Transformer Minimum Efficiency (DOE)

The minimum efficiencies that distribution transformers sold in the US must meet, and the per-unit load at which each is measured.

Low-voltage dry-type — 2016 efficiency % at 35% load

1φ kVA	η %	3φ kVA	η %
15	97.70	15	97.89
25	98.00	30	98.23
37.5	98.20	45	98.40
50	98.30	75	98.60
75	98.50	112.5	98.74
100	98.60	150	98.83
167	98.70	225	98.94
250	98.80	300	99.02
333	98.90	500	99.14
		750	99.23
		1000	99.28

Liquid-immersed — 2016 efficiency % at 50% load

1φ kVA	η %	3φ kVA	η %
10	98.70	15	98.65
15	98.82	30	98.83
25	98.95	45	98.92
37.5	99.05	75	99.03
50	99.11	112.5	99.11
75	99.19	150	99.16
100	99.25	225	99.23
167	99.33	300	99.27
250	99.39	500	99.35
333	99.43	750	99.40
500	99.49	1000	99.43
667	99.52	1500	99.48
833	99.55	2000	99.51
		2500	99.53

Medium-voltage dry-type, 20–45 kV BIL — 2016 efficiency % at 50% load

1φ kVA	η %	3φ kVA	η %
15	98.10	15	97.50
25	98.33	30	97.90
37.5	98.49	45	98.10
50	98.60	75	98.33
75	98.73	112.5	98.52
100	98.82	150	98.65
167	98.96	225	98.82
250	99.07	300	98.93
333	99.14	500	99.09
500	99.22	750	99.21
667	99.27	1000	99.28
833	99.31	1500	99.37
		2000	99.43
		2500	99.47

Medium-voltage dry-type, 46–95 kV BIL — 2016 efficiency % at 50% load

1φ kVA	η %	3φ kVA	η %
15	97.86	15	97.18
25	98.12	30	97.63
37.5	98.30	45	97.86
50	98.42	75	98.13
75	98.57	112.5	98.36
100	98.67	150	98.51
167	98.83	225	98.69
250	98.95	300	98.81
333	99.03	500	98.99
500	99.12	750	99.12
667	99.18	1000	99.20
833	99.23	1500	99.30
		2000	99.36
		2500	99.41

Medium-voltage dry-type, ≥ 96 kV BIL — 2016 efficiency % at 50% load

1φ kVA	η %	3φ kVA	η %
75	98.53	225	98.57
100	98.63	300	98.69
167	98.80	500	98.89
250	98.91	750	99.02
333	98.99	1000	99.11
500	99.09	1500	99.21
667	99.15	2000	99.28
833	99.20	2500	99.33

Low-voltage dry-type — 2029 efficiency % at 35% load

1φ kVA	η %	3φ kVA	η %
15	98.39	15	98.31
25	98.60	30	98.58
37.5	98.74	45	98.72
50	98.81	75	98.88
75	98.95	112.5	98.99
100	99.02	150	99.06
167	99.09	225	99.15
250	99.16	300	99.22
333	99.23	500	99.31
		750	99.38
		1000	99.42

Liquid-immersed (non-submersible) — 2029 efficiency % at 50% load

1φ kVA	η %	3φ kVA	η %
10	98.77	15	98.92
15	98.88	30	99.06
25	99.00	45	99.14
37.5	99.10	75	99.22
50	99.15	112.5	99.29
75	99.23	150	99.33
100	99.29	225	99.38
167	99.46	300	99.42
250	99.51	500	99.38
333	99.54	750	99.43
500	99.59	1000	99.46
667	99.62	1500	99.51
833	99.64	2000	99.53
		2500	99.55
		3750	99.54
		5000	99.53

Medium-voltage dry-type, 20–45 kV BIL — 2029 efficiency % at 50% load

1φ kVA	η %	3φ kVA	η %
15	98.29	15	97.75
25	98.50	30	98.11
37.5	98.64	45	98.29
50	98.74	75	98.50
75	98.86	112.5	98.67
100	98.94	150	98.79
167	99.06	225	98.94
250	99.16	300	99.04
333	99.23	500	99.18
500	99.30	750	99.29
667	99.34	1000	99.35
833	99.38	1500	99.43
		2000	99.49
		2500	99.52
		3750	99.50
		5000	99.48

Medium-voltage dry-type, 46–95 kV BIL — 2029 efficiency % at 50% load

1φ kVA	η %	3φ kVA	η %
15	98.07	15	97.46
25	98.31	30	97.87
37.5	98.47	45	98.07
50	98.58	75	98.32
75	98.71	112.5	98.52
100	98.80	150	98.66
167	98.95	225	98.82
250	99.06	300	98.93
333	99.13	500	99.09
500	99.21	750	99.21
667	99.26	1000	99.28
833	99.31	1500	99.37
		2000	99.42
		2500	99.47
		3750	99.44
		5000	99.43

Medium-voltage dry-type, ≥ 96 kV BIL — 2029 efficiency % at 50% load

1φ kVA	η %	3φ kVA	η %
75	98.68	225	98.71
100	98.77	300	98.82
167	98.92	500	99.00
250	99.02	750	99.12
333	99.09	1000	99.20
500	99.18	1500	99.29
667	99.24	2000	99.35
833	99.28	2500	99.40
		3750	99.40
		5000	99.39

How this is generated

Values are the US Department of Energy standards in 10 CFR 431.196. Federal regulations are not subject to copyright (17 U.S.C. §105). The tables were generated from the regulation text and checked value by value. For a kVA between listed ratings, the regulation itself says to interpolate linearly.

Loss at the test load Pt = kVA · Lt · (1/η − 1) kW Lt = 0.35 low-voltage dry-type; 0.50 liquid-immersed and medium-voltage dry-type

The 2029 levels apply to units manufactured from April 23, 2029. For liquid-immersed units they are the non-submersible values; submersible units keep the 2016 levels. The 2029 three-phase liquid-immersed values dip at 500 kVA — that is how the regulation reads.

Electrical Room Heat Load uses these values for transformer losses (2016 by default, 2029 selectable).

Sources: 10 CFR 431.196 (energy conservation standards for distribution transformers), compliance from January 1, 2016; 10 CFR 431.196 (energy conservation standards for distribution transformers), units manufactured on or after April 23, 2029. © RokBench.