NEC 450.21–450.28 — Transformer Installation by Type (Article 450 Part II)

Articles 51 and 52 covered the protection core (450.3 / 450.4 / 450.6) and the general installation core (450.8–450.14) of Article 450. This article is Part II — "Specific Provisions Applicable to Different Types of Transformers" — the rules that decide where a transformer may physically live, keyed on its insulation type: 450.21 dry-type indoors (the 112½ kVA transformer-room boundary + the Class 155 exceptions), 450.22 dry-type outdoors (weatherproof enclosure + the 12-in combustible-material rule), 450.23 less-flammable liquids (the 300 °C fire point; the 35,000 V indoor cap), 450.24 nonflammable fluids (indoors or outdoors; vault over 35,000 V), 450.25 askarel (the 25 kVA vent / 35,000 V vault pair), 450.26 oil-insulated indoors (the vault rule + all six Exceptions), 450.27 oil-insulated outdoors (the four fire safeguards), and 450.28 modification marking. Part III — the vault construction itself (450.41–450.48) — is Article 54 →.

The sections, in full (2017)

All eight sections of Part II, quoted verbatim from the 2017 NEC:

The insulation-type key — the boundaries at a glance

The 112½ kVA figure appears three times (450.21(A)/(B) indoors, 450.22 outdoors, and 450.26 Ex 1's concrete vault) — it is the recurring "small transformer" boundary of Part II. The 35,000 V figure is the recurring medium-voltage boundary (dry-type → Part III vault; less-flammable indoor routes cap; askarel → vault). The 75 kVA figure is the recurring oil-indoors exception boundary (fire-resistant no-vault; furnace; accelerator).

450.21 — Dry-type indoors: the 112½ kVA boundary

This is the section people hit first when a dry-type unit lands inside a building:

• (A) Not over 112½ kVA. A separation of at least 300 mm (12 in.) from combustible material — or a fire-resistant, heat-insulated barrier in place of the distance. The Exception is the one that puts pad-mount-style dry-type units against walls legally: at 1000 V nominal or less and completely enclosed except for ventilating openings, even the 12 inches is not required.
• (B) Over 112½ kVA. A transformer room of fire-resistant construction — 1-hour minimum fire rating (2023 fuses the rating into the rule; 2017 stated it as a term definition). Exception No. 1: Class 155 or higher insulation systems with a fire-resistant, heat-insulating barrier or separation of not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically. Exception No. 2: Class 155 or higher, completely enclosed except for ventilating openings. This is why the temperature class on the nameplate (450.11(A)(8) — Article 52) is a Part II input: a Class 155 dry-type unit at 150 kVA can sit in a normal room; a Class 120 unit at 150 kVA needs the 1-hour room.
• (C) Over 35,000 V. Part III vault — regardless of the size or insulation class. The medium-voltage boundary.

450.22 — Dry-type outdoors

Two sentences: weatherproof enclosure, and for units exceeding 112½ kVA, no location within 300 mm (12 in.) of combustible materials of buildings — unless Class 155 insulation or higher + completely enclosed except for ventilating openings. Note the asymmetry with 450.21: outdoors there is no barrier alternative to the 12 inches, and the insulation-class escape is Class 155-or-higher (450.21(B)'s barrier-or-distance Exception has no outdoor counterpart).

450.23 — Less-flammable liquid (the 300 °C fire point)

The gateway sentence: the liquid must be listed with a fire point of not less than 300 °C. (Fire point, not flash point — the liquid is permitted to be less flammable than oil, not nonflammable like 450.24's fluids.)

• (A) Indoor. Three routes: (1) Type I or Type II building where all of — 35,000 V or less, no combustible materials stored, liquid confinement area provided, installation complies with the listing's restrictions (the IN enumerates them: maximum tank pressure, pressure relief valve, appropriate fuse types, proper OCPD sizing); (2) an automatic fire extinguishing system + liquid confinement area, 35,000 V or less; (3) in accordance with 450.26 (the oil-insulated vault route). The 35,000 V cap is the boundary: a 40 kV unit has only route (3) — a vault.
• (B) Outdoor. Attached to, adjacent to, or on the roof of buildings — per (1) the listing's restrictions (Type I/II buildings) or (2) in accordance with 450.27 (the outdoor oil safeguards, by reference). The 2017 (B)(2) Informational Note No. 1 defines "Type I and Type II buildings" via NFPA 220-2015 (2023 re-cites NFPA 220-2021) and defines combustible materials as those not classified noncombustible or limited-combustible.

450.24 — Nonflammable fluid

The top of the fluid hierarchy: a dielectric fluid identified as nonflammable — the section's own definition: one that does not have a flash point or fire point and is not flammable in air. Permitted indoors or outdoors. The three indoor obligations: vault if over 35,000 V; a liquid confinement area + pressure-relief vent; and a means for absorbing gases generated by arcing inside the tank, or the pressure-relief vent connected to a chimney or flue carrying the gases to an environmentally safe area. The Informational Note suggests fire hazard analyses "may" increase safety — permissive, not mandatory.

450.25 — Askarel (PCB) — the legacy fluid

Askarel is the older PCB-based fluid (polychlorinated biphenyls) — still present in service on existing units, which is why the rule survives. Two boundaries: over 25 kVA indoors → pressure-relief vent (the word "over" matters: at exactly 25 kVA no vent is required); poorly ventilated place → gas absorption or the vent routed to a chimney/flue outside the building; over 35,000 V → vault. Word-identical 2017 → 2023.

450.26 — Oil-insulated indoors: the vault + six Exceptions

The default is the strictest in Part II: indoor oil-filled = Part III vault. The six Exceptions, in the order an inspector will ask about them:

1. 112½ kVA or less → the vault may be reinforced concrete, not less than 100 mm (4 in.) thick, instead of full Part III construction (the 4-inch concrete vault — the cheapest legal indoor oil-filled option).
2. 1000 V nominal or less + suitable arrangements to prevent an oil fire from igniting other materials → no vault up to 10 kVA in a section classified as combustible, or 75 kVA where the surrounding structure is fire-resistant. This is the exception that covers most small indoor pad-mount-adjacent oil units.
3. Electric furnace transformers, total rating ≤ 75 kVA → no vault in a fire-resistant building/room, with suitable arrangements.
4. Charged-particle-accelerating equipment (medical linear accelerators and the like): ≤ 75 kVA, supply 1000 V or less, integral to the equipment → no vault in noncombustible or fire-resistant construction, with suitable arrangements.
5. Detached building that does not comply with Part III — if it presents no fire hazard to other buildings/property, is used only in supplying electric service, and the interior is accessible only to qualified persons (the "transformer shack" exception).
6. Portable and mobile surface mining equipment (electric excavators): no vault if (1) draining leaking fluid to the ground, (2) safe egress, (3) a 6-mm (1/4-in.) steel barrier for personnel protection.

450.27 — Oil-insulated outdoors: the four fire safeguards

Outdoor oil-filled units (rooftop, pole-adjacent, pad-mounted next to the building) must safeguard combustible material, buildings, fire escapes, and door/window openings from transformer-origin fires. Where the installation presents a fire hazard, apply one or more — "according to the degree of hazard involved" is the AHJ's latitude clause:

1. Space separations
2. Fire-resistant barriers
3. Automatic fire suppression systems
4. Enclosures that confine the oil of a ruptured tank — fire-resistant dikes, curbed areas or basins, or trenches filled with coarse, crushed stone; trapped drains where the exposure and quantity of oil make removal important (a trapped drain keeps fire-extinguishing foam/chemicals in and keeps the oil from running off-site).

The Informational Note points to the National Electrical Safety Code for pole/structure/underground installations — 2017 cites ANSI C2-2007; 2023 cites ANSI/IEEE C2-2017.

450.28 — Modification of transformers

The catch-all for the retrofits: if a modification to an existing installation changes the transformer's type with respect to Part II (e.g., a fluid swap that moves the unit from oil to less-flammable, or a tank replacement that changes the insulation class), the transformer shall be marked to show the type of insulating liquid installed, and the modified installation shall comply with the applicable requirements for that type. The marking is the audit trail: a 450.11(A)(7) nameplate item (amount and kind of insulating liquid) that 450.28 enforces at modification time.

Edition history: 2017 → 2023

The headline: every boundary number and every installation rule is unchanged — 112½ kVA, 300 mm (12 in.), 1000 V, 1 hour, 1.83 m (6 ft) / 3.7 m (12 ft), Class 155, 35,000 V, 300 °C, 25 kVA, 10 kVA / 75 kVA, 100 mm (4 in.), 6 mm (1/4 in.) — all present in both editions. A targeted machine check (verify_art53.py, 192 phrase-level assertions reading the on-disk 2017 NFPA scan and the on-disk 2023 NEC directly) confirms it, item by item:

The verified-substantive core (2017 ≈ 2023)

2020 position — stated, not asserted: no on-disk 2020 full-code text covers Article 450 (the on-disk 2020 scan ends before Article 450's Part II body; the 2020/2023 online viewer is login-gated). So this page documents 2017 → 2023 and does not assert a 2020 word-identity for Part II. The identity claim rests on the two on-disk full sources (2017 NFPA scan + 2023 CSV), the standard this series uses. No Mike Holt 2023 change-summary entries exist for any Part II section (the on-disk Mike Holt 2023 summaries cover only 450.1 and 450.10 in Article 450) — every delta below is on-disk-verified against the two full-text sources, stated as such.

2017 → 2023 deltas in 450.21–450.28 (all verified on disk)

Five worked examples (core-computed)

Every rated current and conductor pick below is computed by the shipped cores under node (compute_art53.js → art53_numbers.json): three-phase rated current as kVA×1000 ÷ (√3 × V), and pickConductor31016 for the Table 310.16 (75 °C Cu column) conductor picks. The boundary figures are the code numbers above; the kVA / voltage values are representative — read your specific transformer's nameplate.

EX1 — 450.21(A)/(B) the 112½ kVA boundary, 480 V primary, core-computed

The pedagogical point of the boundary: stepping one kVA over 112½ doesn't change the conductors (169.15 A and 169.90 A both pick 2/0 AWG Cu at 175 A) — it changes the building. The 450.21(B) trigger is about fire exposure, not ampacity: the same 2/0 Cu feed serves both units, but the 113 kVA unit needs a 1-hour room unless it is Class 155.

EX2 — 450.26 Exception 2: the no-vault indoor oil boundaries (≤ 1000 V), 480 V

The two boundaries are not interchangeable: a 25 kVA indoor oil unit at 480 V cannot use Exception 2 in a combustible section (10 kVA max) — but it can use Exception 1: a 4-inch reinforced-concrete vault instead of the full Part III vault. The 112½ kVA concrete-vault route and the 75 kVA fire-resistant no-vault route are the two "escape hatches" an indoor oil-filled design normally lands on.

EX3 — 450.25 askarel: the 25 kVA vent boundary + the 35,000 V vault boundary

Both askarel boundaries are "over" boundaries: exactly 25 kVA needs no vent; exactly 35,000 V needs no vault. And the vault trigger is voltage, not size — the 100 kVA / 38 kV unit carries only 1.52 A of rated current but still goes in a vault.

EX4 — 450.23(A): the 35,000 V indoor cap, 5 MVA unit

The cap is the design pivot: the same 5 MVA transformer at 34.5 kV can sit in a Type I/II room (with no storage, a confinement area, and the listing's restrictions), while the 38 kV version of the same unit must go into a 450.26 vault. The 35,000 V number is the recurring Part II medium-voltage boundary (it also caps 450.21(C) for dry-type and 450.24/450.25 vaults).

EX5 — 450.26 Exception 6: surface-mining equipment, 250 kVA @ 480 V

Exception 6 is the only Part II exception that does not mention a building at all: the "vault" is replaced by draining + egress + a 6-mm steel screen. The 250 kVA / 480 V mining unit (300.70 A rated, 500 kcmil Cu feed) would need a full Part III vault indoors — but on the excavator it is legal without one, provided the three conditions hold.

Where 450.21–450.28 fit — the type key of Article 450

• The OCPD core these rules sit around: the 450.3 transformer overcurrent article (Article 51) → — Table 450.3(A)/(B) (whose "over 1000 V" and "1000 V and less" split is the same boundary that splits 450.21(A)/(C)), 450.4 autotransformers, 450.6 secondary ties. Article 51 sizes the protection; this article sizes the envelope.
• The general installation rules these type rules assume you already satisfy: the 450.8–450.14 transformer installation article (Article 52) → — guarding, ventilation, grounding, the eight nameplate items (item 7 — amount and kind of insulating liquid — is the 450.28 audit trail; item 8 — temperature class — drives the 450.21(B) Class 155 escapes), the 50 kVA hollow-space boundary (450.13(B) cross-references 450.21(A) directly), and the disconnecting means.
• The separately-derived-system grounding a transformer secondary ties into: the 250.26–250.30 separately derived systems article →.
• The ampacity table behind every conductor pick: the Table 310.16 ampacity article →, with the 240.6 standard ampere ratings → for device picks.

Scope boundary, stated: Part II is exactly these eight sections — there is no 450.15 through 450.20 (verified against both on-disk sources: the sections are absent from the 2017 scan and have no 2023 CSV rows), and Part II ends at 450.28 — Part III (Transformer Vaults, 450.41–450.48) starts at 450.41. 2026 edition (flagged honestly): no on-disk source confirms the Article 450 section numbers moved for the 2026 cycle, so this page cites 450.21–450.28 as stable across 2017–2023 and presents the 2017↔2023 deltas above. Verify section numbers against the NEC edition adopted in your jurisdiction.

Method, sources & honesty notes

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