Frequently Asked Questions

If your question is not answered with this list, please contact us.

Can I connect a single-phase transformer to a three-phase source?

Yes, and the transformer output will be single-phase. Simply connect any two wires from a 3- or 4-wire source to the transformer’s two primary leads. Three single-phase transformers can be used for three-phase applications. They can be used in delta-connected primary and wye or delta-connected secondary. To avoid an unstable secondary voltage, NEVER connect wye primary to delta secondary.

Can I use a transformer to change three-phase to single-phase?

It is not possible for a transformer to present a balanced load to the supply and deliver a single-phase output. Changing three-phase to two-phase, and vice-versa, can be done using special circuitry with standard dual-wound transformers.

Can Transformers be used in parallel?

It is very common for transformers to be placed in parallel service. To provide maximum efficiency and voltage, impedance values must match closely for each transformer involved. A failure to match voltage and impedances will cause unbalanced loading for the transformers and may lead to “overheating” or premature failure.

What is meant by a transformer’s temperature rise?

A transformer’s rated tempature rise (degrees Celsius) is the avarage temperature of the transformer’s windings over an ambient temperature of 40 °C.

Why is the transformer case hot?

Transformers are designed to operate at a specific load. As transformers are overloaded, losses generated increases which reults in a potential case for heating. If a transformer is properly sized for a specific application, no excessive heating should be present.

How do I know when the temperature rise is too high?

Touch is a poor indicator of proper transformer opertaing temperature. Properly designed transformers can reach 50 °C (112 °F) above ambient temperature. In an ambient temperature of 20 °(60 °F) the total temperature can reach 70 °C (190 °F), which is too hot to touch. Thermometers are the best way to determine the temperature.

Do I need special transformers for high ambient temperatures?

If you have an immediate need that cannot wait for a custom-built transformer, you can de-rate a standard transformer. For each 10 °C above 30°C, de-rate the maximum loading by 4% (30°C = 100% ; 40 °C = 96%; 50 °C = 92% ; 60 °C = 88% ).

What about transformers in reverse?

Transformers connected in reverse, to proper input voltages, will provide correct nameplate voltage output, albeit reversed. However, for transformers rated 2 kVA and below, the output voltage would be less than the nameplate rating, since smaller kVA transformers have a greater turns ratio compensation on their low voltage windings.

CAUTION: When reverse connecting a delta-wye transformer, a wye primary will be created. Wye primaries may cause problems and are not recommended. If a wye primary must be used, do not connect the neutral.

Why do you not recommend reverse connecting a delta-wye transformer?

You can reverse wire the delta-wye, the primary wye is connected without using the neutral (XO) and it turns into a delta-delta.

What would happen if you connected the neutral on the wye primaries if reverse connected? Would this short, overheat?

The problem is that you could get a fault current on the neutral which may not trip the breaker in case of a problem.

What is an isolating transformer?

An isolating transformer has the primary and secondary windings connected magnetically, but not electrically. Also referred to as an “insulating” transformer.

What is a non-linear (K-factor) transformer?

A transformer that is designed to handle the odd harmonic current loads caused by much of today’s modern office equipment. A non-linear transformer has a K-factor rating that is an index of its ability to supply harmonic content in its load current while remaining within its operating temperature limit.

What is a drive isolation transformer?

A drive isolation transformer is designed for use with motor drives. It must isolate the motor from the line and handle the overcurrent mode during motor startup. It is important to heed the drive manufacturer’s recommendations for transformer kVA.

What is a buck boost transformer?

Buck-Boost transformers are single-phase isolated distribution transformers having four windings instead of two. They can be connected as an autotransformer to buck (reduce) or boost (raise) the line voltage from 5 – 20%. Typical reduced secondary voltages are 12, 16, 24, 32, or 48 volts. Commonly found raised secondary voltages are 208 to 230 or 240 volts.

What are the differences between copper and aluminum windings?

We design our copper and aluminum units to meet the same specifications. Aluminum coils need to use larger wires, but the user does not see any difference. The copper coils are physically smaller, but we put them in the same enclosure as the Aluminum, so the user will not see any difference. The terminals are the same, so the user will not see any difference. Most wiring lugs are tin-plated so there is no problem connecting the transformer to aluminum or copper wires.

Copper has better conductivity but less life expectancy, correct?

Conductivity is addressed in the design by using larger wire in aluminum units. There is no difference in life expectancy for copper.

Anything else?

The only real difference is that Copper costs more.

How do I determine the electrical load?

Obtain the following standard nameplate or instruction manual data for the equipment (the load) to be powered:

  • Voltage required by the equipment
  • Amperes or kVA capacity required by the equipment
  • Required frequency of source voltage in Hz (cycles per second)
  • Determine whether the load is designed to operate on a single- or three-phase supply
  • Others to order

What is the supply voltage?

The supply voltage may be higher or lower than the voltage required by the load. However, the frequency of the two may not differ.

If your load ratings are not expressed in kVA, use the load voltage and amperage to determine the KVA.

For single-phase: VA = volts x amperes kVA = VA/1000

For three-phase: VA = volts x amperes x 1.73 kVA = VA/1000

Once you have a kVA rating, then select a transformer from the charts in the appropriate section of this catalog by matching the primary and secondary voltages determined above.

What is voltage regulation in a transformer?

The voltage difference between loaded and unloaded output. To provide the proper secondary load voltage, extra primary windings cause the no-load secondary voltage to be 3-5% higher than the load voltage. Also known as “compensated windings.”

What will happen if transformers are operated at non-nameplate voltages?

A transformer is designed using specific ratios that relate to the rated kVA, primary voltage and secondary voltage proportionally. Operating a transformer above or below the nominally designed primary voltage will reflect a proportional increase or decrease in secondary output levels. Extreme caution must be observed when overvoltage levels exist. Excessive input voltage will cause higher core losses, increased noise and elevated temperatures. Overvoltages for any extended period of time have a significant effect on insulation breakdown and transformer failures. Transformers can be specifically designed for extreme voltage conditions if initial specifications state those requirements.

Are your transformers used in residential applications?


Are there additional standards required for residential applications (building codes, etc.)?

We meet the general NEC requirements as far as residential applications, but there could be specific local code requirements.

What is the general life expectancy of your transformers under normal use?

20-40 years for typical use.

What type of terminations are provided on Jefferson Electric transformers?

Jefferson Electric dry type transformers are provided with the following primary and secondary terminations:

  • Encapsulated wire leads
  • Ventilated terminals
  • Machine Tool Terminals
  • Control leads
  • Others to order

Can transformers be operated at different frequencies?

A 60 Hz design is physically smaller than a 50 Hz design. DO NOT use 60 Hz rated transformers on 50 Hz service. Without special designs, higher losses and greater heat rise will result. Operating 60 Hz transformers at higher frequencies may simply provide less voltage regulation.

What would be the result of overloading dry type transformers?

All Jefferson Electric transformers are designed to accommodate short periods of overloading. As the overload becomes excessive and the duration increases, the transformer will experience a percent loss of life. Prolonged overloading generates excessive heating which results in insulation deterioration and ultimately transformer failure. Contact your Jefferson Electric application engineer to determine loading for your unique application.

Can I achieve specific sound levels in a transformer?

Whenever noise is a concern, but before selecting a transformer, assure yourself that the sound levels represented have been measured in accordance with the NEMA standards. If your requirement is lower than that available from the manufacturer’s standard product, request a specific sound level on your RFQ or bid. (See Transformer Sound.)

Why do the energy efficiency ratings only apply to ventilated transformers?

This is what the government says. The law targets the “Distribution” transformers which are the most common and are typically oversized which results in the “wasted” electricity of powering a large unit 24/7.

Are the TP-1 Energy Efficiency ratings exceeded?

We need to meet the ratings, so some may be exceeded slightly. We see no reason to provide a higher level of efficiency since this adds to the cost of the units. It is very expensive to get the next 1% of efficiency and we are already over 97 to 98% depending on the size of the unit.

Are these transformers more efficient than any of our competitors?

Everyone needs to meet the same requirements.

Can you provide any efficiency information, now that you have obtained the Canadian High Efficiency Approval? For example, different loading conditions besides 35% load.

35% was chosen as the test requirements for the new law. There are calculators on the web to check the energy usage and cost savings for switching to a TP-1 transformer, but no one really cares anymore since you must choose the TP-1 units today. Also, to use the calculators, you need to know the specific core loss and coil loss for each model of transformer and this is not typically available.

Do you have any examples that may show cost savings of this transformer?

Our TP-1-Whitepaper shows a typical ROI of 5 years to recover the cost on installing a new TP-1 transformer, but most users will not change a transformer.