Transformer Sizing & OCPD Calculator

Size low-voltage dry-type and liquid transformers (kVA), calculate primary/secondary Full-Load Amps (FLA), size 125% OCPD per NEC §450.3(B), and estimate secondary fault current.

NEC edition

Inputs

Phases

Primary voltage

Secondary voltage

Load (or select transformer kVA)

kVA

Enter connected load kVA or desired transformer nameplate kVA

Transformer type

Protection scheme

Where the secondary is protected at 125%, Table 450.3(B) permits the primary device to go to 250% — routinely needed to ride through inrush

Impedance (%Z)

Typical 5–6% for dry-type; used for fault current calculation

Result

Calculated result

50 kVA transformer

⚠️ ℹ️ The fault current shown assumes an infinite primary bus, so it is the worst case. Use the utility-supplied available fault current for coordination work.

⚠️ ℹ️ Adding secondary protection would permit a primary device up to 250% of 60.1 A, which is usually what it takes to ride through magnetising inrush.

Code references

NEC §450.3 Overcurrent protection for transformers
NEC §450.3(B) Transformers 1000V or less — 125% / 167% / 250% / 300% allowances
NEC §240.21(C) Transformer secondary conductor tap rules
NEC §110.24 Available fault current marking requirement
NEC §240.6(A) Standard ratings, including the 1, 3, 6, 10 and 601 A fuse ratings

Load 45 kVA

Standard transformer 50 kVA §450

Phases Three-phase

Primary voltage 480V

Secondary voltage 208V

Primary FLA 60.1 A

Secondary FLA 138.8 A

Transformer type Dry-type §450.21

Protection scheme Primary only Table 450.3(B)

Primary OCPD percentage 125% — primary protection only Table 450.3(B)

Primary OCPD 80A (75.2A calculated) Table 450.3(B)

Secondary OCPD percentage 125% Table 450.3(B)

Secondary OCPD 175A (173.5A calculated) Table 450.3(B)

Impedance (%Z) 5.75%

Available fault current (secondary) 2,414 A (infinite primary bus assumed) §110.24

⚡ Test your knowledge

NEC Article 220 was renumbered in the 2026 code cycle. What is the new article number?

A. Art. 100
B. Art. 120
C. Art. 200
D. Art. 230

💡 NEC 2026 renumbered Article 220 (Branch-Circuit, Feeder, and Service Calculations) to Article 120.

Energy Management Systems were moved from Article 750 to which article in NEC 2026?

A. Art. 100
B. Art. 125
C. Art. 130
D. Art. 150

💡 NEC 2026 moved Energy Management Systems from Article 750 to Article 130.

Which NEC standard size overcurrent device comes after 90A?

A. 95A
B. 100A
C. 110A
D. 105A

💡 Per NEC §240.6(A), the standard sizes are …90, 100, 110, 125… There is no 95A standard size.

How it works & the NEC rules behind it

Step-down transformers (such as standard 480V Delta to 208Y/120V Three-Phase commercial dry-type units) require careful coordination of primary and secondary overcurrent protection under NEC Article 450 and transformer secondary conductor tap rules under NEC §240.21(C).

Overcurrent Protection Rules (NEC Table 450.3(B)):

Primary Only Protection: Where secondary protection is not provided, the primary OCPD must not exceed 125% of primary FLA (or the next standard size up if current is 9A or more).
Primary and Secondary Protection: Where secondary OCPD is provided at 125% of secondary FLA, the primary OCPD can be sized up to 250% of primary FLA to prevent nuisance tripping from transformer magnetic inrush current!
Secondary Conductor Taps (§240.21(C)): Secondary conductors cannot be protected by the primary OCPD in 3-phase systems. They must terminate in a single OCPD sized to protect the conductors within 10 feet or 25 feet.

Formulas & equations

Single-Phase FLA = (kVA × 1,000) / Voltage Three-Phase FLA = (kVA × 1,000) / (√3 × Voltage) Secondary Fault Current (Isc) = Secondary FLA / (%Z / 100)

Where:
• %Z: Transformer percent impedance from nameplate (typically 3.0% to 5.75% for dry-type units).

NEC reference table

Transformer kVA (3-Phase)	480V Primary FLA	Primary 125% Breaker	208V Secondary FLA	Secondary 125% Breaker
15 kVA	18.0 A	25 A	41.6 A	60 A
30 kVA	36.1 A	50 A	83.3 A	110 A
45 kVA	54.1 A	70 A	124.9 A	175 A
75 kVA	90.2 A	125 A	208.2 A	300 A
112.5 kVA	135.3 A	175 A	312.3 A	400 A
150 kVA	180.4 A	250 A	416.4 A	600 A
225 kVA	270.6 A	350 A	624.6 A	800 A
300 kVA	360.8 A	500 A	832.7 A	1200 A

Worked example, step by step

Example Calculation: 45 kVA 480V to 208Y/120V 3-Phase Transformer

Scenario: Sizing primary and secondary circuit breakers and secondary available fault current for a 45 kVA dry-type transformer (5.0% impedance).

Primary FLA (480V 3Ø): 45,000 / (1.732 × 480) = 54.1 Amps. Primary Breaker (125%) = 54.1 × 1.25 = 67.6A → 70A Breaker.
Secondary FLA (208V 3Ø): 45,000 / (1.732 × 208) = 124.9 Amps. Secondary Main Breaker (125%) = 124.9 × 1.25 = 156.1A → 175A Breaker.
Available Fault Current (Isc): 124.9A / 0.05 = 2,498 Amps. A standard 10k or 22k AIC secondary distribution panel is fully adequate.

Frequently asked questions

When energized, transformers draw an instantaneous magnetizing inrush current that can reach 10 to 20 times normal full-load current for a few cycles. Sizing the primary breaker with an adequate time-delay trip curve or providing primary protection at 250% (with a 125% secondary OCPD per Table 450.3(B)) prevents nuisance tripping.

No. Under NEC §240.21(C), transformer secondary conductors are taps and do not have overcurrent protection at their point of supply. Therefore, they must comply with the 10-foot or 25-foot transformer secondary tap rules and terminate in a single set of fuses or circuit breaker.

Inputs

Take this toolkit with you on the job

Studying for your electrical exam?

How it works & the NEC rules behind it

Overcurrent Protection Rules (NEC Table 450.3(B)):

Formulas & equations

NEC reference table

Worked example, step by step

Frequently asked questions

Why does a transformer trip the primary breaker when first energized? −

Do transformer secondary conductors have overcurrent protection at their supply end? +

Related NEC calculators

Fault Current / AIC Calculator

Overcurrent Protection Selector

Wire Gauge Calculator