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Distribution Transformer Efficiency

Until 1996 Industry Standards for the Efficiency of Distribution Transformers did not exist. During 1996 NEMA® (National Electrical Manufactures Association) published NEMA TP 1 “Guide for Determining Efficiency for Distribution Transformers” or distribution transformer efficiency. The Scope of this standard included Single Phase and Three Phase Dry Type and Liquid filled Distribution Transformers. NEMA TP-1 was updated during 2002 and the requirements of TP-1 were included into the Energy Star program.

Effective January 1st 2007 The Energy Policy Act of 2005 as interpreted in Federal Regulations 10 CFR Part 431 – “Energy Efficiency Program For Certain Commercial and Industrial Equipment” Subpart K – Distribution Transformers” requires Low Voltage Dry Type Distribution Transformers intended for installation in the United States and U.S. Territories to meet minimum distribution transformer efficiency standards as set forth in the program and as indicted in Table 4-2 of NEMA Standard TP-1 2002.

Low Voltage Distribution Transformers are defined as follows:

  1. Has input voltage of 600 volts or less
  2. Is Air Cooled
  3. Does not use oil as coolant

Table I

Guide for Determining the Energy Efficiency of Distribution Transformers
NEMA Standard TP 1 -2002
Low Voltgage Transformers (600 volts and below)

KVASingle Phase Efficiency %KVAThree Phase Efficiency %
1597.71597.0
2598.03097.5
37.598.24597.7
5098.37598.0
7598.5112.598.2
10098.615098.3
16798.722598.5
25098.830098.6
33398.950098.7
75098.8
100098.9

Efficiency values are at 35% of nameplate rated load and at a reference temperature of 75° C

The term “Distribution Transformer” does not include the following:

  1. Auto Transformer
  2. Drive Isolation Transformer
  3. Grounding Transformer
  4. Machine Tool Transformer
  5. Non-Ventilated Transformer
  6. Rectifier Transformer
  7. Regulating Transformer
  8. Sealed Transformer
  9. Special Impedance Transformer
  10. Testing Transformer
  11. Transformer with tap Range of 20% or more
  12. Uninterruptible Power Supply Transformers
  13. Welding Transformers

One significant difference between the TP-1-1996 requirements and the DOE 2007 requirements is that the TP-1 requirements excluded “high harmonic transformers” (K factor rated) and “Retrofit Transformers”, the DOE 2007 requirements do not exclude these two types of Transformers.

October 2007 the U.S. Department of Energy published “Energy Conservation Program for Commercial Equipment: Distribution Transformer Energy Conservation Standards, Final Rule. This ruling mandated efficiency requirements for Dry Type and Liquid Filled Distribution Transformers effective as of  January 1st, 2010. This mandated requirement is commonly referred to as DOE 2010.

Per Federal Regulations 10 CFR Part 431 Subpart K a Distribution Transformer is defined as:

  1. Has an input voltage of 34.5 kV or less
  2. Has an output voltage of 600 V or less
  3. Is rated of operation at a frequency of 60HZ
  4. Has a capacity of 10 kVA to 2500 kVA for liquid-immersed units and 15 kVA to 2500 kVA for dry type units

The exceptions are defined by Federal Regulations 10 CFR 431 ¶431.192 (5) which are the same as the exceptions as indicated in the DOE 2007 requirements.

April 2013 a new ruling was published by the Department of Energy raising the efficiency required by DOE 2010. These increased efficiencies were effective on January 1st 2016. These new increased efficiencies are commonly referred to as DOE 2016. The scope and exceptions remained the same as for DOE 2010.

A comparison of the DOE 2010 and DOE 2016 transformer efficiencies are as in indicated in the following tables II, III and IV.

Table II

Low Voltage Transformers (600 volts and below)
DOE 2010 and 2016 Transformer Efficiency Standards

KVASingle Phase Efficiency % DOE 2010Single Phase Efficiency % DOE 2016KVAThree Phase Efficiency % DOE 2010Three phase Efficiency % DOE 2016
1597.797.71597.097.89
2598.098.03097.598.23
37.598.298.24597.798.4
5098.398.37598.098.6
7598.598.5112.598.298.74
10098.698.615098.398.83
16798.798.722598.598.94
25098.898.830098.699.02
33398.998.950098.799.14
75098.899.23
100098.999.28

Efficiency values are at 35% of nameplate rated load and at a reference temperature of 75° C

Table III – Single Phase

Medium Voltage Dry Type Distribution Transformers – DOE 2010 vs DOE 2016

KVA20-45 BIL45-95 BIL>= 96 BIL
kVA SinglePhaseDOE 2010DOE 2016%DIF20102016%DIF20102016%DIF
1598.198.1097.8697.860NANANA
2598.3398.33098.1298.120NANANA
37.598.4998.49098.398.30NANANA
5098.698.6098.4298.420NANANA
7598.7398.73098.5798.57098.5398.530
10098.8298.82098.6798.67098.6398.630
16798.9698.96098.8398.83098.898.80
25099.0799.07098.9598.95098.9198.910
33399.1499.14099.0399.03098.9998.990
50099.2299.22099.1299.12099.0999.090
66799.2799.27099.1899.18099.1599.150
83399.3199.31099.2399.23099.299.20

Table IV – Three Phase

KVA20-45 BIL45-95 BIL>= 96 BIL
kVA Three Phase20102016%DIF20102016%DIF20102016%DIF
1597.597.5097.1897.180NANANA
3097.997.9097.6397.630NANANA
4598.198.1097.8697.860NANANA
7598.3398.33098.1298.130.01NANANA
112.598.4998.520.0398.398.360.06NANANA
15098.698.650.0598.4298.510.09NANANA
22598.7398.820.0998.5798.690.1298.5398.570.04
30098.8298.930.1198.6798.810.1498.6398.690.06
50098.9699.090.1398.8398.990.1698.898.890.09
75099.0799.210.1498.9599.120.1798.9199.020.11
100099.1499.280.1499.0399.20.1798.9999.110.12
150099.2299.370.1599.1299.30.1899.0999.210.12
200099.2799.430.1699.1899.360.1899.1599.280.13
250099.3199.470.1699.2399.410.1899.299.330.13

Efficiency values are at 50% of nameplate rated load and at a reference temperature of 75° C

One of the exceptions for Distribution Transformers per DOE 2016 are those with special impedance. Normal Impedance percentages for Dry Type Distribution Transformers as defined in 10 CFR 431 are as listed in the following Table.

Table V

Normal Impedance Range for Dry Type Transformers
Per Federal Regulations 10 CFR Part 431 Subpart K Table 2

Single PhaseThree Phase
kVAImpedance (%)kVAImpedance (%)
151.5-6.0151.5-6.0
251.5-6.0301.5-6.0
34.51.5-6.0451.5-6.0
501.5-6.0751.5-6.0
752.0-7.0112.51.5-6.0
1002.0-7.01501.5-6.0
1672.5-8.02253.0-7.0
2503.5-8.03003.0-7.0
3333.5-8.05004.5-8.0
5003.5-8.07505.0-8.0
6675.0-8.010005.0-8.0
8335.0-8.015005.0-8.0
20005.0-8.0
25005.0-8.0