Efficiency Finder Tool
Used to determine BIL class for Dry-Type transformers only.
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Overview of the Standards
The U.S. Department of Energy (DOE) establishes minimum energy conservation standards for distribution transformers to reduce energy losses and enhance national energy efficiency. These regulations have evolved, with significant updates implemented in 2010 and 2016. The 2016 standard, which became effective on January 1, 2016, requires higher efficiency levels than the preceding 2010 rules.
These standards apply to transformers manufactured for sale in or imported into the United States. Compliance is determined by measuring efficiency at a specific percentage of the transformer's nameplate-rated load. This guide provides the detailed efficiency tables for both the 2010 and 2016 guidelines.
How Efficiency is Calculated and Tested
Transformer efficiency (η) is the ratio of the useful power output to the total power input, expressed as a percentage. The difference between the input and output power is due to energy losses within the transformer, which are dissipated as heat.
The fundamental formula is:
Efficiency (η) = (Power Output / Power Input) x 100%
Because Power Input equals Power Output plus the transformer's internal losses (No-Load and Load losses), the formula can be expanded for calculation purposes. To ensure a consistent standard for comparison, the DOE specifies the exact conditions under which this efficiency must be determined:
- Load Level: Efficiency for all medium-voltage transformers must be calculated at exactly 50% of the nameplate-rated load.
- Power Factor: The calculation assumes a unity power factor of 1.0 (where voltage and current are perfectly in phase).
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Reference Temperature: This is a critical factor, as winding resistance changes with temperature, affecting load losses. The DOE specifies different reference temperatures based on the transformer type:
- For Dry-Type Transformers, load losses are corrected to a reference temperature of 75°C.
- For Liquid-Immersed Transformers, load losses are corrected to a reference temperature of 55°C.
- No-Load Losses: These core losses are measured separately, typically at an ambient temperature of 20°C.
By standardizing these conditions, manufacturers can be held to a consistent benchmark, and consumers can accurately compare the efficiencies of different transformer models regardless of where or how they were tested.
DOE 2016 Efficiency Standards
These standards represent the current mandatory minimum efficiencies and are more stringent than the 2010 levels.
Liquid-Immersed (Oil-Filled) Transformers
| Single-Phase | Three-Phase | ||
|---|---|---|---|
| kVA | Efficiency (%) | kVA | Efficiency (%) |
| 10 | 98.70 | 15 | 98.65 |
| 15 | 98.82 | 30 | 98.83 |
| 25 | 98.95 | 45 | 98.94 |
| 37.5 | 99.05 | 75 | 99.08 |
| 50 | 99.11 | 112.5 | 99.17 |
| 75 | 99.20 | 150 | 99.23 |
| 100 | 99.26 | 225 | 99.31 |
| 167 | 99.36 | 300 | 99.36 |
| 250 | 99.42 | 500 | 99.43 |
| 333 | 99.46 | 750 | 99.48 |
| 500 | 99.52 | 1000 | 99.51 |
| 667 | 99.55 | 1500 | 99.55 |
| 833 | 99.58 | 2000 | 99.58 |
| - | - | 2500 | 99.60 |
Efficiency measured at 50% of rated load.
Dry-Type Transformers
| Single-Phase | Three-Phase | ||||||
|---|---|---|---|---|---|---|---|
| kVA | 20-45 kV BIL (%) | 46-95 kV BIL (%) | ≥ 96 kV BIL (%) | kVA | 20-45 kV BIL (%) | 46-95 kV BIL (%) | ≥ 96 kV BIL (%) |
| 15 | 98.10 | 97.86 | - | 15 | 97.50 | 97.18 | - |
| 25 | 98.33 | 98.12 | - | 30 | 97.90 | 97.63 | - |
| 37.5 | 98.49 | 98.30 | - | 45 | 98.10 | 97.86 | - |
| 50 | 98.60 | 98.42 | - | 75 | 98.33 | 98.13 | - |
| 75 | 98.73 | 98.57 | 98.53 | 112.5 | 98.52 | 98.36 | - |
| 100 | 98.82 | 98.67 | 98.63 | 150 | 98.65 | 98.51 | - |
| 167 | 98.96 | 98.83 | 98.80 | 225 | 98.82 | 98.69 | 98.57 |
| 250 | 99.07 | 98.95 | 98.91 | 300 | 98.93 | 98.81 | 98.69 |
| 333 | 99.14 | 99.03 | 98.99 | 500 | 99.09 | 98.99 | 98.89 |
| 500 | 99.22 | 99.12 | 99.09 | 750 | 99.21 | 99.12 | 99.02 |
| 667 | 99.27 | 99.18 | 99.15 | 1000 | 99.28 | 99.20 | 99.11 |
| 833 | 99.31 | 99.23 | 99.20 | 1500 | 99.37 | 99.30 | 99.21 |
| - | - | - | - | 2000 | 99.43 | 99.36 | 99.28 |
| - | - | - | - | 2500 | 99.47 | 99.41 | 99.33 |
Efficiency measured at 50% of rated load.
DOE 2010 Efficiency Standards
These standards were the precursor to the 2016 update and applied to transformers manufactured after January 1, 2010.
Liquid-Immersed (Oil-Filled) Transformers
| Single-Phase | Three-Phase | ||
|---|---|---|---|
| kVA | Efficiency (%) | kVA | Efficiency (%) |
| 10 | 98.62 | 15 | 98.36 |
| 15 | 98.76 | 30 | 98.62 |
| 25 | 98.87 | 45 | 98.78 |
| 37.5 | 98.98 | 75 | 98.95 |
| 50 | 99.04 | 112.5 | 99.07 |
| 75 | 99.13 | 150 | 99.14 |
| 100 | 99.20 | 225 | 99.23 |
| 167 | 99.29 | 300 | 99.29 |
| 250 | 99.35 | 500 | 99.36 |
| 333 | 99.40 | 750 | 99.42 |
| 500 | 99.46 | 1000 | 99.46 |
| 667 | 99.49 | 1500 | 99.50 |
| 833 | 99.52 | 2000 | 99.54 |
| - | - | 2500 | 99.56 |
Efficiency measured at 50% of rated load.
Dry-Type Transformers
Note: The DOE 2010 standards for Dry-Type transformers were identical to the 2016 standards. The primary changes in 2016 focused on liquid-filled units and low-voltage dry-type transformers. For consistency, the 2016 table above serves for both standards.