Transformer Sizing Calculator
Size the maximum primary and secondary overcurrent device for a transformer rated 1000 V or less per NEC Table 450.3(B). Enter the kVA, phase, and winding voltages; the tool finds each winding's rated current, applies the table percentage, and rounds to a standard NEC 240.6(A) breaker or fuse.
How to use this calculator
- Enter the transformer kVA rating, phase, and primary voltage. The tool computes the primary rated current from the nameplate kVA, not a load reading.
- Choose the protection method. Primary only holds the primary device to the tighter 125/167/300 percent limits; primary and secondary lets the primary reach 250 percent and adds a secondary device.
- Pick breaker or fuse. Both use the NEC 240.6(A) standard ratings; fuses add the 1, 3, 6, 10, and 601 A ratings for small transformers.
- Read the maximum device on each winding, the percentage applied, and the ceiling. The notes flag any Note 1 round-up and where a ceiling forces a round-down.
NEC reference
This calculator follows NEC 2020 Table 450.3(B), Maximum Rating or Setting of Overcurrent Protection for Transformers 1000 Volts and Less. Rated current is the nameplate kVA divided by the winding voltage (and by the square root of 3 for a three-phase line current). The table percentages and Note 1 (the 240.6(A) next-higher-rating allowance on the 125 percent cells) are identical in the 2017, 2020, and 2023 editions. Standard device ratings come from NEC 240.6(A).
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
How transformer overcurrent protection is sized
Transformer protection under NEC 450.3 is expressed as a percentage of the transformer's rated current, not of the connected load. The rated current is a nameplate quantity: for a single-phase transformer it is the volt-amperes divided by the winding voltage, and for a three-phase transformer the line current is the volt-amperes divided by the square root of 3 times the line voltage. Table 450.3(B) then sets the maximum device on each winding.
The percentages are generous because a transformer draws a large magnetizing inrush at energization; the device has to ride through that inrush without opening while still protecting the transformer against a sustained overload or fault. Providing a device on the secondary as well as the primary lets the primary device sit higher (up to 250 percent) because the secondary device now backs up the transformer and its secondary conductors.
Table 450.3(B) maximum percentages
| Protection method | Winding and rated current | Max device |
|---|---|---|
| Primary only | Primary, 9 A or more | 125% |
| Primary only | Primary, 2 A to less than 9 A | 167% |
| Primary only | Primary, less than 2 A | 300% |
| Primary and secondary | Primary, any rated current | 250% |
| Primary and secondary | Secondary, 9 A or more | 125% |
| Primary and secondary | Secondary, less than 9 A | 167% |
Note 1 permits the next higher standard 240.6(A) rating only on the 125 percent cells. The 167, 250, and 300 percent values are maximums, so those devices round down to the next standard size. Note 2 allows the secondary device to be up to six breakers or fuse sets grouped in one location.
Worked example: 45 kVA, 480V to 208V, three-phase
- Primary rated current: 45,000 / (1.732 x 480) = 54.1 A.
- Secondary rated current: 45,000 / (1.732 x 208) = 124.9 A.
- With primary and secondary protection, the primary is limited to 250 percent: 54.1 x 250 percent = 135.3 A, so the largest standard breaker not exceeding it is 125 A.
- The secondary is 9 A or more, so it is limited to 125 percent: 124.9 x 125 percent = 156.1 A. Note 1 rounds that up to the next standard 175 A breaker.
Frequently asked questions
What size primary breaker for a 45 kVA, 480V three-phase transformer?
The primary rated current is 54.1 A. With primary-only protection Table 450.3(B) allows 125 percent, so 54.1 x 125 percent = 67.7 A, which Note 1 rounds up to a 70 A breaker. With secondary protection added the primary may go up to 250 percent instead.
Is the percentage based on the load or the transformer?
On the transformer. Table 450.3(B) percentages apply to the transformer's rated (nameplate) current on each winding, not to the connected load current. Compute the rated current from the kVA and the winding voltage.
When can I round the device up?
Only on the 125 percent cells, under Note 1. Everywhere else the percentage is a ceiling, so you take the largest standard 240.6(A) rating that does not exceed the calculated value. This tool applies each rule automatically and says which it used.
Does this size the transformer conductors?
No. This calculator sizes the overcurrent devices only. Primary and secondary conductors, including the 240.21(B) and 240.21(C) tap rules, are separate; use the Wire Size or Wire Gauge calculators for conductor ampacity.
What about transformers over 1000 volts?
Those use Table 450.3(A), which adds supervised-location columns and different percentages, and often involves available fault current. That is outside this tool's scope.
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