Transformer Overcurrent Protection (Over 1000 V)
Medium-voltage transformer protection has three more dials than the low-voltage table: whether the location is supervised, the transformer's rated impedance, and whether the device is a circuit breaker or a fuse. This chart lays out NEC Table 450.3(A) — maximum device rating or setting as a percentage of transformer rated current, primary and secondary — including the supervised-only row where secondary protection is not required at all.
Maximum OCPD as % of rated current — over 1000 V
| Location | Impedance | Pri. brkr | Pri. fuse | Sec. >1 kV brkr | Sec. >1 kV fuse | Sec. ≤1 kV brkr/fuse | Next size up? |
|---|---|---|---|---|---|---|---|
| Any | ≤ 6% | 600% | 300% | 300% | 250% | 125% | Yes |
| Any | 6–10% | 400% | 300% | 250% | 225% | 125% | Yes |
| Supervised only | Any | 300% | 250% | — | — | — | Yes |
| Supervised only | ≤ 6% | 600% | 300% | 300% | 250% | 250% | No |
| Supervised only | 6–10% | 400% | 300% | 250% | 225% | 250% | No |
How supervision changes the table
The primary limits don't move — 600% / 300% breaker-or-fuse at low impedance, 400% / 300% above 6% — whether or not the location is supervised. What supervision buys is downstream freedom: the low-voltage secondary limit relaxes from 125% to 250%, and the any-impedance row appears, where a primary breaker at 300% (fuse at 250%) is allowed to protect the transformer with no secondary overcurrent device at all. The theory is that qualified staff, maintenance programs, and coordinated relaying stand in for the prescriptive device. Note that even where the transformer's secondary protection is omitted, the secondary conductors and any panelboard still have their own protection rules — omitting the device is a 450.3 allowance, not a blanket one.
Breaker vs. fuse, and why the columns differ
Breakers get the higher primary allowance (600% vs. 300% at low impedance) because a breaker's inverse-time curve is set as a setting on a relay or trip unit, while a fuse's rating is the fuse — the code holds fuses closer to the load current so a secondary fault reflected through the transformer still melts them in useful time. If your fuses are the electronically actuated kind set to open at a chosen current, they count as breakers here (Note 4). For the transformers most crews install — 1000 V and less — use the low-voltage transformer protection chart instead, and size the windings with the transformer sizing calculator.
Common questions
What counts as a supervised location?
Per the table’s own note, one “where conditions of maintenance and supervision ensure that only qualified persons will monitor and service the transformer installation” — think a fenced substation yard or a locked electrical room in an industrial plant with an electrical staff. It is a judgment the AHJ has to accept, not a box you check; if the room is accessible to general building staff, plan on the any-location rows.
Why does transformer impedance change the allowed percentages?
Impedance limits how much fault current the transformer can pass through itself. A low-impedance unit (6% or less) lets large faults through, which the big 600% primary breaker allowance can still clear — while a higher-impedance unit (over 6 to 10%) chokes fault current down, so the device has to sit closer to the load current to see the fault at all: 400% breaker, and tighter secondary-side numbers too.
What do breaker and fuse maxima look like on a real unit?
Take a 1500 kVA, 12,470 V to 480Y/277 V transformer in an unsupervised location, impedance under 6%: primary FLA is about 69 A, so the primary breaker may be set up to 600% ≈ 417 A, or fuses up to 300% ≈ 208 A. The 480 V secondary (about 1804 A FLA) is held to 125% ≈ 2255 A — in practice the secondary main. In a supervised plant, that same secondary could go to 250%, or with the any-impedance row, be omitted entirely.
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