Oil-Filled Transformer Sizing Chart: Complete kVA Selection Guide - Power Transformer Technical Guide & Educational Resources

Comprehensive sizing chart for oil-immersed distribution transformers. Find specifications, dimensions, oil
volume, and selection guidelines for all standard kVA ratings from 10 kVA to 2500 kVA.

Oil-filled transformers are the most widely used type for utility distribution, industrial facilities, and
commercial applications. The mineral oil provides excellent insulation and cooling, enabling higher
efficiency and longer lifespan compared to dry-type units. This comprehensive sizing chart helps engineers and
contractors select the correct kVA rating.

✅ Key Advantages of Oil-Filled Transformers: Higher efficiency (up to 99.4%), better overload
capability, lower acoustic noise, and longer service life (30+ years with proper maintenance).

Single-Phase Oil-Filled Transformer Sizing Chart

Single-phase oil-filled transformers are commonly used for residential distribution, rural electrification,
and commercial applications. Primary voltage: 7200V, Secondary: 120/240V.

kVA	Dimensions (H×W×D)	Weight (lbs)	Oil (gal)	BIL	Specs
10 kVA	29″ × 21″ × 15″	300 lbs	15 gal	60 kV	View Details →
15 kVA	32″ × 23″ × 16″	400 lbs	20 gal	60 kV	View Details →
25 kVA	36″ × 25″ × 18″	640 lbs	26 gal	60 kV	View Details →
37.5 kVA	39″ × 28″ × 20″	800 lbs	32 gal	60 kV	View Details →
50 kVA	42″ × 30″ × 22″	950 lbs	38 gal	60 kV	View Details →
75 kVA	46″ × 32″ × 24″	1,200 lbs	50 gal	95 kV	View Details →
100 kVA	49″ × 35″ × 26″	1,500 lbs	60 gal	95 kV	View Details →
167 kVA	55″ × 39″ × 30″	2,000 lbs	80 gal	95 kV	View Details →
250 kVA	61″ × 43″ × 33″	2,500 lbs	100 gal	95 kV	View Details →
333 kVA	66″ × 46″ × 37″	3,000 lbs	120 gal	95 kV	View Details →
500 kVA	74″ × 52″ × 41″	4,000 lbs	150 gal	125 kV	View Details →

Three-Phase Oil-Filled Transformer Sizing Chart

Three-phase oil-filled transformers serve industrial plants, commercial buildings, utility substations, and
grid infrastructure. Primary voltage: 12470Y/7200V.

kVA	Dimensions (H×W×D)	Weight (lbs)	Oil (gal)	Secondary	Cooling	Specs
15 kVA	34″ × 24″ × 18″	500 lbs	20 gal	208Y/120V	ONAN	Details →
30 kVA	39″ × 28″ × 21″	750 lbs	30 gal	208Y/120V	ONAN	Details →
45 kVA	42″ × 31″ × 23″	950 lbs	38 gal	208Y/120V	ONAN	Details →
75 kVA	46″ × 35″ × 26″	1,250 lbs	50 gal	208Y/120V	ONAN	Details →
112.5 kVA	51″ × 38″ × 29″	1,600 lbs	65 gal	208Y/120V	ONAN	Details →
150 kVA	55″ × 41″ × 32″	1,900 lbs	80 gal	208Y/120V	ONAN	Details →
225 kVA	60″ × 46″ × 35″	2,400 lbs	100 gal	208Y/120V	ONAN	Details →
300 kVA	65″ × 50″ × 39″	3,000 lbs	120 gal	208Y/120V	ONAN	Details →
500 kVA	73″ × 56″ × 44″	4,000 lbs	160 gal	208Y/120V	ONAN	Details →
750 kVA	80″ × 61″ × 48″	5,000 lbs	200 gal	480Y/277V	ONAN	Details →
1000 kVA	87″ × 66″ × 53″	6,500 lbs	250 gal	480Y/277V	ONAN	Details →
1500 kVA	96″ × 74″ × 59″	9,000 lbs	350 gal	480Y/277V	ONAF	Details →
2000 kVA	104″ × 80″ × 65″	12,000 lbs	450 gal	480Y/277V	ONAF	Details →
2500 kVA	111″ × 86″ × 70″	15,000 lbs	550 gal	480Y/277V	ONAF	Details →

Oil Volume & Specifications by kVA

Proper oil management is critical for oil-filled transformer performance and longevity. Use this chart for oil volume
planning and maintenance.

kVA Range	Oil Volume	Oil Type	Sample Interval	Filtering Interval
10-50 kVA	15-40 gallons	Mineral Oil (IEEE C57.106)	Every 3 years	Every 10 years
75-167 kVA	50-80 gallons	Mineral Oil (IEEE C57.106)	Every 2 years	Every 7 years
225-500 kVA	100-160 gallons	Mineral Oil (IEEE C57.106)	Every 1-2 years	Every 5 years
750-2500 kVA	200-550 gallons	Mineral Oil (IEEE C57.106)	Annually	Every 3-5 years

⚠️ Oil Testing Standards: Oil should be tested per IEEE C57.106 for
dielectric breakdown voltage, moisture content, acidity, and dissolved gas analysis (DGA).

How to Select the Right Oil-Filled Transformer Size

Step 1: Calculate Total Connected Load

Add up all electrical loads that will be served by the transformer:

Motors and industrial equipment (consider starting current)
Lighting and general receptacles
HVAC systems
Future expansion (typically 20-25% reserve)

Step 2: Apply Demand Factor

Application	Typical Demand Factor
Residential/Rural	35-50%
Commercial	60-70%
Industrial/Manufacturing	70-85%
Utility Substation	80-90%

Step 3: Consider Environment

Oil-filled transformers are ideal for:

Outdoor installations (pole-mount, pad-mount, substation)
High ambient temperature environments (excellent cooling)
Applications requiring 24/7 operation with overload capability
Long service life requirements (30+ years)

Example: An industrial plant has 400 kVA connected load with 80% demand factor and 0.9 power
factor.
Required kVA = (400 × 0.80) ÷ 0.9 = 356 kVA → Select 500 kVA three-phase oil-filled

Frequently Asked Questions

What is the difference between ONAN and ONAF cooling?

ONAN (Oil Natural, Air Natural) uses natural convection only. ONAF (Oil Natural,
Air Forced) adds fans for higher capacity. Larger units (1500+ kVA) typically use ONAF for enhanced heat
dissipation.

How long do oil-filled transformers last?

With proper maintenance, oil-filled transformers have a typical service life of 30-40 years. Regular
oil testing and filtering can extend life beyond 50 years.

What oil is used in transformers?

Most distribution transformers use mineral insulating oil per IEEE C57.106. Some newer units use
FR3 natural ester fluid for enhanced fire safety and environmental benefits.

Related Resources

📊 More Reference Charts:

Oil-Filled Full
Load Amps Chart – FLA for all voltages
Transformer
Oil Complete Guide – Types, testing, maintenance

Standards & References

IEEE C57.12.00 –
General Requirements for Liquid-Immersed Transformers
IEEE C57.106 – Guide
for Acceptance of Insulating Oil
DOE 10 CFR 431 – Energy Conservation Standards

Disclaimer: Specifications shown are typical reference values per IEEE standards. Actual specifications vary by
manufacturer. Always consult manufacturer datasheets for exact dimensions and oil volumes. Last updated: January
2026