Transformer Secondary Conductor Calculator

Why a transformer secondary is not a feeder tap

A feeder tap connects to a feeder at the same voltage, so its ampacity is a fraction of the feeder overcurrent device directly. A transformer secondary conductor is different: a transformer sits between the primary overcurrent device and the conductor, and it changes the current. Because a transformer conserves apparent power, the secondary current is the primary current scaled by the primary-to-secondary voltage ratio. The primary overcurrent device therefore reflects onto the secondary as a different number, and every 240.21(C) secondary ampacity is a fraction of that reflected value, not of the primary rating itself.

Reflected primary OCPD = primary device rating times the primary voltage divided by the secondary voltage. For a step-down transformer the reflected value is larger than the primary rating, because the secondary current is higher. Swapping the two voltages inverts the ratio and undersizes the conductor badly, which is the most common mistake in these calculations.

The three secondary rules

240.21(C)(1) lets the primary overcurrent device protect the secondary at any length, but only for a single-phase two-wire or three-phase delta-delta three-wire secondary, and only when the conductor is sized to the full reflected OCPD. 240.21(C)(2) is the 10-foot secondary rule: the conductor ampacity has to at least match the single device it feeds, and where it leaves the enclosure it also has to be at least one-tenth of the reflected OCPD. 240.21(C)(6) is the 25-foot secondary rule: it trades a larger minimum ampacity, one-third of the reflected OCPD, for the longer allowable run, and requires the conductors to land on a single breaker or fuse set.

Worked example: 75 kVA, 480 to 208 V, 25-foot secondary

1. The transformer has a 125 A device on its 480 V primary and a 208Y/120 V secondary that runs 25 feet, so the 25-foot rule applies.
2. Reflected primary OCPD is 125 times 480 divided by 208, which is 288.5 A.
3. Required secondary ampacity is one-third of that: 288.5 / 3 = 96.2 A.
4. In the 75 degrees C column of Table 310.16, 4 AWG copper is 85 A, which falls short of 96.2 A.
5. 3 AWG copper is 100 A, which meets it, so the minimum secondary conductor is 3 AWG copper.
6. The secondary still has to terminate in a single device rated no more than 100 A and be protected from physical damage.

Why 240.4(D) does not limit a secondary conductor

For an ordinary branch conductor, 240.4(D) caps overcurrent protection at 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG copper regardless of the table ampacity. Those caps do not apply to transformer secondary conductors: 240.4 makes an exception for conductors sized under 240.21, so a secondary conductor is sized on its full Table 310.16 ampacity. This calculator applies that exception and does not impose the 240.4(D) cap.

Conditions beyond ampacity

Meeting the ampacity fraction is necessary but not sufficient. The results panel lists the remaining conditions for the applied rule: for a 25-foot secondary, termination in a single overcurrent device and protection from physical damage; for a 10-foot secondary, not extending beyond the equipment supplied and, where it leaves the enclosure, the 10-times reflected-device limit. A conductor that meets the ampacity but fails a condition is not a compliant secondary.

Frequently asked questions

What is the 25-foot transformer secondary rule?

NEC 240.21(C)(6) allows a transformer secondary conductor up to 25 feet where the ampacity is at least one-third of the reflected primary overcurrent device, the conductors terminate in a single breaker or fuse set, and they are protected from physical damage.

How do I figure the reflected primary OCPD?

Multiply the primary overcurrent device rating by the primary voltage divided by the secondary voltage. A 125 A device on a 480 to 208 V transformer reflects to 125 times 480 divided by 208, or 288 A.

Does the primary breaker protect the secondary conductors?

Only for a single-phase two-wire or delta-delta three-wire secondary sized to the full reflected OCPD, under 240.21(C)(1). Other configurations need a 10-foot or 25-foot rule or a secondary overcurrent device.

Does NEC 240.4(D) apply to secondary conductors?

No. 240.4 excepts conductors sized under 240.21 from the small-conductor limits, so a secondary conductor is sized on its full table ampacity rather than the 15, 20, or 30 A caps for 14, 12, and 10 AWG copper.

Related calculators

• Feeder Tap Conductor Size Calculator: the 240.21(B) sibling for taps at the same voltage, with no transformer in between.
• Breaker Size Calculator: the standard 240.6(A) device and minimum conductor for a general-use load.
• Wire Ampacity Calculator: the Table 310.16 ampacity of a chosen conductor, with ambient and fill derating.
• Wire Size Calculator: size a conductor for a load by ampacity and voltage drop together.