Transformer Sizing Calculator
Convert kVA to primary and secondary amps, then size OCPD per NEC 450.3(B). Free, no account required.
Transformer Details
Common sizes: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000 kVA.
54.1 A / 124.9 A
Primary and secondary full-load current at rated kVA.
NEC 450.3(B) Reference
- Primary only (current ≥ 9 A): Maximum primary OCPD = 125% of rated primary current. If not a standard rating, next higher standard size is permitted.
- Primary only (current 2 to 9 A): Maximum primary OCPD = 167%. Below 2 A, 300%.
- Primary + secondary protection: Primary OCPD up to 250%, secondary OCPD up to 125% of rated secondary current.
- Conductor sizing: Primary and secondary conductors are sized per NEC 215 / 240, typically at 125% of rated current for continuous load.
- Note: 450.3(A) covers transformers over 1,000 V (different table, different rules). This tool focuses on 1,000 V and less, the dominant exam scenario.
Common transformer sizes (3-phase, primary 480V, secondary 208V, primary-only OCPD)
| kVA | Primary A | Secondary A | Primary OCPD |
|---|---|---|---|
| 15 | 18.0 A | 41.6 A | 25 A |
| 30 | 36.1 A | 83.3 A | 50 A |
| 45 | 54.1 A | 124.9 A | 70 A |
| 75 | 90.2 A | 208.2 A | 125 A |
| 112.5 | 135.3 A | 312.3 A | 175 A |
| 150 | 180.4 A | 416.4 A | 250 A |
| 225 | 270.6 A | 624.5 A | 350 A |
| 300 | 360.8 A | 832.7 A | 500 A |
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This calculator follows NEC 2023 Article 450.3(B) for transformers rated 1,000 V and less. For transformers over 1,000 V refer to 450.3(A). Always verify with your AHJ and the NEC edition adopted in your jurisdiction.
How the transformer sizing calculation works
Transformer problems are two calculations wearing one hat. First you convert kVA to current on each winding, which is arithmetic. Then you size overcurrent protection from Table 450.3(B), which is a rules lookup that changes depending on whether you are protecting the primary only or the primary and secondary both.
For a single-phase transformer, amps equal VA divided by volts. For three-phase, amps equal VA divided by the product of volts and 1.732. Run it once at the primary voltage and once at the secondary voltage. The kVA is the same on both sides, so the currents are inversely proportional to the voltages.
Table 450.3(B) covers transformers of 1,000 V or less. With primary protection only, the primary device may not exceed 125 percent of primary current when that current is 9 A or more, with a next-standard-size allowance in Note 1. Where secondary protection is provided at not more than 125 percent of secondary current, the primary device is permitted up to 250 percent of primary current. That 250 percent allowance is the detail exams test, because it looks wrong until you remember the secondary device is doing the close-in protection.
Step by step
- 1
Convert kVA to primary amps
Single-phase: VA / primary volts. Three-phase: VA / (primary volts x 1.732).
- 2
Convert kVA to secondary amps
Same formula at the secondary voltage.
- 3
Choose the protection scheme
Primary only, or primary plus secondary. This selects the row in Table 450.3(B).
- 4
Apply the percentage
125 percent primary-only for currents of 9 A or more. Up to 250 percent primary where secondary protection at 125 percent or less is provided.
- 5
Round per Note 1
Where 125 percent does not correspond to a standard rating, the next higher standard rating in 240.6(A) is permitted.
- 6
Size the secondary conductors
240.21(C) governs secondary conductor protection and the tap rules that apply to them.
Worked example
A 45 kVA, three-phase, 480 V to 208Y/120 V dry-type transformer with primary and secondary protection. Size both devices.
- Primary amps = 45,000 / (480 x 1.732) = 54.1 A.
- Secondary amps = 45,000 / (208 x 1.732) = 124.9 A.
- Secondary device, Table 450.3(B) at 125 percent: 124.9 x 1.25 = 156.1 A. Next standard size is 175 A, permitted by Note 1.
- Because secondary protection is provided at not more than 125 percent, the primary device may be up to 250 percent: 54.1 x 2.50 = 135.3 A.
- Largest standard rating not exceeding 135.3 A is 125 A.
Answer: 125 A primary and 175 A secondary. Conductors are sized separately, and the secondary conductors have to satisfy 240.21(C).
How this is tested on the exam
Transformer sizing is a staple of master exams and appears on some journeyman exams. Two things decide the outcome: whether you used 1.732 for three-phase, and whether you noticed that the presence of secondary protection unlocks the 250 percent primary allowance. Note 1 rounding applies to the 125 percent cases but the 250 percent primary figure is a not-to-exceed ceiling, so you round down there.
Frequently asked questions
What is the 1.732 multiplier?
It is the square root of 3, which relates line and phase quantities in a balanced three-phase system. Three-phase amps equal VA divided by the line voltage times 1.732. Leaving it out inflates the current by about 73 percent.
Why can the primary device be 250 percent?
Table 450.3(B) permits it only where the secondary is protected at not more than 125 percent of secondary current. The secondary device handles overload and close-in faults, so the primary device is free to be sized generously enough to ride through magnetizing inrush without nuisance tripping.
Do these percentages protect the conductors?
No. Table 450.3(B) protects the transformer. Conductors are protected separately, primary conductors under 240.4 and secondary conductors under 240.21(C), which includes the 10-foot and 25-foot secondary tap rules.
Does this apply to transformers over 1,000 volts?
No. This calculator implements Table 450.3(B), which covers transformers rated 1,000 V or less. Table 450.3(A) covers over 1,000 V and uses different percentages with supervised-condition allowances.