Transformer Sizing — kVA and 450.3(B)
Two things about transformer protection surprise people. The primary device may be sized at 250% of primary current, not 125% — because magnetising inrush would trip anything tighter. And Article 450 protects the transformer, not the secondary conductors; those are 240.21(C)'s problem, which is where the tap rules come from.
Size the transformer
kVA, not kW.
Transformer size
Next standard rating above the required kVA
Primary FLA
90.21 A
Secondary FLA
208.2 A
Primary OCPD max
225 A
Secondary OCPD max
300 A
See the breakdown
With secondary protection at 125%, Table 450.3(B) lets the primary device go to 250%.
The method, explained in plain English
Inrush is why 250% exists
Energising a transformer draws a brief spike many times full-load current. A device at 125% trips on it, which is why the code trades primary headroom for secondary protection.
450.3 protects the transformer
Not the secondary conductors. Those need 240.21(C) — the 10-foot and 25-foot tap rules exist precisely because a compliant primary device can leave them unprotected.
Size in kVA, never kW
The winding is limited by current. A load list in kW understates the requirement on anything that is not purely resistive.
Impedance cuts both ways
Lower impedance gives better voltage regulation but a much higher available fault current at the secondary. Check the interrupting rating downstream.
Worked examples
How the protection ceiling moves with the arrangement and the size.
60 kVA of load, 480 V to 208 V — the defaults
primary: 75,000 ÷ (1.732 × 480) = 90.21 A · secondary: ÷ (1.732 × 208) = 208.2 A
primary OCPD: 250% × 90.21 = 225.5 A → 225 A
secondary OCPD: 125% × 208.2 = 260.2 A → 300 A (Note 1)
Result: the secondary carries 2.31× the primary current — exactly the voltage ratio. That is why the secondary panel and feeder dwarf the primary feed.
The same transformer, primary protection only
125% × 90.21 = 112.8 A instead of 225.5 A
Result: half the headroom. A 100 A primary device on a 75 kVA transformer will very likely trip on energisation, which is why fitting secondary protection is the normal answer to inrush tripping.
A small control transformer
between 2 A and 9 A → ceiling is 167%
167% × 3.61 = 6.0 A
Result: the small-current bands exist because inrush is proportionally harder to distinguish from a fault at low current. Below 2 A the allowance rises again, to 300%.
Standard sizes and full-load current
Computed from the same data the calculator uses. Impedance figures are planning values — always use the nameplate.
| kVA | 480 V 3φ primary | 208 V 3φ secondary | 480 V 1φ | Typical %Z |
|---|---|---|---|---|
| 15 | 18.04 A | 41.64 A | 31.25 A | 2.5% |
| 25 | 30.07 A | 69.39 A | 52.08 A | 2.5% |
| 30 | 36.08 A | 83.27 A | 62.50 A | 2.5% |
| 37.5 | 45.11 A | 104.1 A | 78.13 A | 2.5% |
| 45 | 54.13 A | 124.9 A | 93.75 A | 2.5% |
| 50 | 60.14 A | 138.8 A | 104.2 A | 2.5% |
| 75 | 90.21 A | 208.2 A | 156.3 A | 2.5% |
| 100 | 120.3 A | 277.6 A | 208.3 A | 3.5% |
| 112.5 | 135.3 A | 312.3 A | 234.4 A | 3.5% |
| 150 | 180.4 A | 416.4 A | 312.5 A | 3.5% |
| 167 | 200.9 A | 463.5 A | 347.9 A | 3.5% |
| 225 | 270.6 A | 624.5 A | 468.8 A | 3.5% |
| 300 | 360.8 A | 832.7 A | 625.0 A | 3.5% |
| 500 | 601.4 A | 1,388 A | 1,042 A | 4.5% |
| 750 | 902.1 A | 2,082 A | 1,563 A | 4.5% |
| 1,000 | 1,203 A | 2,776 A | 2,083 A | 5.75% |
Sources & standards: NEC (NFPA 70) 2023 — Article 450 and Table 450.3(B) for transformers of 1000 V or less, including Note 1 (next higher standard rating on the 125% entries) and Note 3 (primary protection satisfying the secondary requirement only on a two-wire-to-two-wire transformer); 240.21(C) for secondary conductor protection; 450.11 for nameplate marking; and Table 310.16 for conductors. Impedance values are typical dry-type planning figures, not code values. Local amendments override the model code, and the AHJ has final say.
Frequently asked questions
Common questions about sizing and protecting a transformer.
How do I size a transformer in kVA?
Total the connected load in kVA, not kW — a transformer is limited by the current it carries, and only unity-power-factor loads make the two equal. Add a growth margin (20–25% is typical) and round up to the next standard rating. Sizing from kW understates the requirement on any motor or electronic load.
Why can the primary breaker be 250% of primary current?
Because of transformer magnetising inrush — energising a transformer draws a brief current spike many times its full-load rating, and a device sized at 125% would trip on it. Table 450.3(B) permits the primary device to go to 250% provided secondary protection is present at no more than 125% of secondary current. With primary protection only, the ceiling drops to 125% for primary currents of 9 A or more.
What if the primary current is very small?
Table 450.3(B) gets more generous as the current gets smaller, because inrush is proportionally harder to distinguish from a fault. With primary protection only: 167% where primary current is 2 A up to 9 A, and 300% where it is under 2 A. A 3 kVA 480 V three-phase transformer draws just 3.6 A, so its primary device may go to 167%.
Does 450.3 protect the secondary conductors?
No, and this is the distinction that catches people. Article 450 protects the transformer. The secondary conductors are a separate problem governed by 240.21(C), which is why the 10-foot and 25-foot tap rules exist. A compliant 450.3(B) primary device can leave secondary conductors unprotected — you have to satisfy both requirements.
Can I count the primary device as the secondary protection?
Only for a two-wire-to-two-wire transformer, where 450.3(B) Note 3 lets primary protection satisfy the secondary requirement because the current ratio is fixed and predictable. On a three-phase or multi-wire secondary it does not work, because an unbalanced secondary fault does not reflect predictably to the primary.
What impedance should I assume?
Always use the nameplate. As a planning figure, small dry-type units run about 2.5% up to 75 kVA, 3.5% to 300 kVA, 4.5% to 750 kVA, and 5.75% above that. Impedance matters twice: it sets the voltage regulation under load, and it sets the available fault current at the secondary — lower impedance means a stiffer supply and a much higher fault current.
Why is the secondary current so much higher than the primary?
Because power is conserved while voltage steps down, so current steps up by the same ratio. A 480 V to 208 V transformer has a 2.31:1 voltage ratio, so its secondary carries 2.31 times the primary current for the same kVA. This is why the secondary conductors and panel on a step-down transformer are dramatically larger than the primary feed.
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