A transformer secondary is where two code jobs get confused. NEC 450.3(B) sizes the device protecting the transformer winding; it says nothing about the conductors leaving the secondary terminals, and NEC 240.4(F) states outright that those conductors are not considered protected by the primary device except on a 2-wire-to-2-wire single-phase or delta-delta 3-wire transformer. That leaves the secondary run to NEC 240.21(C), five length-based allowances with ampacity floors of their own — one-tenth of the primary device rating at 10 ft, one-third at 25 ft, each multiplied by the primary-to-secondary voltage ratio. Canada does not use length at all: CEC Rule 26-256(2)(a) sets a flat 125% of rated secondary current. On a 150 kVA, 208Y/120 V secondary that is the difference between a 346 A floor and a 520 A floor — and in general between at most 83% of rated secondary current and a flat 125%. These are the rules behind the secondary current and device ratings produced by the transformer sizing and overcurrent protection calculator.
Why doesn’t the primary device protect the secondary conductors?
A transformer changes both voltage and current, so a secondary fault reaches the primary device reduced by the turns ratio and shifted by the winding connection. On a delta-wye transformer a single-phase secondary fault does not present that device with a proportional current, which is why NEC 240.4(F) withholds the assumption of protection. Only the symmetrical cases survive it — a single-phase transformer with a 2-wire secondary, and a three-phase delta-delta with a 3-wire secondary — and for those NEC 240.21(C)(1) permits the primary device to protect the secondary feeder, provided the protection complies with 450.3 and does not exceed the secondary conductor ampacity multiplied by the secondary-to-primary voltage ratio.
Every other transformer needs either a secondary overcurrent device or one of the length-based allowances below. The secondary main is usually there anyway: NEC 408.36 requires a panelboard to be protected by an overcurrent device rated not greater than the panelboard, located within or on its supply side.
What does NEC 240.21(C) actually allow?
Five subsections follow 240.21(C)(1), each pairing a maximum length with an ampacity floor and a termination condition. Numbering below is the 2023 NEC; Canadian references are to CSA C22.1.
| Subsection | Max length | Minimum secondary conductor ampacity | Key conditions |
|---|---|---|---|
| 240.21(C)(2) | 10 ft | Computed load, and the rating of the device supplied; where conductors leave the tap enclosure, 1/10 × primary OCPD × voltage ratio | Do not extend beyond the equipment supplied; enclosed in raceway or metal enclosure |
| 240.21(C)(3) | 25 ft | Not less than the transformer secondary current rating | Industrial installations only, qualified persons servicing |
| 240.21(C)(4) | Unlimited | Sized to the load and termination | Outside conductors; protected from physical damage; single device integral to the disconnect; readily accessible disconnect outside or inside nearest the point of entrance, including conductors deemed outside under 230.6 |
| 240.21(C)(5) | 25 ft, primary plus secondary combined | Per the feeder tap rule invoked | Secondary conductors from a feeder-tapped transformer |
| 240.21(C)(6) | 25 ft | 1/3 × primary OCPD × voltage ratio | Single circuit breaker or set of fuses limiting load current to the conductor ampacity permitted by 310.14 (Table 310.16); protected from physical damage |
Two things here are load-bearing. First, the floors in (C)(2) and (C)(6) scale with the primary overcurrent device, not the load — a decision made upstream sets the minimum copper downstream. Second, NEC 240.4(B), the next-size-up allowance that lets a 100 A breaker protect 95 A conductors on an ordinary circuit, is effectively unavailable here, because each tap rule independently requires the terminating device rating to sit at or below the conductor ampacity.
A worked example: 150 kVA, 480 V to 208Y/120 V at 20 feet
Take a three-phase dry-type distribution transformer feeding a 208Y/120 V panelboard 20 ft away, protected on both windings. Conductors are copper at 75 °C terminations, permitted above 100 A by NEC 110.14(C)(1)(b).
- Winding currents. 150 000 ÷ (√3 × 208) = 150 000 ÷ 360.3 = 416 A secondary; 150 000 ÷ (√3 × 480) = 150 000 ÷ 831.4 = 180 A primary.
- Device ratings, carried over. NEC Table 450.3(B) gives a 450 A primary device (250% × 180.4 = 451 A, rounded down to a NEC 240.6(A) standard rating) and a 600 A secondary main (125% × 416.4 = 520 A, rounded up under Note 1). Both are derived step by step in sizing transformer overcurrent protection; this article starts where that one stops.
- Voltage ratio. 480 ÷ 208 = 2.31.
- The 240.21(C)(6) floor at 20 ft. (1 ÷ 3) × 450 × 2.31 = 150 × 2.31 = 346 A.
- The termination test. The single 600 A main must not exceed the conductor ampacity, and 240.4(B) offers no relief. The governing minimum is therefore 600 A, not 346 A.
- Conductor selection. From NEC Table 310.16, 75 °C copper: 350 kcmil = 310 A, so two parallel sets give 2 × 310 = 620 A ≥ 600 A. Two sets of 300 kcmil would give 2 × 285 = 570 A and fail.
I = kVA × 1000 ÷ (√3 × V)
Minimum ampacity, 25 ft rule = primary OCPD ÷ 3 × (V primary ÷ V secondary)
Minimum ampacity, 10 ft rule = primary OCPD ÷ 10 × (V primary ÷ V secondary)
At 8 ft, NEC 240.21(C)(2) would apply instead. Its 1/10 floor of 0.1 × 450 × 2.31 = 104 A bites only where the conductors leave the enclosure in which the tap is made; inside it, (C)(2) demands the computed load and the rating of the device supplied. Either way the 600 A termination governs — the length rules relax the floor, never the device-to-conductor match.
Where the one-third floor actually bites
The 25 ft floor binds only when a modest panel hangs off a large transformer. Assume both windings protected and the primary device set at the standard rating just below 250%:
| Transformer (480 V to 208Y/120 V) | Primary FLA | Primary OCPD at 250% | 10 ft floor, conductors leaving the enclosure | 25 ft floor |
|---|---|---|---|---|
| 75 kVA | 90 A | 225 A | 52 A | 173 A |
| 150 kVA | 180 A | 450 A | 104 A | 346 A |
| 500 kVA | 601 A | 1200 A | 277 A | 923 A |
That 500 kVA dry-type unit draws 500 000 ÷ (√3 × 208) = 1388 A, so an 800 A panelboard is only part of its load. Put that panel 20 ft away and the arithmetic turns: the 800 A main would allow 800 A conductors under the 10 ft rule, but (C)(6) demands 1200 ÷ 3 × 2.31 = 923 A. Three parallel sets of 300 kcmil copper give 3 × 285 = 855 A and fail; three sets of 350 kcmil give 3 × 310 = 930 A and pass. That is 8.8% more ampacity, bought with about 17% more copper by cross-sectional area, purely because the run crossed 10 ft. Because the 250% primary figure is a ceiling rather than a target, choosing a smaller primary device lowers the floor directly.
Where Canada differs: CEC Rule 26-256
The Canadian Electrical Code (CSA C22.1) reaches the same safety outcome by a different mechanism. CEC Rule 26-256(1)(a) requires primary conductors to have an ampacity of not less than 125% of the rated primary current, and 26-256(2)(a) requires secondary conductors to have an ampacity of not less than 125% of the rated secondary current. There is no length-based reduction and no one-third floor.
Holding one machine constant — 150 kVA, 208Y/120 V secondary, 416 A rated secondary current — and running both codes across two primary voltages isolates the difference:
| 150 kVA, 208Y/120 V secondary (416 A) | NEC 240.21(C)(6) floor at 25 ft | CEC 26-256(2)(a) floor, any length |
|---|---|---|
| 480 V primary — 180 A, 450 A device | 346 A | 520 A |
| 600 V primary — 144 A, 350 A device | 337 A | 520 A |
| Basis | 1/3 × primary OCPD × voltage ratio | 125% × rated secondary current |
| Length dependence | Yes — 10 ft, 25 ft, or unlimited outside | None |
The 600 V row runs on the service voltage common in Canadian buildings: 150 000 ÷ (√3 × 600) = 144 A primary, a 350 A device at 250%, a 600 ÷ 208 = 2.88 ratio, and 350 ÷ 3 × 2.88 = 337 A. The Canadian primary conductor minimum is 1.25 × 144.3 = 180 A; the secondary minimum is 1.25 × 416.4 = 520 A either way.
The general rule falls out of the two percentages. The NEC floor is one-third of a device already capped at 250% of primary current, and primary current times the voltage ratio is secondary current — so the 25 ft floor can never exceed 0.333 × 2.50 = 83% of rated secondary current, and rounding the device down to a standard rating pushes it lower still. The CEC starts at 125% of that same current and stays there: the NEC floor moves with the primary device, the CEC floor moves with nothing. On the protection side, CEC Rule 26-254(1) limits the primary device to 125% of rated primary current, 26-254(2) permits the primary feeder to reach 300% where a secondary device is set at not more than 125% of rated secondary current, and 26-254(3) permits the next higher standard rating where 125% does not land on one. Where the transformer’s overcurrent protection is selected under 26-250(1) or (2) or 26-254(3), 26-256(4) sends the conductors to Rules 14-100 and 14-104 — and 14-104(1) holds the device rating to the allowable ampacity of the conductors it protects, subject to the Table 13 next-standard-size exception in 14-104(1)(a). The reduced-size tap allowance in Rule 14-100 item (f) caps the combined length of one primary plus one secondary conductor at 7.5 m. Confirm the current subrule against CSA C22.1 and the local AHJ or ESA.
What to specify
- Secondary full-load current at the actual secondary voltage — the basis for every percentage below, and what the calculator produces
- Primary overcurrent device rating, because the 240.21(C)(2) and (C)(6) floors are computed from it, not from the load
- Run length and route — under or over 10 ft, under or over 25 ft, inside or outside, and whether the conductors leave the tap enclosure
- Terminating device — a single circuit breaker or set of fuses, rated at or below the conductor ampacity, with no NEC 240.4(B) next-size-up
- Termination temperature rating — the 75 °C column of NEC Table 310.16 per 110.14(C)(1)(b) above 100 A, and ampacity after ambient or conduit-fill adjustment
- Panelboard or switchboard rating and its main, to satisfy NEC 408.36, plus the available fault current and matching AIC and SCCR from available fault current and short-circuit ratings
- Code basis on the drawing — the NEC 240.21(C) subsection or CEC 26-256 subrule named explicitly, with physical protection called out where (C)(2) or (C)(6) requires it
Where Entogo fits
Entogo builds three-phase dry-type and pad-mounted distribution transformers and the low-voltage switchboards and panelboards they feed in its own Toronto-area factory, designed and built to ANSI/IEEE C57, UL 1561 / UL 1562, UL 67 and UL 891; UL (cULus) / CSA certifiable on request. Because the transformer and the assembly its secondary conductors land in are engineered together, the secondary main rating, the bus ampacity and the termination provisions are settled as one specification rather than reconciled on site.
Run the winding currents and device ratings in the transformer sizing and overcurrent protection calculator, then apply the 240.21(C) floor or the 26-256 percentage to the run. Turn the result into a specification with the transformer configurator, browse the transformer and substation range, or see the same coordination inside a packaged lineup under substations and power distribution. For the sizing decision that precedes all of this, see how to size a transformer.
Transformer secondary conductors reward a habit rather than a formula: name the rule before picking the wire. Establish whether the primary device may be credited at all under 240.4(F), pick the subsection matching the run length, compute the floor from the device and the voltage ratio, then check the terminating device against the conductor ampacity. In Canada, skip the length question and start at 125%.
