NEC 450.3 · 2023 Code

Transformer Fuse & Breaker Size Chart

Q: Why did my 69.7 A result round up to 70 A, but a 135.3 A result round down to 125 A?

Because Note 1 to Table 450.3(B) applies only to the 125% cells . There, rounding up to the next higher standard rating is permitted. Every other cell is a maximum, so the device rounds down to the largest standard rating that does not exceed it. A 135.3 A maximum therefore becomes 125 A, not 150 A.

Q: Does every cell in Table 450.3(A) let me round up to the next size?

No. Only the cells carrying Note 1 may round up. In the supervised-with-secondary rows the primary cells show no Note 1 reference, so this calculator treats them as maxima and rounds down . That matches the 2023 table text — but as with everything on this page, confirm against the edition your jurisdiction adopted before you commit to a size.

Enter the transformer rating and protection method. You get the compliant amp rating, the arithmetic behind it, and whether the selection passes.

1Transformer

Rating

kVA

Primary voltage

Above 1000 V — Table 450.3(A) applies, so the settings below are needed.

Phase

Settings that change the answerRequired above 1000 V

Secondary protection

The single biggest lever here: it doubles the permitted primary rating. Above 1000 V it also opens up the higher primary-only row, but only in a supervised location.

Secondary voltage

This secondary is 1000 V or less, so Table 450.3(A) gives it either 125% (any location) or 250% (supervised with secondary protection).

Overcurrent device

Both use the same percentage below 1000 V; they differ in available amp ratings.

Location

Decides which row of Table 450.3(A) applies. A supervised location can use the primary-only row, and with secondary protection it can raise a 1000 V-or-less secondary from 125% to 250%.

Transformer impedance (%Z)

Confirm from the nameplate — it changes the allowable rating. Above 10% the table has no row at all.

Class R assumed. Table 450.3(B) does not vary the percentage by fuse class — what differs is coordination, since only Class R may sit at 125% without nuisance operation on inrush.

Primary OCPD (NEC 450.3(A))

110AClass R

Class R fuse

✓PASS

The selected rating is within the range permitted by NEC 450.3.

Full-load current (FLA): 41.8 A
Applied coefficient: 250%
Computed value: 104.6 A
Rounding rule: Next higher — note permits it
Standard rating: 110 A

Rounded UP to 110 A — the next higher commercially available rating. Above 1000 V the NEC lists no standard ratings, so confirm this size is actually stocked.

Secondary conductors (NEC 240.21(C))

Secondary FLA: 1202.8 A
Applied coefficient: 125%
Computed value: 1503.6 A
Rounding rule: Next higher — note permits it
Standard rating: 1600 A

Rounded UP to the next higher standard rating (NEC 240.6(A)) — permitted by the note attached to this cell. NEC 240.21(C) requires the secondary conductors to be protected separately regardless of what protects the primary.

NoteAbove 1000 V, primary-only protection is limited to supervised locations. This is reported using the supervised figure, but an unsupervised installation must protect the secondary as well — which changes the answer.
NoteTransformer %Z is assumed. Confirm the actual impedance from the nameplate — it changes the allowable OCPD rating.
NoteAbove 1000 V the NEC lists no standard ampere ratings, so Note 1 defers to the next higher commercially available rating. The size shown comes from the medium-voltage series normally stocked for switchgear and fusible gear — confirm it against the device your supplier actually offers.

Code references

NEC 450.3(A) Table 450.3(A) — Primary voltage 13800 V exceeds 1000 V — Table 450.3(A) applies.
Table 450.3(A) Note 1 — Where a rating is limited to a maximum, the next higher commercially available rating is permitted. Above 1000 V the NEC publishes no list of standard ratings, so the selectable sizes are whatever is manufactured and stocked.
NEC 450.3(B) secondary coefficient, Table 450.3(A) has no entry — Supervised location, primary protection only, any %Z; secondary coefficient 125%.

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Primary fuse

110A

✓ PASS

Transformer fuse size chart by kVA

Primary-side maximum overcurrent protection for transformers 1000 V or less, primary-only protection, per NEC 450.3(B)(1) Table 450.3(B). Filter by phase and secondary voltage.

kVA	FLA	Coeff.	Computed	Max fuse (240.6A)
15	18.0 A	125%	22.6 A	25 A
25	30.1 A	125%	37.6 A	40 A
30	36.1 A	125%	45.1 A	50 A
37.5	45.1 A	125%	56.4 A	60 A
45	54.1 A	125%	67.7 A	70 A
50	60.1 A	125%	75.2 A	80 A
60	72.2 A	125%	90.2 A	100 A
75	90.2 A	125%	112.8 A	125 A
100	120.3 A	125%	150.4 A	175 A
125	150.4 A	125%	187.9 A	200 A
150	180.4 A	125%	225.5 A	250 A
167	200.9 A	125%	251.1 A	300 A
200	240.6 A	125%	300.7 A	350 A
250	300.7 A	125%	375.9 A	400 A
333	400.5 A	125%	500.7 A	600 A
375	451.1 A	125%	563.8 A	600 A
400	481.1 A	125%	601.4 A	700 A
500	601.4 A	125%	751.8 A	800 A
600	721.7 A	125%	902.1 A	1000 A
750	902.1 A	125%	1127.7 A	1200 A
1000	1202.8 A	125%	1503.6 A	1600 A
1250	1503.6 A	125%	1879.5 A	2000 A
1500	1804.3 A	125%	2255.3 A	2500 A
1675	2014.8 A	125%	2518.5 A	3000 A
2000	2405.7 A	125%	3007.1 A	4000 A
2500	3007.1 A	125%	3758.9 A	4000 A
3000	3608.5 A	125%	4510.7 A	5000 A
3330	4005.5 A	125%	5006.9 A	6000 A

Primary-side values based on NEC 450.3(B)(1) Table 450.3(B) — primary-only protection, next higher standard rating per NEC 240.6(A). Transformer protection does not by itself protect the secondary conductors; those must be sized separately.

NEC 450.3(A) vs 450.3(B) — which table applies

These two tables get confused constantly. The test is a single number.

	Table 450.3(A)	Table 450.3(B)
Applies when	Primary voltage over 1000 V	Primary voltage 1000 V or less
Coefficient depends on	Transformer %Z, supervised status, device type	Rated current only (over 9 A, 2–9 A, under 2 A)
Typical range	13.8 kV / 11 kV distribution feeders	480 V / 208 V commercial and industrial
Rounding direction	Only cells carrying Note 1 may round up; the rest are maxima and round down	Note 1 reaches the 125% cells only; every other cell rounds down

Only the 125% cells may round up

Note 1 to Table 450.3(B) applies to the 125% cells and nothing else. Every other cell is amaximum, so the device rounds down to the next standard rating. Two adjacent columns of the same table therefore round in opposite directions.

Protection method	Winding and current	Max OCPD	Rounding
Primary only	Primary, 9 A or more	125%	Next higher — Note 1
Primary only	Primary, 2 A to less than 9 A	167%	Next lower — maximum
Primary only	Primary, less than 2 A	300%	Next lower — maximum
Primary and secondary	Primary, any current	250%	Next lower — maximum
Primary and secondary	Secondary, 9 A or more	125%	Next higher — Note 1
Primary and secondary	Secondary, less than 9 A	167%	Next lower — maximum

Worked example, 45 kVA 480/208 V three phase with secondary protection: the primary is capped at 250% × 54.1 A = 135.3 A, and 125 A is the largest standard rating that does not exceed it. The secondary sits on the 125% cell, so 156.1 A may round up to 175 A.

Above 1000 V — Table 450.3(A)

Medium-voltage percentages run far higher because inrush is a much larger multiple of full-load current. The primary and the secondary are sized separately, and each is split again by whether that side is over or under 1000 V — so a 13.8 kV primary feeding a 480 V secondary reads a different secondary column from a transformer that steps 13.8 kV down to 4.16 kV.

Location	%Z	Primary over 1000 V		Secondary over 1000 V		Secondary 1000 V or less
Location	%Z	Breaker	Fuse	Breaker	Fuse	Either
Any location	Not more than 6%	600% ¹	300% ¹	300% ¹	250% ¹	125% ¹
Any location	Over 6%, not more than 10%	400% ¹	300% ¹	250% ¹	225% ¹	125% ¹
Supervised only	Any	300% ¹	250% ¹	Not required
Supervised, with secondary	Not more than 6%	600%	300%	300% ⁵	250% ⁵	250% ⁵
Supervised, with secondary	Over 6%, not more than 10%	400%	300%	250% ⁵	225% ⁵	250% ⁵

Superscripts are the table's own notes. Note 1 permits the next higher standard rating where the computed value does not land on one. Note 5 permits omitting secondary protection altogether on a transformer with coordinated thermal overload protection from its manufacturer — it is not a rounding permission, so the 250% cells it appears on remain maxima.

Above 1000 V the NEC lists no standard ratings

NEC 240.6(A) covers 1000 V and below only. Above 1000 V, Note 1 defers to the next highercommercially available rating, which is whatever the manufacturer builds and the supplier stocks. This calculator uses the medium-voltage series normally carried for switchgear and fusible gear, so confirm the size against the device you can actually get.

Standard fuse amp ratings — NEC 240.6(A)

Only these values may be selected. When a computed percentage does not land on one of them, NEC 450.3 Note 1 permits moving to the next standard rating.

Fuses

13610152025303540455060708090100110125150175200225250300350400450500600700800100012001600200025003000400050006000

Circuit breakers

152025303540455060708090100125150175200225250300350400450500600700800100012001600200025003000400050006000

Above 800 A, NEC 240.4(C) requires the ampacity of the protected conductors to be at least equal to the overcurrent device setting, so the wire size becomes the limiting factor.

Data sources

Coefficients and limits are implemented from the code itself. Manufacturer documents were used only to cross-check the logic; no proprietary table data was copied.

NEC 2023 Article 450.3 with Tables 450.3(A) and 450.3(B), including table Notes 1 and 5
NEC 2023 Article 240.6(A) — standard ampere ratings, 1000 V and below
NEC 2023 Articles 240.4(C) and 240.21(C)
Above 1000 V the NEC publishes no list of standard ampere ratings. The sizes used there are the medium-voltage series normally stocked for switchgear and fusible gear, which is a market convention rather than code text — confirm against your supplier. The row percentages and note markers above were checked against the 2023 table text: the supervised-with-secondary rows carry no Note 1 reference, and their secondary cells carry Note 5.
Logic cross-checked against application notes from Eaton, Mersen, Hubbell/Acme, Rockwell Automation and Sola/Hevi-Duty. No proprietary table data was copied.

Questions people actually ask

What size fuse does a 100 kVA transformer need?

For a 100 kVA transformer at 480 V three phase the primary full-load current is 120.3 A. The 125% value is 150.4 A, and because 150 A sits just below that figure the next standard rating under NEC 240.6(A) is 175 A. At 240 V three phase the current rises to 241 A, the 125% value becomes 301 A, and the standard rating is 350 A.

What is the difference between NEC 450.3(A) and 450.3(B)?

The only test is the primary voltage. Table 450.3(B) applies when the primary is 1000 V or less and its coefficients depend only on the rated current. Table 450.3(A) applies when the primary is over 1000 V and its coefficients depend on the transformer impedance and whether the location is supervised.

Why did my 69.7 A result round up to 70 A, but a 135.3 A result round down to 125 A?

Because Note 1 to Table 450.3(B) applies only to the 125% cells. There, rounding up to the next higher standard rating is permitted. Every other cell is a maximum, so the device rounds down to the largest standard rating that does not exceed it. A 135.3 A maximum therefore becomes 125 A, not 150 A.

Can I protect the primary side only?

Yes in general, but there are limits. When the required primary overcurrent device rating reaches 1000 A or more, primary-only protection is not permitted and secondary protection must be provided. Secondary conductors must also be protected separately under NEC 240.21(C).

What does Class R mean for transformer fusing?

Class R fuses are specifically designed to coordinate with transformers. Because of their time-current characteristics, a Class R fuse can be selected at up to 125% of the transformer rating without nuisance operation on inrush current. Other classes such as Class RK may require a larger rating.

Does this cover 13.8 kV transformers?

Yes. Once the primary exceeds 1000 V the calculator switches to NEC 450.3(A) and asks for the transformer impedance and the supervised or unsupervised status, because those two facts change the allowable rating substantially. One caveat above 1000 V: the NEC publishes no list of standard ampere ratings at those voltages, so Note 1 defers to the next higher commercially available rating. The size shown comes from the medium-voltage series normally stocked for switchgear and fusible gear — confirm it against what your supplier actually offers.

Does every cell in Table 450.3(A) let me round up to the next size?

No. Only the cells carrying Note 1 may round up. In the supervised-with-secondary rows the primary cells show no Note 1 reference, so this calculator treats them as maxima and rounds down. That matches the 2023 table text — but as with everything on this page, confirm against the edition your jurisdiction adopted before you commit to a size.

Disclaimer. Based on the 2023 NEC. Not engineering advice and not a substitute for the code text. Verify against the edition adopted in your jurisdiction and with your Authority Having Jurisdiction — protection decisions affect life safety.