4.7 Transformer Secondary Conductors & Selective Coordination (0303)
Key Takeaways
- A transformer primary overcurrent device generally does not protect the secondary conductors, because the primary device sees secondary current scaled by the turns ratio — which is why 240.21(C) exists.
- 240.21(C)(1) permits a primary device to protect 2-wire to 2-wire secondary conductors where the primary device rating multiplied by the primary-to-secondary voltage ratio does not exceed the secondary conductor ampacity.
- 240.21(C)(2) allows 10 ft secondary conductors, and 240.21(C)(6) allows 25 ft secondary conductors, each with its own minimum-ampacity and termination conditions.
- Selective coordination means only the overcurrent device nearest the fault opens; it is mandatory for elevator feeders (620.62), emergency systems (700.32), legally required standby (701.32), critical operations power systems (708.54), and the essential electrical system of a health care facility.
- Coordination is verified from time-current curves supplied by the manufacturer, not from device ratings alone — two devices of different amperage are not automatically coordinated.
Why the Primary Device Usually Cannot Protect the Secondary
The overcurrent device on the primary of a transformer sees primary current. The secondary conductors carry secondary current. Those two are related by the voltage ratio, so a device that looks generous on the primary can be enormous relative to the secondary conductors.
Take a 45 kVA, 480 V to 208Y/120 V transformer with a 70 A primary device (the value computed in Section 6.4). Referred to the secondary, that 70 A device does not open until the secondary current reaches roughly 70 × (480 ÷ 208) = 161 A, and on a three-phase transformer the relationship is further modified by the winding configuration. Secondary conductors sized for the transformer's 125 A rated secondary current would be running well above their ampacity before the primary device ever noticed.
That is the entire reason 450.3 protects the transformer and 240.21(C) protects the secondary conductors — two different jobs, two different sections. Never answer a secondary-conductor question out of Table 450.3(B).
The 240.21(C) Options
| Rule | Application | Key conditions |
|---|---|---|
| 240.21(C)(1) | 2-wire primary to 2-wire secondary | The primary device is considered to protect the secondary conductors where the primary device rating multiplied by the primary-to-secondary voltage ratio does not exceed the secondary conductor ampacity. Available only for single-phase, 2-wire to 2-wire arrangements |
| 240.21(C)(2) | Industrial installations, up to 25 ft | Qualified persons service the installation; secondary conductor ampacity at least equal to the secondary full-load current and at least the sum of the connected loads; conductors protected from physical damage |
| 240.21(C)(3) | Secondary conductors not over 10 ft | Ampacity not less than the combined calculated loads, and not less than the rating of the device or termination supplied; conductors terminate in a single device rated no more than the conductor ampacity; conductors in a raceway; and the secondary conductor ampacity is at least one-tenth of the primary device rating multiplied by the voltage ratio |
| 240.21(C)(4) | Outside secondary conductors | The conductors are outside the building except at the point of termination, protected from damage, and terminate at a single device that limits the load to the conductor ampacity, with the disconnect at a readily accessible location |
| 240.21(C)(6) | Secondary conductors not over 25 ft | Secondary conductor ampacity at least equal to the secondary full-load current, and at least one-third of the primary device rating multiplied by the voltage ratio; conductors terminate in a single device rated no more than the conductor ampacity; conductors protected from physical damage |
The shape is familiar from feeder taps in Section 4.3: a 10 ft option with a one-tenth ampacity floor, and a 25 ft option with a one-third floor. The difference is that here the length is measured from the transformer secondary terminals to the overcurrent device, and the primary device rating must be translated through the voltage ratio before either fraction means anything.
Field pattern. The common commercial installation is a dry-type transformer feeding a panelboard mounted directly beneath it, with a main breaker in the panel. That is a 10 ft secondary conductor set terminating in a single device — 240.21(C)(3). The two things to check are that the conductors are in a raceway for the whole run and that the panel main does not exceed the secondary conductor ampacity.
Selective Coordination
Selective coordination is defined in Article 100 as the localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents.
Read the definition carefully, because it is written to close a loophole. Coordination is not "the upstream device is bigger." It must hold across every current magnitude the system can produce and across every clearing time — including the instantaneous region, where two circuit breakers can easily overlap even though their ampere ratings are far apart.
Where the NEC requires it
| Application | Section |
|---|---|
| Elevator, dumbwaiter, escalator, and moving walk feeders where more than one driving machine disconnecting means is supplied by a single feeder | 620.62 |
| Emergency systems — the emergency system overcurrent devices must be selectively coordinated with all supply-side devices | 700.32 |
| Legally required standby systems | 701.32 |
| Critical operations power systems (COPS) | 708.54 |
| Health care essential electrical systems — coordination requirements in Article 517 for the essential electrical system | Article 517 |
Everywhere else, coordination is a design preference, not a Code requirement.
How an inspector verifies it
You cannot verify coordination by looking at nameplates. The verification package is a set of time-current curves from the device manufacturers, plotted together, showing no overlap over the range from the minimum expected overload to the maximum available fault current at that point in the system. For fuses, coordination is commonly demonstrated by manufacturer-published selectivity ratios. For circuit breakers, it often requires devices with adjustable instantaneous settings or zone-selective interlocking, because two thermal-magnetic breakers with fixed instantaneous trips will usually both open on a high-magnitude fault no matter how their ampere ratings compare.
What the inspector should ask for: a coordination study stamped by the engineer of record, the available fault current at the equipment (which 110.24 already requires to be field marked on service equipment other than dwellings), and evidence that the as-installed device settings match the study. A breaker shipped at its factory default when the study called for a raised instantaneous setting is a coordination failure even though the device is the right catalog number.
Coordination versus series rating — do not confuse them
Series rating (240.86) lets a downstream device with a lower interrupting rating be used behind a tested upstream device — the two devices are expected to open together on a high fault. Selective coordination requires exactly the opposite: only the downstream device opens. A series-rated combination is therefore, by definition, not selectively coordinated in the range where the upstream device participates. Where coordination is mandatory — an elevator feeder, an emergency system — a series-rated combination is not an acceptable solution.
A 45 kVA, 480 V to 208Y/120 V transformer has a 70 A primary overcurrent device. Which section governs the protection of the SECONDARY conductors?
Which of the following applications does the NEC require to be selectively coordinated?
Why is a series-rated combination under 240.86 not a way to achieve selective coordination?