What is the difference between kVA and kW on a transformer nameplate?

kVA (kilovolt-amps) is apparent power: the total electrical load the transformer can deliver. kW (kilowatts) is real power: the portion that does actual work. The difference is power factor. A 15 kVA transformer feeding a 0.80 power factor load delivers 12 kW of real power. Fuse sizing always uses kVA, not kW, because the transformer windings and fuses must handle the full apparent current regardless of power factor. Our kVA to amps calculator converts the rating to current for wire sizing.

Can I use circuit breakers instead of fuses for transformer protection?

NEC 450.3 covers overcurrent protection by fuses or circuit breakers. This calculator sizes fuses only, from the fuse cells of Table 450.3(B) and the standard fuse ratings; for a breaker, read its rating or setting from your adopted code. Breakers offer the advantage of being resettable after a trip, while fuses must be replaced. For critical applications, some engineers prefer fuses because they have no moving parts and provide faster clearing times on high-fault currents. The choice often depends on the installation environment and maintenance accessibility.

Why do transformers have different fuse sizes on primary and secondary?

Because the currents are different on each side. A step-down transformer that converts 480V to 120V multiplies the current by 4. A 15 kVA unit draws about 31A on the primary but delivers 125A on the secondary. A secondary fuse is required only with primary and secondary fuses: then each fuse is sized from the current on its own side, the primary up to 250% and the secondary up to 125% (Note 1). With a primary fuse only, the default here, the table requires no secondary fuse, and the primary fuse, up to 125% (Note 1), protects the transformer.

What does "next standard size up" mean for fuse selection?

Fuses come in fixed ampere ratings (15A, 20A, 25A, 30A and so on). The table gives a maximum, and the largest standard rating at or below it always complies. Where the maximum comes from a 125% cell and falls between two standard ratings, like 39.1A, Note 1 permits the next higher standard rating (40A). The 167%, 250% and 300% cells carry no such note, so read the rating at or below the maximum there.

Transformer Fusing Calculator - NEC 450.3 Sizing

8 min read

Size the fuses for any power transformer. Enter the kVA rating, the primary and secondary voltages, and whether the transformer has a primary fuse only or primary and secondary fuses. The calculator returns the full-load current on each side, the maximum fuse from the cells of NEC Table 450.3(B) and the standard fuse ratings around that maximum. It covers transformers of 1000 V and under; read a transformer over 1000 V from your adopted code. For background on apparent power vs true power, our kVA vs kW guide explains the power triangle and why fuse sizing uses kVA. To see how the NEC 450.3 percentages are chosen and read straight from the code tables, our transformer fuse sizing guide walks through the full method.

Two transformer coils with a winding line and a fuse symbol on the lead at each side, titled Transformer Fusing.

Why Transformer Fusing Exists

Every transformer on this page needs a primary fuse; whether it needs a secondary fuse depends on the scheme. The primary fuse protects the transformer itself from short circuits and overloads on the secondary side. With a primary fuse only, the default here, NEC Table 450.3(B) requires no secondary fuse: at the default 15 kVA the primary fuse may be up to 125% of full-load current (Note 1). With primary and secondary fuses, the primary fuse may be up to 250% and a secondary fuse is required, up to 125% of secondary current at the default.

Without proper fusing, a short circuit on the secondary side would cause the transformer to draw excessive current from the primary supply. The transformer's windings overheat, insulation breaks down, and at worst the transformer fails catastrophically, with smoke, fire, or oil rupture on larger units.

The primary fuse also limits the available fault current downstream. A utility service might deliver 10,000+ amps of short circuit current. Without a properly sized primary fuse, that entire fault current flows through the transformer into whatever caused the short.

NEC Article 450 governs transformer protection. The rules differ based on transformer size, impedance, and whether overcurrent protection exists on both sides. Getting this right matters for both safety and code compliance: an inspector will check fuse sizing on any permitted installation.

Transformer fuse diagram, primary fuse only: primary up to 125% of full-load current, secondary fuse not required. — Drawn for the default: 15 kVA single-phase, 480/120 V, primary fuse only. NEC Table 450.3(B) sets the primary at up to 125% of full-load current at 9A or more (where that is not a standard rating, Note 1 permits the next higher one), 167% from 2A to under 9A and 300% under 2A, and requires no secondary fuse. With primary and secondary fuses, the primary is up to 250% and the secondary up to 125% of secondary current (167% where the secondary current is under 9A). Your adopted code governs.

Step-by-Step Fuse Sizing for Transformers

Calculate the full load current (FLC) on each side. FLC = (kVA x 1000) / Voltage for single-phase, and (kVA x 1000) / (Voltage x √3) for three-phase. A 15 kVA single-phase transformer at 480V primary draws 31.25A. The same transformer at 120V secondary delivers 125A. The current ratio is the inverse of the voltage ratio: step down the voltage, step up the current.
Read the primary cell. With a primary fuse only, NEC Table 450.3(B) sets the primary maximum by primary current: 125% at 9A or more, 167% from 2A up to 9A, and 300% under 2A. So 31.25A x 125% = 39.1A. The largest standard rating at or below that is 35A. Where 125% does not land on a standard rating, Note 1 permits the next higher standard rating, here 40A.
Read the secondary cell. With primary and secondary fuses, the primary maximum is 250% and the secondary maximum is set by secondary current: 125% at 9A or more (with the same Note 1) and 167% under 9A. With a primary fuse only, the table requires no secondary fuse. For the same 15 kVA unit with both fuses, the secondary maximum is 125A x 125% = 156.3A: 150A at or below it, 175A by Note 1.
Select standard fuse sizes. The standard fuse sizes per NEC 240.6, as Bussmann prints them, are 1, 3, 6, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 601, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000 and 6000A. The largest rating at or below the maximum always stays inside the table. Only the 125% cells carry Note 1, and when the maximum is itself a standard rating, the next larger size is not permitted.
Verify coordination. If the primary and secondary fuses do not coordinate, a small secondary fault can trip the primary, taking down everything on that transformer rather than just the faulted circuit.

NEC 450.3(B) Fuse Cells Reference

Protection scheme	Current band	Primary (% of FLC)	Secondary (% of FLC)
Primary fuse only	Primary 9A or more	125% (Note 1)	Not required
Primary fuse only	Primary 2A to under 9A	167%	Not required
Primary fuse only	Primary under 2A	300%	Not required
Primary and secondary fuses	Secondary 9A or more	250%	125% (Note 1)
Primary and secondary fuses	Secondary under 9A	250%	167%

Note 1: where 125% of the current does not correspond to a standard rating, a higher rating that does not exceed the next higher standard rating is permitted. The cells above are the 2017 edition as Bussmann's handbook prints them; the percentages read the same in the 2020 text on the publisher's host. Your adopted code governs. This calculator sizes fuses only.

The calculator defaults to a primary fuse only, the scheme whose primary cell is the smallest. Choose primary and secondary fuses when the transformer has a fuse on each side. This page covers transformers of 1000V and under (Table 450.3(B)); read a transformer over 1000V from your adopted code.

For critical electrical installations, always have a licensed electrician verify the fuse sizing against the specific NEC edition adopted by your jurisdiction. Local amendments may modify these values.

Worked Examples

Fusing a 45 kVA Lighting Transformer

Context

A commercial building has a 45 kVA 3-phase dry-type transformer stepping 480V down to 208V for lighting panels, with a fuse on each side. You need to select primary and secondary fuses per NEC Table 450.3(B).

Calculation

Primary FLC (3-phase): 45,000 / (480 × √3) = 45,000 / 831.38 = 54.13 A

Primary fuse (250%, primary and secondary fuses): 54.13 × 250% = 135.3 A maximum → standard rating at or below: 125 A

Secondary FLC (3-phase): 45,000 / (208 × √3) = 45,000 / 360.27 = 124.91 A

Secondary fuse (125%): 124.91 × 125% = 156.1 A maximum → standard rating at or below: 150 A; Note 1 permits the next higher standard rating, 175 A

Interpretation

With both fuses, the primary maximum is 135.3 A and the secondary maximum 156.1 A. With a primary fuse only, the same transformer reads the 125% cell: 54.13 × 125% = 67.7 A maximum, 60 A at or below it, and Note 1 permits 70 A. The 3-phase formula divides by √3 (≈1.732), which produces lower per-line currents than the single-phase formula for the same kVA, because three conductors share the load.

Takeaway

The loads downstream of this transformer still need individual breakers. Use our amps draw calculator to verify each lighting circuit's current draw before selecting branch breakers. For the wire run between the transformer and lighting panel, check the voltage drop to confirm the conductors are heavy enough for the distance.

Fusing a 500 kVA Transformer at 480V

Context

A 500 kVA 3-phase transformer steps 480V down to 208V, with a fuse on each side. A transformer with a primary over 1000V is outside this calculator: read it from your adopted code.

Calculation

Primary FLC (3-phase): 500,000 / (480 × √3) = 500,000 / 831.38 = 601.41 A

Primary fuse (250%, primary and secondary fuses): 601.41 × 250% = 1,503.5 A maximum → standard rating at or below: 1,200 A

Secondary FLC (3-phase): 500,000 / (208 × √3) = 500,000 / 360.27 = 1,387.86 A

Secondary fuse (125%): 1,387.86 × 125% = 1,734.8 A maximum → standard rating at or below: 1,600 A; Note 1 permits the next higher standard rating, 2,000 A

Interpretation

At these currents the standard ratings step in wide gaps. The secondary maximum of 1,734.8 A sits between 1,600 A and 2,000 A; Mersen prints a 1,600 A secondary fuse for this transformer, the rating at or below the maximum.

Takeaway

For the motors downstream, read each one's full-load current from the printed motor table with our motor FLA calculator, which also gives the conductor minimum at 125 percent of it; the branch-circuit protection is a separate step the motor page does not size.

Frequently Asked Questions

Glossary

Full Load Current

The current a transformer draws at its rated kVA and voltage. FLC on the primary and secondary sides differ because of the voltage ratio: lower voltage means higher current for the same power.

NEC Fuse Multiplier

NEC Article 450.3 specifies maximum fuse sizes as percentages of the transformer FLC. The multiplier depends on voltage level, transformer type, and whether primary protection alone is used or both primary and secondary.

Inrush Current

The surge of magnetizing current a transformer draws when first energized. Mersen's transformer protection note gives the fuse withstand values chosen for it, not the inrush itself: "To accommodate this inrush current, fuses are often selected which have time-current withstand values of at least 12 times transformer primary rated current for .1 second and 25 times for .01 second." Fuses must be sized large enough to ride through this inrush without blowing.

Planning a battery backup for transformer-fed equipment? Our battery runtime calculator estimates how long your UPS would sustain the load during an outage.

Transformer fuse sizing is a code-compliance requirement, not a suggestion. If the transformer feeds a solar installation or UPS battery backup system, the fusing still follows the same NEC rules. Oversized fuses fail to protect the transformer. Undersized fuses blow under normal load. The values this calculator produces are starting points. Verify against the specific NEC edition your jurisdiction enforces and the transformer manufacturer's recommended protection. For any permitted installation, have a licensed electrician specify the final fuse sizes. These results are estimates based on the kVA and voltage values you enter.

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Browse all electrical calculators: Motor FLA, wire sizing, voltage drop, load calculations, and transformer fusing based on NEC standards.

Last updated: October 26, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, wind power, and EV ownership/driving, all from building, maintaining, and operating his own systems.

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.

Methodology reviewed by Doc. dr. sc. Danijel Jerković-Štil, Assistant Professor, FERIT Osijek.