GuideCalculators and sizing10 min read
Transformer fuse and breaker sizing
The NEC sets the largest overcurrent device a transformer may have in Table 450.3: for a transformer at 1,000 V or less, a primary device at 125% of full-load current (or 250% with a secondary device at 125%); for a medium-voltage primary, a fuse at 300% or a breaker at 600% of primary current, with the secondary at 125%. Those are maximums. The fuse actually chosen must also ride through the inrush when the transformer is energized, which is why pad-mount bayonet fuses come from the manufacturer’s table rather than from the code alone.
The short version
- Transformers at 1,000 V and below (Table 450.3(B)): primary only at 125% of primary full-load amps (167% if under 9 A, 300% if under 2 A); or primary at 250% with a secondary device at 125%.
- Primary over 1,000 V (Table 450.3(A), any location, impedance up to 6%): primary fuse 300% or breaker 600%; secondary over 1,000 V: fuse 250% or breaker 300%; secondary at 1,000 V or less: 125%.
- Where 125% doesn’t land on a standard rating, the next standard size up is allowed; the 250% and 300% figures are hard ceilings.
- These protect the transformer. The secondary conductors are protected by 240.21(C), which is usually what sets the secondary breaker.
- A pad-mount’s bayonet fuses are sized from the maker’s table for the kVA and voltage, typically 140 to 200% of full-load current, with current-limiting backup fuses for high fault currents.
- Inrush is 8 to 12 times full-load current for about a tenth of a second (up to 25 times on small units), and a fuse that melts under it is the most common nuisance trip on a new installation.
Calculator
Enter the transformer’s rating and voltages. The calculator gives its full-load current on each side and the NEC’s maximum device for each case: primary alone, primary with a secondary device, and the secondary.
Full-load current: 46.3 A on the 12,470 V side, 1,203 A on the 480 V side. Table 450.3(A), any location, impedance up to 6%:
| Device | Rule | Maximum |
|---|---|---|
| Primary fuse | 300% of 46.3 A | 138 A |
| Primary breaker | 600% of 46.3 A | 277 A |
| Secondary device | 125% of 1,203 A (next standard size up allowed) | 1,600 A |
Maximums, not selections: the conductors have their own rules (240.4 and 240.21(C)), and the fuse chosen must also ride through inrush.
The NEC rules, in one table
Article 450.3 protects the transformer itself from overload and from faults on its secondary. The percentages are of the transformer’s rated full-load current on that side.
| Arrangement | Primary device | Secondary device |
|---|---|---|
| Primary only, primary current 9 A or more | 125% (next standard size up allowed) | None required |
| Primary only, 2 A to 9 A | 167% | None required |
| Primary only, under 2 A | 300% | None required |
| Primary and secondary, secondary 9 A or more | 250% | 125% (next standard size up allowed) |
| Primary and secondary, secondary under 9 A | 250% | 167% |
| Transformer impedance | Primary fuse | Primary breaker | Secondary over 1,000 V | Secondary 1,000 V or less |
|---|---|---|---|---|
| Up to 6% | 300% | 600% | Fuse 250% or breaker 300% | 125% |
| Over 6% to 10% | 300% | 400% | Fuse 225% or breaker 250% | 125% |
Supervised locations (qualified people on site) have a second set of rows that allow the secondary device to be omitted with a tighter primary setting, or looser secondary settings. The utility’s own transformers follow the utility’s practice, not the NEC.
Two readings of the table matter in practice. The 125% figures carry a note allowing the next standard rating up (so 113 A becomes 125 A), while the 250%, 300% and 600% figures are ceilings. And the secondary percentage is of the secondary current, which on a 480 V to 208 V transformer is 2.3 times the primary’s.
Chart: 480 V to 208Y/120 V dry-type transformers
The common building transformer, with its full-load currents and the NEC maximums rounded to standard ratings.
| kVA | Primary amps | Primary only, 125% | Primary with secondary, 250% | Secondary amps | Secondary, 125% |
|---|---|---|---|---|---|
| 15 | 18 | 25 A | 45 A max | 41.6 | 60 A |
| 30 | 36.1 | 50 A | 90 A max | 83.3 | 110 A |
| 45 | 54.1 | 70 A | 135 A max | 125 | 175 A |
| 75 | 90.2 | 125 A | 225 A max | 208 | 300 A |
| 112.5 | 135 | 175 A | 338 A max | 312 | 400 A |
| 150 | 180 | 250 A | 451 A max | 416 | 600 A |
| 225 | 271 | 350 A | 676 A max | 625 | 800 A |
| 300 | 361 | 500 A | 902 A max | 833 | 1,200 A |
| 500 | 601 | 800 A | 1,503 A max | 1,388 | 2,000 A |
The 125% columns use the next-standard-size-up allowance; the 250% column is a ceiling, so the standard rating at or below it. A 15 kVA unit’s primary draws 18 A, so 125% is 22.6 A and a 25 A breaker.
Chart: 12.47 kV pad-mount primaries
For a pad-mount fed at 12.47 kV with a 480Y/277 V secondary, at the 5.75% impedance most of these units carry.
| kVA | Primary amps | Primary fuse, 300% max | Primary breaker, 600% max | Secondary amps at 480 V | Secondary, 125% |
|---|---|---|---|---|---|
| 300 | 13.9 | 41 A | 83 A | 361 | 500 A |
| 500 | 23.1 | 69 A | 138 A | 601 | 800 A |
| 750 | 34.7 | 104 A | 208 A | 902 | 1,200 A |
| 1,000 | 46.3 | 138 A | 277 A | 1,203 | 1,600 A |
| 1,500 | 69.4 | 208 A | 416 A | 1,804 | 2,500 A |
| 2,000 | 92.6 | 277 A | 555 A | 2,406 | 4,000 A |
| 2,500 | 116 | 347 A | 694 A | 3,007 | 4,000 A |
Ceilings. The fuses fitted to a pad-mount are chosen from the maker’s table (below) and land well under them: a 1,000 kVA unit at 46 A primary typically carries bayonet links in the 65 to 100 A range.
The conductors have their own rules
Article 450.3 says nothing about the wires. The primary conductors are protected by the primary device under 240.4 in the ordinary way. The secondary conductors are the awkward case, because a primary device can’t see a secondary overload properly through the ratio: 240.21(C) lets secondary conductors run unprotected for 10 feet (with limits on ampacity and where they end) or 25 feet to a single device, and otherwise requires a secondary device at the conductors’ ampacity. In most installations that device is the secondary main breaker, and its rating is set by the conductor ampacity and 240.21(C), not by the 125% in 450.3. The two rules are checked separately and the smaller result wins.
Bayonet fuses on pad-mount transformers
A pad-mount’s primary protection lives inside its tank. Most North American units carry one bayonet fuse per phase (Bay-O-Net is Eaton’s name, from Cooper Power Systems, and it has become the generic word): an expulsion fuse link in a holder that slides into a well through the tank wall, under the oil, and comes out with a hot stick so the link can be changed without opening the tank or draining anything.
- Current-sensing link
- Responds to overcurrent alone, with an inverse time curve. Sized to the transformer’s kVA and voltage from the maker’s table.
- Dual-sensing link
- Responds to current and to oil temperature, so a long overload that heats the oil opens it before the windings are damaged. The usual choice for utility units.
- Dual-element link
- A time-delay element for inrush and a fast element for faults; the choice where nuisance operation on energizing has been a problem.
- Current-limiting backup fuse
- A bayonet fuse interrupts only a few thousand amps. Where the available fault current is higher, a partial-range current-limiting fuse (ELSP, CLX and the like) sits in series under the oil: it never sees normal load, only the large faults the bayonet can’t clear, and it limits their energy. It isn’t replaceable from outside and a blown one means the tank is opened.
- Drywell canister fuses
- Full-range current-limiting fuses in a canister through the tank wall, on some units instead of the bayonet-plus-backup pair; removable from outside.
The links are sized from the manufacturer’s table, which lists a link for each kVA and primary voltage: for 12.47 kV units the table runs from about 8 A at 75 kVA to about 140 A at 2,500 kVA, roughly 140 to 200% of full-load current. A listing for a used pad-mount should say what it carries; the bayonet type and link size are on the nameplate or the fuse label, and the backup fuse’s rating on a plate by the well.
Inrush: why the fuse is bigger than the load
When a transformer is energized, the core’s flux starts from wherever it was left and can swing to twice its normal peak, driving the core into saturation. For the first few cycles the magnetizing current is enormous: 8 to 12 times full-load current on a distribution transformer, up to 25 times on small units, decaying over a tenth of a second to a second or more. It looks like a fault to anything watching the current.
The protection is set so that it isn’t fooled:
- Fuses: the minimum-melt curve must lie above the inrush points, taken as 12 times full-load current at 0.1 second and 25 times at 0.01 second. Transformer-rated time-delay fuses (E-rated medium-voltage fuses, dual-element low-voltage fuses) are shaped for this.
- Breakers: the instantaneous pickup is set above about 12 times full-load current; a magnetic-only breaker on a transformer primary is usually set near its maximum.
- Relays: a differential relay on a large transformer uses second-harmonic restraint, because inrush is rich in second harmonic and fault current isn’t.
The other boundary is the transformer’s through-fault damage curve (IEEE C57.109), which says how long the windings can carry a given multiple of rated current. The fuse or breaker must clear below it. A device that sits above the inrush points and below the damage curve is the whole art of transformer protection.
Fuse curve: above 12 × FLA at 0.1 s and 25 × FLA at 0.01 s; below the C57.109 damage curve
Coordination with what’s upstream
A transformer’s fuse should clear a fault on its secondary before the breaker feeding it trips, so that one failed transformer doesn’t take down a feeder. That’s checked on a time-current plot: the fuse’s total clearing curve has to sit to the left of the upstream device’s curve across the range of fault currents the transformer can deliver, which the impedance guide works out. Where the upstream device is a utility recloser, the utility sets the rules and usually the fuse.
On a used transformer, the fuses are part of what you’re buying. Our listings record the bayonet and backup fuses where the seller gives them, and Magic Search reads them off sellers’ pages across the web.
Questions people ask
How do you size a fuse for a transformer?
Work out the primary full-load current (kVA × 1,000 ÷ volts, with a 1.732 for three-phase), apply the NEC’s percentage from Table 450.3 for the transformer’s voltage class and whether it has secondary protection, and round to a standard rating as the table allows. Then confirm the fuse’s minimum-melt curve sits above the inrush point (12 times full-load current at 0.1 second) and below the transformer’s through-fault damage curve from IEEE C57.109.
What size breaker do I need for a 75 kVA transformer?
A 75 kVA three-phase transformer with a 480 V primary draws 90 A. With primary protection only, the NEC allows 125%, or 113 A, and the next standard size, so a 125 A breaker. With a secondary breaker as well, the primary may be up to 250% (225 A) and the 208 V secondary, at 208 A full load, gets 125%, or 260 A, rounded up to 300 A; in practice the secondary breaker is usually set by the conductors at 250 or 300 A.
What size fuse for a 1,000 kVA transformer at 12.47 kV?
The primary full-load current is 46 A. The NEC’s ceiling is 300% for a fuse, 139 A, and 600% for a breaker. The fuse actually fitted, from the maker’s table for a 12.47 kV, 1,000 kVA pad-mount, is typically a bayonet fuse in the 65 to 100 A range, backed by a current-limiting fuse, chosen to ride through inrush and clear a secondary fault within the transformer’s damage curve.
What is a bay-o-net fuse?
Bay-O-Net is Eaton’s (formerly Cooper Power Systems’) name for the oil-immersed expulsion fuse used on most pad-mount transformers in North America, and the term has become generic. The fuse element sits in a holder that pushes into the tank through a well in the wall, under oil, and is drawn out with a hot stick to replace the link without opening the tank. Dual-sensing links respond to current and to oil temperature; dual-element links give a time delay for inrush. A bayonet fuse interrupts only a few thousand amps, so a current-limiting fuse in series handles the big faults.
Why does my transformer blow its fuse when it’s energized?
Inrush. Energizing a transformer drives the core into saturation for a few cycles, and the current can reach 8 to 12 times full-load current, more if the switch closes near a voltage zero and the core was left magnetized. A fuse sized tightly to the running load melts under it. The fix is a fuse with a time delay rated for transformer duty, sized by the maker’s table, or a breaker whose instantaneous setting sits above about 12 times full-load amps.
Do I need both primary and secondary protection?
Not always. Table 450.3(B) allows a transformer at 1,000 V or below to have primary protection only, at 125%. Secondary protection lets the primary device be larger (250%), which rides through inrush more easily, and it’s nearly always there anyway as the secondary main breaker that protects the conductors and feeds the panel. Above 1,000 V, Table 450.3(A) requires a secondary device unless the installation is a supervised location, where a tighter primary setting is allowed instead.
Sources: NEC Article 450.3 and Tables 450.3(A) and (B); NEC 240.4, 240.6 and 240.21(C); IEEE C57.109 (through-fault current duration); Eaton Cooper Power Systems Bay-O-Net fuse selection tables; S&C and Eaton fuse application guides.