Transformer Overcurrent Protection (1000 V or Less)

The maximum overcurrent device for a transformer is a percentage of its rated current, set by NEC Table 450.3(B): protect from the primary side alone, or protect both windings and buy a bigger primary device. Compute the winding's full-load amps, apply the percentage below, and pick the device from the standard 240.6(A) ratings — rounding up only where Note 1 says you may.

Maximum OCPD as % of rated current

NEC 450.3(B)

Maximum overcurrent-device rating or setting as a percentage of the winding's rated current, for transformers 1000 V and less (values per NEC Table 450.3(B)). The scheme describes which OCPDs are installed — every transformer has a secondary; 'primary OCPD only' means the primary device is the sole overcurrent protection. Round up = Note 1: where 125% doesn't land on a standard 240.6(A) rating, the next higher standard rating is permitted; all other cells are hard ceilings.

Maximum overcurrent-device rating or setting as a percentage of the winding's rated current, for transformers 1000 V and less (values per NEC Table 450.3(B)). The scheme describes which OCPDs are installed — every transformer has a secondary; 'primary OCPD only' means the primary device is the sole overcurrent protection. Round up = Note 1: where 125% doesn't land on a standard 240.6(A) rating, the next higher standard rating is permitted; all other cells are hard ceilings.
Protection scheme	Winding	Rated current	Max OCPD	Next size up?
Primary OCPD only	Primary	9 A or more	125%	Yes — Note 1
Primary OCPD only	Primary	Less than 9 A	167%	No
Primary OCPD only	Primary	Less than 2 A	300%	No
Primary + secondary OCPDs	Primary	Any	250%	No
Primary + secondary OCPDs	Secondary	9 A or more	125%	Yes — Note 1
Primary + secondary OCPDs	Secondary	Less than 9 A	167%	No

The 250% primary cell also carries Note 3: a transformer with coordinated thermal overload protection by the manufacturer can exceed it under conditions spelled out in the note. Check the nameplate before leaning on that.

Worked maximum sizes — three-phase, 480 V Δ to 208Y/120 V

450.3(B) × 240.6(A)

Maximum permitted OCPD per NEC 450.3(B) for the standard dry-type kVA ladder, computed from exact full-load currents and the standard 240.6(A) ratings. Primary-only column at 125% with Note 1 round-up; primary-with-secondary at the 250% ceiling rounded down; secondary at 125% with Note 1 round-up.
kVA	Primary FLA (A)	Max primary — no secondary OCPD	Max primary — with secondary OCPD	Secondary FLA (A)	Max secondary OCPD
15	18.0	25 A	45 A	41.6	60 A
30	36.1	50 A	90 A	83.3	110 A
45	54.1	70 A	125 A	124.9	175 A
75	90.2	125 A	225 A	208.2	300 A
112.5	135.3	175 A	300 A	312.3	400 A
150	180.4	250 A	450 A	416.4	600 A
225	270.6	350 A	600 A	624.5	800 A
300	360.8	500 A	800 A	832.7	1200 A
500	601.4	800 A	1200 A	1387.9	2000 A
750	902.1	1200 A	2000 A	2081.8	3000 A
1,000	1202.8	1600 A	3000 A	2775.7	4000 A

Maximums, not recommendations — smaller devices are always compliant and often chosen for coordination. Every row's currents are 9 A or more, so the 125% / 250% / 125% cells govern throughout.

Worked maximum sizes — single-phase, 480 V to 120/240 V

450.3(B) × 240.6(A)

Maximum permitted OCPD per NEC 450.3(B) for common single-phase kVA ratings, computed from exact full-load currents and the standard 240.6(A) ratings, same rules as above.
kVA	Primary FLA (A)	Max primary — no secondary OCPD	Max primary — with secondary OCPD	Secondary FLA (A)	Max secondary OCPD
15	31.3	40 A	70 A	62.5	80 A
25	52.1	70 A	125 A	104.2	150 A
37.5	78.1	100 A	175 A	156.3	200 A
50	104.2	150 A	250 A	208.3	300 A
75	156.3	200 A	350 A	312.5	400 A
100	208.3	300 A	500 A	416.7	600 A

Maximums, not recommendations — smaller devices are always compliant and often chosen for coordination. Every row's currents are 9 A or more, so the 125% / 250% / 125% cells govern throughout.

One OCPD or two — what the schemes mean

Every transformer has a secondary winding; the scheme names describe how many overcurrent devices you install. Primary OCPD only means the breaker or fuses ahead of the transformer are the sole protection — so they're held to a tight 125%, because they must catch secondary-side overloads reflected through the windings. Primary + secondary OCPDs adds a device on the secondary (typically the panelboard main); that device handles overload at 125% of secondary FLA, which frees the primary device to run as large as 250% — enough to ride through magnetizing inrush without nuisance tripping. That's the scheme nearly every delta-wye install uses in practice, since the secondary equipment needs a main anyway.

450.3 protects the transformer — not everything behind it

This table answers one question: how big may the device be so the transformer survives. It deliberately says nothing about the secondary conductors — those follow the tap rules of 240.21(C) — or a panelboard on the secondary, which needs its own protection per 408.36. On a common delta-wye transformer the primary device cannot protect either one, which is why real installs almost always end up in the primary-and-secondary row even though the primary-only column exists. Size the windings first with the transformer sizing calculator, then apply this table to each side.

Over 1000 volts — a different table

Medium-voltage transformers take NEC Table 450.3(A), whose limits swing on three extra variables — whether the location is supervised, the transformer's impedance band, and whether the device is a fuse or a breaker — with separate columns again for secondaries over and under 1000 V. See the MV transformer protection chart for the full 450.3(A) grid.

Common questions

What size breaker for a 75 kVA, 480 V to 208Y/120 V transformer?

Primary full-load current is 75,000 ÷ (480 × 1.732) ≈ 90 A; with primary-and-secondary protection the primary breaker may be up to 250% ≈ 225 A, or with primary-only protection 125% ≈ 113 A → a 125 A breaker via Note 1. Secondary FLA is 75,000 ÷ (208 × 1.732) ≈ 208 A; 125% ≈ 260 A → a 300 A device via Note 1. Remember 450.3 protects the transformer — the secondary conductors and panelboard have their own rules (240.21(C), 408.36).

When can I skip the secondary overcurrent device?

Table 450.3(B) allows primary-only protection at 125% (167% / 300% for small primaries). In practice you rarely get to use it on three-phase installs: the secondary conductors still need protection per 240.21(C), and a panelboard on the secondary requires protection per 408.36 — a delta-wye transformer primary OCPD cannot protect those. The primary-only column mostly earns its keep on single-phase 2-wire and control transformers.

Why do small transformers get 167% and 300%?

Magnetizing inrush. A transformer with a primary current under 9 A would need a tiny OCPD at 125%, and a tiny OCPD trips on the inrush every time the transformer energizes. The code trades protection margin for startability: up to 167% below 9 A and 300% below 2 A — and those cells are hard ceilings, with no Note 1 round-up.

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