Bulletin No.2
International Line Voltages and Frequencies

Supply Voltages Worldwide
The power used by transformers in electronic equipment is single phase sinewave AC taken from a wall outlet, with the voltage and the frequency delivered by the local power utility.

The US Department of Commerce has published a booklet "Electric Current Abroad"ä which lists the voltages and frequencies used in each city around the world. Using this booklet as source, we have developed the following list of all single phase voltages and frequencies used and the number of countries where each voltage is used:

The allowable variation in voltage is normally ± 10% and the frequency is normally stable to better than .1%. The voltages and frequencies used in most industrialized countries are in bold. (Japan uses 100V 50Hz and 100V 60Hz.)

Some countries use different voltages and frequencies in different locations such countries are included in the country count more than once. To see the name of the country(countries) corresponding to each voltage, click here.

Nominal Primary Voltages
Ideally, the nominal primary voltage for a transformer should be equal to the nominal supply voltage, and the rated frequency should be equal to the supply frequency. It would obviously be impractical to follow these rules strictly and it is not necessary.

The transformer design does not impose any lower limit on the supply voltage. The output voltage from the transformer does, however, decrease with decreasing supply voltage, so a lower limit is set by the tolerance required for the secondary voltages. This tolerance is usually in the order of -10%

At the rated frequency, supply voltages higher than the nominal primary voltage will eventually cause noticeable mechanical hum or saturation effects, but a properly designed transformer should allow for more than + 10% headroom before this happens.

The transformer design does not impose any practical upper limit on how much the supply frequency can exceed the rated frequency, but a supply frequency lower than the rated frequency has the same effect as an overvoltage. The voltage headroom will be decreased by the percentage reduction in supply frequency below rated frequency, and a frequency larger than the rated frequency will increase the headroom. (A 50Hz transformer will have 6% headroom when run at 48Hz, but more than 32% headroom at 60Hz.)

As a general rule it can be assumed that it is safe to use a transformer for nominal supply voltages between about 95% and 105% of the transformer's nominal primary voltage. The listing below shows selected nominal primary voltages and corresponding ranges of supply voltages supported by each:

Multiple Primary Voltages
A transformer must be provided with more than one primary winding if it is to be used for several nominal voltages. One way to do this is to make a winding with several taps, but other methods are often more economical, as the following example will show:

A parallel/series connection works only when the high voltage is a whole multiple of the low voltage. Taps are needed for 100/120V or 220/240V, but the wasted winding space is quite small in these cases. If the primary must cover a wide range of voltages, it is always best to combine small taps with the series/parallel connection of the tapped windings.

Standard Primaries
At TOROID we use three types of primary windings as standard:

The color coding of these primaries is as shown.

A Quad primary has two blue leads and two red leads. Leads with like colors must be joined outside the transformer to form a blue and a red tap. This way of forming a tap is standard in all TOROID transformers.

The standard Quad primary is designed to have the two 100V windings in parallel for 100V operation, so a 6-lead voltage selector should be used. If the standard Quad primary is used with a 5-lead voltage selector, the rated VA for the transformer at the 100V tap must be reduced by about 4%. ( The 5-lead selector uses one half of the primary winding as an autotransformer for the 100V tap, so the primary copper losses are higher than for parallel 100V windings.)

Cores with primary windings of the three standard types are kept in inventory by TOROID for better economy and faster turnaround of small orders.

Upon request we will make primaries for any voltage or voltage combination, and with custom color coding.

How to Reduce Cost and Size of a Transformer
Correct specification of the secondary data is the most important rule for cost and size reduction of a transformer. Tips on how to specify secondary data for several transformer applications will be given in future issues of Transformer Facts.

The cost of a transformer increases with the size of the transformer and with the number of primary windings, so savings can also be made by following these rules for specifying primary data:

a. Do not specify a lower rated frequency than needed. (A 60Hz transformer has 20% higher rated power than a 50Hz transformer of the same size and weight.)

b. Do not specify more primary windings than needed. (Taps in the primary windings waste winding space, and thus make it necessary to make the transformer bigger.)

c. Do not specify more voltage headroom than needed. (The size of the transformer increases if the maximum supply voltage specified is larger than 110% of the nominal primary voltage.)

Conclusions:
Do not include extra safety margins. Tell us what you need, and let us decide what margins to allow.
You can save money by using a single-primary 60Hz transformer in USA and Canada, and using a 50Hz Dual primary or Quad primary only in other markets. We can design the two transformers to give identical output data, but your mechanical design must leave space for the larger 50Hz version.

Appendix to Section 1: Supply Voltages Worldwide -
Used in at least one location in the country