Short answer: Transformer current is kVA × 1,000 divided by the voltage, and also by 1.732 for three-phase. For transformers up to 1000 V, Article 450 allows up to 125% of primary current with primary protection only, or up to 250% on the primary and 125% on the secondary with protection on both sides.
Transformer questions mix two currents and two percentages. The PSI outline lists Transformers in the services area of both Texas Journeyman exam parts. The Texas Journeyman Practice Pack includes 299 calculation practice questions with step-by-step solutions across exam topics. Start with the worked examples below to calculate transformer primary and secondary currents, then use timed exam practice to review the broader services and separately derived systems topics. US$39 once.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the method in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC.
Where transformers sit in the code
Article 450 covers transformers and transformer vaults. The protection rules for transformers up to 1000 volts are in a table in Part I (Table 450.5(B)). The secondary conductors that leave a transformer follow the tap rules in Article 240 (240.21(C)), and whether a transformer secondary is a separately derived system depends on how it is connected to other systems, not simply on whether it has a neutral. For a typical 480 V delta to 208Y/120 V transformer, review the separately derived system rules in Article 250, including 250.30. Texas uses the 2026 NEC for installations and exams from September 1, 2026 (TDLR rule adoption notice, PSI Candidate Information Bulletin).
If you only need the current, our transformer kVA calculator does the arithmetic. This page explains the steps so you can do them by hand on the exam.
kVA to amps
| System | Formula | Example |
|---|---|---|
| Single-phase | Amps = kVA × 1,000 ÷ volts | 25 kVA at 240 V = 104.2 A |
| Three-phase | Amps = kVA × 1,000 ÷ (volts × 1.732) | 75 kVA at 480 V = 90.2 A |
Use the line-to-line voltage for three-phase: 480 V, 208 V, not 277 V or 120 V.
Protection rules in plain words
These are the parts of the table apprentices use most, for transformers up to 1000 V. Check the full table in your own book.
| Protection arrangement | Primary current 9 A or more | Secondary current 9 A or more |
|---|---|---|
| Primary only | Up to 125% of primary current; next standard size up is allowed if 125% is not a standard size | No separate secondary overcurrent device is required for transformer protection under this arrangement. Secondary conductor protection must still comply with Article 240. |
| Primary and secondary | Up to 250% of primary current | Up to 125% of secondary current; next standard size up is allowed |
Smaller currents use higher percentages; those rows are in the same table.
Worked example 1: 75 kVA, primary protection only
A 75 kVA three-phase transformer has a 480 V delta primary and a 208Y/120 V secondary. For this example, calculate the transformer’s maximum primary overcurrent protection using the primary-only arrangement in Table 450.5(B). This protects the transformer itself; the primary and secondary conductors must also satisfy their applicable overcurrent-protection rules in Article 240. Do not assume that primary-only transformer protection eliminates the need to protect the secondary conductors.
- Primary current: 75,000 ÷ (480 × 1.732) = 75,000 ÷ 831.4 = 90.2 A.
- Maximum primary protection: 90.2 × 1.25 = 112.8 A.
- 112.8 A is not a standard size. The next standard size up is 125 A, so a 125 A primary breaker is allowed.
Check: 90.2 × 831.4 = 75,000 (74,992 with rounding), so the current is right. And 110 A, the standard size below, would also comply; 125 A is the largest allowed.
Worked example 2: same transformer, primary and secondary protection
Now the designer protects both sides, which lets the primary device be larger.
- Secondary current: 75,000 ÷ (208 × 1.732) = 75,000 ÷ 360.3 = 208.2 A.
- Secondary protection: 208.2 × 1.25 = 260.2 A. The next standard size up is 300 A.
- Primary protection: up to 250% of 90.2 A = 225.5 A. With this arrangement the rounding-up allowance applies to the secondary, so the primary uses the standard size that does not exceed 225.5 A: 225 A.
Check: the ratio of the two currents should equal the voltage ratio. 208.2 ÷ 90.2 = 2.31, and 480 ÷ 208 = 2.31. They match.
Worked example 3: a single-phase transformer
A 25 kVA single-phase transformer, 480 V primary, 120/240 V secondary, primary protection only.
- Primary current: 25,000 ÷ 480 = 52.1 A.
- Secondary current: 25,000 ÷ 240 = 104.2 A.
- Primary protection: 52.1 × 1.25 = 65.1 A. Next standard size up: 70 A.
Check: 52.1 × 480 = 25,008 and 104.2 × 240 = 25,008. Both sides carry the same kVA, as they should.
What else Article 450 asks for
- Ventilation and clearances. Dry-type transformers need the clearances on their nameplate and the ventilation the manufacturer lists.
- Accessibility. Transformers must be readily accessible for inspection, with listed exceptions for some dry-type units above ceilings.
- Secondary conductors. The tap rules in Article 240 (240.21(C)) apply, with length, ampacity and protection conditions that depend on the installation.
- Grounding. A separately derived system gets its system bonding jumper at a single point, at the transformer or at the first disconnecting means, plus a grounding electrode conductor (Article 250, 250.30). Follow the Article 250 rules for the installation; do not assume the transformer case is always the bonding point.
Check yourself
Our own practice questions. Answers below each.
- 45 kVA three-phase, 208 V secondary. Secondary current? Answer: 45,000 ÷ 360.3 = 124.9 A.
- 15 kVA single-phase, 240 V primary. Primary current and primary-only protection? Answer: 15,000 ÷ 240 = 62.5 A; × 1.25 = 78.1 A; next standard size up 80 A.
- 112.5 kVA three-phase, 480 V primary. Primary current? Answer: 112,500 ÷ 831.4 = 135.3 A.
Disclosure: If you buy through the Amazon link below, Tsai Electro may earn a commission at no extra cost to you. As an Amazon Associate I earn from qualifying purchases.
Transformer formulas and diagrams in your pocket
Ugly’s Electrical References, 2026 Edition. The publisher’s description lists mathematical formulas, ampacity and conduit fill information, conversion tables, and transformer and control circuit wiring diagrams. It is a field reference, not a substitute for the NEC, and spiral-bound books are not allowed in the Texas exam room.
FAQ
How do you calculate transformer amps from kVA?
For single-phase, multiply kVA by 1,000 and divide by the voltage. For three-phase, multiply kVA by 1,000 and divide by the line voltage times 1.732. For example, 75 kVA at 480 V three-phase is about 90.2 amps.
What size breaker protects a transformer primary?
With protection on the primary only, up to 125% of the primary current for currents of 9 amps or more, and the next standard size up is allowed. With protection on both sides, the primary may go up to 250%.
Does a transformer secondary need overcurrent protection?
It depends on the arrangement. If the primary is protected at 125% or less, the table does not require secondary protection for the transformer itself, but the secondary conductors still need protection under the tap rules.
Is a transformer a separately derived system?
A typical isolation transformer supplying a 208Y/120 V secondary from a 480 V system is a separately derived system. Its bonding and grounding follow the separately derived system rules in NEC Article 250, which depend on the secondary configuration.
Sources checked October 2026
- TDLR rule adoption notice, 2026 NEC: https://www.tdlr.texas.gov/news/rulemaking/2026/09/01/commission-adopts-rules-12/
- PSI Candidate Information Bulletin for TDLR (updated 9/15/2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- TDLR exam page (spiral-bound books not allowed): https://www.tdlr.texas.gov/electricians/elecexam.htm
- NFPA 70 (NEC), 2026 edition, Articles 240, 250 and 450, read in your own copy: https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70
