Transformers: Ensuring efficient electrical system design

The best approach for design efficiency is to minimize the number of transformers that are contributing to losses.

By Brad Watford July 10, 2017

As clients’ budgets are getting tighter, every penny spent is scrutinized. Both capital and long-term operational costs are evaluated for optimal cost savings. Over the past decade, 3-phase, low-voltage, dry-type transformer efficiency has become a hot topic for owners and specifying engineers seeking to save money on the long-term operational costs within a facility.

How can specifying engineers implement high-efficiency, 3-phase, low-voltage, dry-type transformers into facilities, taking into consideration all of the parameters of the transformer designs, to make a safe and effective electrical distribution system? There is no single, simple solution. It depends on the facility type and how it needs to function. The best approach for design efficiency is to minimize the number of transformers that are contributing to losses. A focus of design is to provide transformation only where necessary-and to think outside the box when considering what kilovolt-ampere size transformer to use.

When a transformer is not working, energy is wasted. The primary function of a transformer is to convert power from one voltage level to another. If there is no load downstream, then the transformer cannot do its job efficiently. The energy consumed by a nonfunctioning transformer can be compared to the human body at rest. No matter what tasks humans complete during the day, we require rest at night-but our bodies do not turn off. This is similar to a facility operating during the day, and then everyone leaving for the night.

Low-voltage, 3-phase, dry-type transformers are designed to be most efficient at 35% loading of the nameplate rating. This design load may be typical for most transformers across all installations, but in more recent designs and installations, one can see an increased average loading of 45% to 55% of the nameplate rating-some designs push even higher.

Design considerations

As electrical distribution system designs are using the new transformers, engineers and designers should consider physical space, rated temperature rise, impedance, maximum inrush current, and arc flash.

Physical space

As energy efficiency has been the focus of transformer design, the physical footprint of the transformer has noticeably become larger to accommodate design revisions. When coordinating with an architect in new construction or a renovation, one should take into consideration the physical footprint, working clearances required per the NFPA 70-2017: National Electrical Code (NEC), and ventilation clearance. Most manufacturers require a minimum 6 in. of clearance from walls and other combustible materials for ventilation and to prevent a fire within the electrical room. This information can be confirmed in the manufacturer’s specifications.

Rated temperature rise

Standard temperature rise is the rated operating temperature above the ambient temperature. When one is converting power from higher to lower voltage or vice versa, heat is generated. Lower temperature rise is achieved by incorporating a more robust core-and-winding configuration. Standard transformer temperature rises are rated for 302°F, 239°F, and 176°F.

The most commonly used temperature-rise rating is 302°F because it has the least amount of material in the core and windings, making it less expensive to manufacture and purchase. However, with this higher temperature rise, more heat is being generated into the electrical room. If higher kilovolt-ampere-rated transformers are used, or if multiple units are located in the same room, one should consider the use of lower temperature-rise transformers. Less heat being generated could prolong the life of the equipment and be less of a burden on the facility cooling systems. The specification of temperature-rise ratings will affect impedance and maximum inrush.


Impedance is the effective resistance of the electric circuit or component to ac, arising from the combined effects of ohmic resistance and reactants. As transformers become more efficient, the result is lower impedance. Some impedance is a useful concept when designing a distribution system. The impedance helps reduce the available fault current between points in the system. At different, specified, temperature-rise ratings, the impedance can vary for the same size transformer. A transformer rated for 112.5 kVA may have an infinite bus let-through greater than 10,000-ampere interrupting capacity (AIC) depending on the temperature rating specified, resulting in downstream equipment needing to be rated for 14,000 AIC or more.

Maximum inrush

Inrush is the sudden influx of power. So, with greater efficiency, modified construction, and lower impedance, the maximum inrush at 100% load can be higher than seen in previous, less efficient transformers. When planning for the installation, the typical approach might be to feed the transformer from a standard, molded-case circuit breaker. Most of these circuit breakers do not have adjustable, instantaneous-trip settings. When sizing the upstream feeder breaker for a new transformer, it may be best to consider an adjustable-trip circuit breaker or the next higher standard rating to accommodate for maximum inrush when the unit is energized under load, as long as that size still meets the requirements set forth by the NEC.

For larger transformers in the 112.5-kVA range and higher, the best solution may be the use of an adjustable trip circuit breaker to allow for adjustments to meet inrush requirements and properly protect the transformer from damage. Usually, the issue of inrush nuisance tripping is seen after the facility has lost normal power and power is restored, either on the generator or through the return of utility power, when the feeder circuit breaker trips during the re-energizing period. This is very seldom caught during start-up of the transformer since the load is still not connected.

Arc flash considerations

Arc flash is caused by available incident energy in an electrical distribution system. It is a type of electrical explosion or discharge that results from a low-impedance connection. Energized electrical work within a facility is often frowned upon by the Environmental Health and Safety (EHS) department. But in some instances, for proper troubleshooting, energized is the only way to find the problem. Incident energy requires some design considerations when dealing with and mitigating higher-than-expected energy values associated with larger transformers. NFPA 70E-2015: Standard for Electrical Safety in the Workplace allows for transformers sized for 240 V at 125 kVA and less to be omitted from incident-energy calculations because the authors determined that the equipment downstream of these transformers could not generate enough incident energy to be harmful.

After engineers complete arc flash hazard analyses and review the data from software calculations, transformers smaller than 125 kVA with lower impedances theoretically can allow enough incident energy in the system to be greater than the NFPA 70E exception contemplates. Taking this into consideration, use of the calculated values for all transformer sizes below 125 kVA is recommended for the engineer’s consideration. This is a more conservative approach to arc flash analysis, but human safety is the highest priority. After actual results are calculated, each panelboard or switchboard downstream of a 3-phase, low-voltage, dry-type transformer should be evaluated on a case-by-case basis.

Brad Watfordelectrical engineer, CRB. This article originally appeared on CRB’s website. CRB is a CFE Media content partner. Edited by Hannah Cox, content specialist, CFE Media,

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