Why does the primary side have much lower FLA than the secondary side in the worked example?

Because the primary voltage (13,800V) is much higher than the secondary voltage (480V) in this example, and for the same power (kVA) transfer, current is inversely proportional to voltage — the higher-voltage side always carries proportionally lower current, which is exactly why utilities and large facilities distribute power at higher voltages before stepping it down close to the point of use.

Does NEC 450.3 apply the same percentage regardless of transformer voltage class?

No — NEC 450.3 has distinct tables and permitted percentages depending on whether the transformer is over or under 600V, and further distinctions based on whether secondary protection is provided. Always check the specific applicable case in the current code edition rather than assuming one percentage applies universally.

Is secondary FLA the same as the ampacity required for the main secondary breaker?

Secondary FLA is the minimum design current the secondary system has to handle at full transformer load — it directly sets minimum conductor ampacity, and it's also a key input (along with any applicable NEC secondary protection rules) for sizing the secondary main overcurrent device, though the exact secondary OCPD sizing rule depends on the specific system configuration and applicable code section.

Engineering·7 min read·August 14, 2026

🔌 Transformer FLA and Overcurrent Protection Sizing per NEC 450

How full-load ampere calculations for the primary and secondary of a transformer feed into overcurrent protective device sizing under NEC Article 450, with a worked example.

By EngineersUniverse Editorial Team

Engineering Editorial & Technical Review Team

Published August 14, 2026

Why FLA Is the Bridge Between kVA Rating and Circuit Protection

Once a transformer's kVA rating is selected, the next practical design step is determining the current — full-load amperes (FLA) — that rating corresponds to on both the primary (source) and secondary (load) side, since overcurrent protective devices (OCPDs, typically circuit breakers or fuses) and conductor sizing are specified in amperes, not kVA. FLA is the conversion between the transformer's power rating and the actual current values that downstream electrical design decisions depend on.

The FLA Formulas

For a three-phase transformer, FLA = (kVA × 1000) / (√3 × V), where V is the line-to-line voltage on the side being calculated. For a single-phase transformer, FLA = (kVA × 1000) / V. Each side of the transformer (primary and secondary) is calculated separately using its own respective rated voltage, since the same kVA rating corresponds to different current values at different voltage levels — a lower-voltage side always carries proportionally higher current for the same power transfer.

Worked Example

For a 225 kVA, three-phase transformer with a 13,800V primary and 480V secondary: Primary FLA = (225 × 1000) / (√3 × 13,800) = 225,000 / 23,902 ≈ 9.4 A. Secondary FLA = (225 × 1000) / (√3 × 480) = 225,000 / 831.4 ≈ 270.6 A. Note the large difference in magnitude — the same kVA rating produces dramatically different current values at the two very different voltage levels, exactly as expected since power equals voltage times current (for a given power transfer, lower voltage requires proportionally higher current).

How NEC 450.3 Sets Maximum OCPD Ratings

NEC Article 450, specifically Section 450.3, establishes maximum permitted overcurrent protective device ratings as a percentage of calculated FLA, with the specific percentage depending on the transformer's voltage class and whether secondary protection is also provided. For transformers 600V and below with secondary protection provided, primary OCPD is commonly permitted up to 125% of primary FLA; without secondary protection, the maximum primary OCPD percentage is typically higher (allowing up to 250% in some configurations) to accommodate the reality that the primary device alone must handle the full range of possible downstream conditions. These percentages and their specific conditions are laid out in detailed tables within NEC 450.3 — always verify the exact applicable percentage against the current code edition and the specific voltage class and protection configuration for a real design, since the rules include several distinct cases.

Applying the Percentage to the Worked Example

Continuing the example above, if this 225 kVA transformer (480V secondary, secondary protection provided) is protected under a rule permitting up to 125% of primary FLA: maximum primary OCPD = 9.4 × 1.25 ≈ 11.75 A — the actual specified breaker or fuse would be the standard OCPD size at or below this calculated maximum (breakers come in standard trip ratings, similar to transformers coming in standard kVA sizes, so the specified device is whichever standard rating doesn't exceed the calculated maximum allowed).

Why Secondary FLA Sets Minimum Conductor Ampacity

Secondary FLA (270.6 A in the worked example) is the minimum ampacity the secondary conductors need to be sized for — conductors carrying the transformer's full rated secondary current have to be selected from standard ampacity tables (accounting for conductor material, insulation type, and any applicable derating for conduit fill, ambient temperature, or number of current-carrying conductors) at or above this FLA value. This is a separate, though related, calculation from the OCPD sizing step — conductor ampacity and overcurrent protection sizing both derive from the same FLA figure but follow their own distinct code rules (conductor ampacity from NEC Article 310, OCPD sizing from Article 450).

Why Getting This Chain Right Matters

The full chain — kVA rating → FLA on each side → OCPD sizing (NEC 450.3) and conductor sizing (NEC 310) — is a sequence where each step depends on the previous one being calculated correctly. An error anywhere in this chain (a wrong FLA calculation, a misapplied OCPD percentage, an under-ampacity conductor) can result in equipment that's improperly protected or conductors that are undersized for the actual current they'll carry, both of which are genuine safety and code-compliance issues, not just design inefficiencies.

Topics covered

transformer FLA calculationNEC 450.3 transformer protectiontransformer OCPD sizingprimary secondary full load amps