Published June 13, 2026

Transformer Sizing 101

Transformer calculations look intimidating because there are several numbers flying around — FLA, turns ratio, OCPD — but each one answers a simple question. Here's what each piece means and how they fit together.

Full Load Amps (FLA): How Much Current Flows on Each Side

FLA tells you how much current the transformer draws (or delivers) at its rated KVA and voltage. The formulas:

Single phase: FLA = (KVA × 1000) ÷ Voltage
Three phase: FLA = (KVA × 1000) ÷ (Voltage × 1.732)

The 1.732 (√3) factor accounts for the relationship between line and phase values in a three-phase system. You'll calculate FLA separately for the primary side (using primary voltage) and the secondary side (using secondary voltage) — same KVA, different voltage, different current.

Turns Ratio: Why the Voltage Changes

The turns ratio is simply the primary voltage divided by the secondary voltage — it reflects the ratio of windings inside the transformer. A 480V-to-120V transformer has a turns ratio of 4:1. This is mostly a "sanity check" number — it confirms the transformer is doing what you expect (stepping voltage down or up by the expected amount) and can help verify nameplate data.

Primary OCPD Sizing — NEC 450.3(B)

This is where a lot of the real-world decision-making happens. NEC Table 450.3(B) governs how the primary overcurrent protective device (breaker or fuse) is sized relative to the primary FLA.

For the common case of primary-only protection (no separate secondary OCPD) on transformers rated 1000V or less with primary current of 9A or more:

Max OCPD = 125% of primary FLA
If 125% doesn't land on a standard breaker/fuse size, you're allowed to round up to the next standard size per NEC 240.6(A)

Worked example: a 75 KVA, 480V three-phase transformer has a primary FLA of 75,000 ÷ (480 × 1.732) ≈ 90.2A. 125% of that is ≈ 112.8A. Since 112.8A isn't a standard breaker size, round up to the next standard size — 125A.

Worth knowing: if a transformer has protection on both primary and secondary, NEC Table 450.3(B) allows the primary OCPD to go up to 250% of primary FLA — but that's a different scenario than the primary-only case most calculators (including ours) are built around.

Use our Transformer Calculator — enter your phase, primary/secondary voltages, and KVA rating, and it'll calculate primary and secondary FLA, turns ratio, and the recommended primary OCPD size per NEC 450.3(B) and 240.6(A).

The Bottom Line

Transformer sizing comes down to three questions: how much current flows on each side (FLA), what voltage relationship the windings create (turns ratio), and how big the primary breaker/fuse needs to be to protect the transformer without nuisance tripping on inrush (OCPD via 450.3(B)). Get those three numbers right, and the rest of the install follows naturally.

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