Managing increased transformer inrush current

The U.S. Department of Energy (DOE) new efficiency levels for low-voltage dry-type distribution transformers came into effect at the beginning of 2016. Technically known as CFR title 10 Chapter II Part 431 (in Appendix A of Subpart K 2016), the new efficiency requirements are more commonly referred to as the DOE 2016 Efficiency levels.

To meet these new efficiency levels, manufacturers can employ various design strategies including the use of high-grade steel, lowering of the induction level of the core, and using different core constructions. However, these design changes also can impact other characteristics of the transformer, including size, cost, and inrush current characteristics.

Although changes in transformer size and cost are significant considerations, this information can be easily obtained from the manufacturer. The third variable identified above, inrush current, can be less obvious but equally important when designing a system with new DOE 2016 compliant transformers.

Inrush current is an important consideration when selecting an overcurrent protection device (OCPD) to protect the transformer. If sized incorrectly, the OCPD may operate during system start-up and prevent the transformer from energizing.

In the past, many design engineers took advantage of NFPA 70: National Electrical Code (NEC) article 450 table 450.3(B) and sized primary OCPDs for low-voltage distribution dry-type transformers at no more than 125% of the transformers full-load primary ampere (FLA). With the primary OCPD sized at 125%, the transformer is fed with the lowest-cost cable and conduit, which provides adequate overload and short-circuit protection for the wire and for the transformer. In addition, the protection is well below the damage curve of the transformer (NEMA Standard 206 requires the transformer to withstand 20 to 25 times full-load current rating for 2 seconds for standard dry-type transformers). The other factor allowing this selection to work is that the transformer inrush currents were typically 4 to 10 times its primary FLA rating.

However, with the advent of the 2010 DOE legislation, the desire to use higher efficiency transformers, and the increased application of K factor and specialty transformers, the industry started to experience some nuisance tripping of the primary OCPD when sized at 125%.

And now with the possibility of even higher inrush currents as a result of DOE 2016, this topic takes on additional importance. Today it is not uncommon for the theoretical maximum transformer inrush currents to be as high as 20 to 30 times the primary FLA of the transformers.

When this possibility for higher inrush currents is compounded with the many variations in construction methods and materials between manufacturers, and even between transformer types/ratings from the same manufacturer, it becomes extremely important for engineers to verify inrush current values.

To help customers with this new challenge, some manufacturers design and test standard (or most common) DOE 2016 transformers to allow the primary OCPD to be sized at 125%. However, this may not be the case with all manufacturers. If the primary OCPD requires to be sized larger than 125%, the design engineer can take advantage of NEC Table 450.3(B) (OCPD sizing up to 250%) and NEC article 240.21 to avoid the need to provide a secondary circuit OCPD at the transformer when feeding a lighting panelboard or load.

It also is important to note that manufacturer published inrush values are most often descriptive of transformers that are energized from the primary winding, which are the outer windings of a transformer. If the transformer is reverse-fed and energized from the secondary winding—the inner windings—you can expect the inrush values to be dramatically greater. To address this issue, engineers should always be cautious of back feeding in applications above 75 kVA and select the largest OCPD allowed by code.

As outlined in this column, it is vital for design engineers to be aware of inrush currents for low voltage dry type distribution transformers, especially in our modern high-efficiency world. Remember to always review NEC labeling and consult with manufacturers before installing.

Greg J. Hausman is a senior application engineer at Eaton, with more than 35 years of experience in the electrical distribution equipment industry. He is an active member of IEEE, International Association of Electrical Inspectors, and NFPA.

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