How do I calculate what size transformer I need?

Add up the total connected load in watts or VA. For single-phase, divide by the secondary voltage to get amps, then multiply amps by voltage and divide by 1000 to get kVA. For three-phase, use kVA = (√3 × V × I) ÷ 1000. Select the next standard transformer kVA rating at or above your calculated value. Common standard sizes are 15, 25, 37.5, 45, 75, 112.5, 150, 225, 300, and 500 kVA.

What is the 125% rule for transformer OCPD?

Per NEC Table 450.3(B), when both primary and secondary overcurrent protection are provided, the primary OCPD can be set at a maximum of 125% of the primary full-load current, and the secondary OCPD at 125% of the secondary full-load current. If the calculated value does not match a standard breaker size, you may round up to the next standard size per NEC 240.6(A).

When can I use the 250% rule for transformers?

NEC Table 450.3(B) allows primary-only protection at up to 250% of the primary full-load current when the primary current is 9 amps or more and no secondary protection is provided. For primary currents under 9 amps, the maximum is 167%. This is typically used when the secondary side feeds a panelboard with its own main breaker that serves as secondary protection.

How do you calculate transformer primary and secondary amps?

For single-phase: I = kVA × 1000 ÷ V. For three-phase: I = kVA × 1000 ÷ (√3 × V). For example, a 45 kVA three-phase transformer with a 480V primary has a primary FLA of 45,000 ÷ (1.732 × 480) = 54.1 amps. The same transformer with a 208V secondary has a secondary FLA of 45,000 ÷ (1.732 × 208) = 124.9 amps.

Transformer Sizing Guide: Calculations, NEC Rules + Free Calculator

Sizing a transformer is a two-part problem: first you calculate how much kVA the load demands, then you size the overcurrent protection per NEC 450.3(B). The kVA calculation is straightforward arithmetic, but the overcurrent protection rules trip people up because they change depending on whether you protect the primary side only, secondary side only, or both. This guide walks through both halves with worked examples so you can handle transformer questions on the exam and in the field.

Quick tool: Use our free Transformer Sizing Calculator to check your work on any problem in this guide.

Part 1: Calculating Transformer kVA

The kVA rating tells you the maximum apparent power a transformer can deliver continuously without overheating. To select the right transformer, you need to know the total connected load in VA or watts and the voltage on both sides.

Single-Phase Formula

kVA = (V × I) ÷ 1000

Or equivalently:

kVA = Total VA ÷ 1000

Three-Phase Formula

kVA = (√3 × V × I) ÷ 1000

Where V is line-to-line voltage and I is line current.

Standard Transformer Sizes

Transformers come in standard kVA ratings. You cannot order a 43 kVA transformer — you select the next standard size above your calculated load. The common dry-type ratings are:

| Single-Phase | Three-Phase | |---|---| | 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100 kVA | 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000 kVA |

Always round up to the next available size. Oversizing by one step is normal and provides headroom for load growth.

Example: Selecting Transformer kVA

A commercial tenant space has the following three-phase loads at 208Y/120V:

Lighting: 18 kW at unity power factor (18 kVA)
Receptacles: 8 kW at 0.95 PF (8.4 kVA)
HVAC unit: 12 kW at 0.85 PF (14.1 kVA)

Total apparent power: 18 + 8.4 + 14.1 = 40.5 kVA

Standard size: The next three-phase standard above 40.5 kVA is 45 kVA.

In practice you would also apply NEC Article 220 demand factors to the load calculation. For exam problems, apply demand factors only when the problem tells you to.

Part 2: Calculating Primary and Secondary Current

Once you know the kVA rating and voltages, calculate the full-load amps on each side. You need these values to size the overcurrent protection devices.

Single-Phase

I = kVA × 1000 ÷ V

Three-Phase

I = kVA × 1000 ÷ (√3 × V)

Example: Finding FLA for a 45 kVA Transformer

A 45 kVA, three-phase, 480V primary to 208Y/120V secondary transformer.

Primary FLA:

I_primary = 45,000 ÷ (1.732 × 480) = 45,000 ÷ 831.4 = 54.1 A

Secondary FLA:

I_secondary = 45,000 ÷ (1.732 × 208) = 45,000 ÷ 360.3 = 124.9 A

These numbers are the starting point for every overcurrent protection calculation that follows.

Part 3: Overcurrent Protection — NEC 450.3(B)

This is where transformer protection gets interesting. NEC 450.3(B) covers transformers rated 1000 volts or less (which is nearly every transformer you will encounter on the exam or in commercial work). The rules are organized in Table 450.3(B), and the protection limits depend on which sides you protect.

Table 450.3(B) Summary

| Protection Arrangement | Primary OCPD Maximum | Secondary OCPD Maximum | |---|---|---| | Primary and secondary protection | 125% of primary FLA | 125% of secondary FLA | | Primary only (≥ 9A primary) | 250% of primary FLA | None required | | Primary only (< 9A primary) | 167% of primary FLA | None required | | Secondary only | Not covered by 450.3(B) | 125% of secondary FLA |

Critical rule: When the calculated OCPD size does not correspond to a standard breaker size from NEC 240.6(A), you are permitted to round up to the next standard size. This is explicitly allowed by 450.3(B) and is more generous than the general rule in 240.4(B).

Standard Breaker Sizes (NEC 240.6(A))

For reference, the standard sizes you will work with most often:

15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600

Worked Examples

Example 1: Primary and Secondary Protection (Most Common)

Given: 45 kVA, 480V to 208Y/120V, three-phase. Both primary and secondary OCPD will be provided.

Step 1 — Primary FLA: 45,000 ÷ (1.732 × 480) = 54.1 A

Step 2 — Primary OCPD: 54.1 × 1.25 = 67.6 A → next standard size up = 70 A

Step 3 — Secondary FLA: 45,000 ÷ (1.732 × 208) = 124.9 A

Step 4 — Secondary OCPD: 124.9 × 1.25 = 156.1 A → next standard size up = 175 A

Answer: 70 A primary breaker, 175 A secondary breaker.

Example 2: Primary Only Protection (250% Rule)

Given: 75 kVA, 480V to 208Y/120V, three-phase. Only primary protection is provided (the secondary panel has its own main breaker that serves as downstream protection).

Step 1 — Primary FLA: 75,000 ÷ (1.732 × 480) = 90.2 A

Step 2 — Check the 9A threshold: 90.2 A is well above 9 A, so the 250% rule applies.

Step 3 — Primary OCPD: 90.2 × 2.50 = 225.5 A → next standard size up = 250 A

Answer: 250 A primary breaker. No separate secondary OCPD is required by 450.3(B), though the panel's main breaker provides de facto secondary protection.

Example 3: Single-Phase Transformer

Given: 25 kVA, single-phase, 240V primary to 120/240V secondary. Both primary and secondary protection.

Step 1 — Primary FLA: 25,000 ÷ 240 = 104.2 A

Step 2 — Primary OCPD: 104.2 × 1.25 = 130.2 A → next standard size up = 150 A

Step 3 — Secondary FLA: 25,000 ÷ 240 = 104.2 A (the secondary is 120/240V, so line-to-line is still 240V)

Step 4 — Secondary OCPD: 104.2 × 1.25 = 130.2 A → next standard size up = 150 A

Answer: 150 A primary breaker, 150 A secondary breaker.

Note that when primary and secondary voltages are the same (as in a 1:1 isolation transformer or a buck-boost transformer at unity ratio), the currents on both sides are equal and the OCPD sizes match.

Example 4: Small Transformer (Under 9A Primary)

Given: 3 kVA, single-phase, 480V to 120V. Primary protection only.

Step 1 — Primary FLA: 3,000 ÷ 480 = 6.25 A

Step 2 — Check the 9A threshold: 6.25 A is below 9 A, so the 167% rule applies (not 250%).

Step 3 — Primary OCPD: 6.25 × 1.67 = 10.4 A → next standard size up = 15 A

Answer: 15 A primary breaker. The 250% rule is NOT available here because the primary current is under 9 A.

Common Exam Traps

Trap 1: Forgetting the √3 factor on three-phase. If you divide kVA by voltage without multiplying by 1.732, you will get an answer that is √3 too high. Every three-phase current calculation needs the √3.

Trap 2: Using 250% when both sides are protected. The 250% rule only applies when there is NO secondary protection. If the question says both primary and secondary protection are provided, you must use 125% on both sides.

Trap 3: Not rounding up. Table 450.3(B) permits the next standard breaker size up when the calculated value falls between standards. If you round down, you will get the wrong answer.

Trap 4: Confusing 450.3(B) with 240.4(B). The general overcurrent protection rule in 240.4(B) only allows rounding up for circuits 800 A and below and has exceptions for receptacle circuits. Table 450.3(B) has its own rounding rules that are more permissive. On the exam, use the 450.3(B) rules for transformer OCPD.

How This Relates to Conductor Sizing

Table 450.3(B) sizes the overcurrent protection, but the conductors feeding the transformer still need to be sized per NEC 215.2 (feeders). The conductor ampacity must be at least 125% of the transformer FLA if the load is continuous, or 100% if non-continuous. Use NEC Table 310.16 to select the conductor based on the required ampacity and the terminal temperature rating.

For a deeper dive into conductor sizing and derating, see our wire size calculator.

When You Need a Transformer on the Exam

Transformer questions appear primarily in the Calculations portion of the Texas journeyman exam. The most common question types are:

Calculate the primary or secondary FLA given kVA and voltage
Size the primary OCPD given the protection arrangement
Size the secondary OCPD given the protection arrangement
Determine which NEC rule applies (125% vs 250% vs 167%)

If you can work through the four examples above without hesitation, you are prepared for any transformer question on the exam. Practice with our free Transformer Sizing Calculator to build speed and confidence before test day.