7.3 Transformer Secondary Conductors — 240.21(C)

Three Separate Questions

When a transformer appears in an exam question, three different rules may be in play and each must be answered separately:

Answering the transformer-protection question when the exam asked about secondary conductors is the most common error on this topic.

Why Secondary Conductors Are Special

A primary overcurrent device sees current on the primary side. A fault on the secondary reflects back to the primary transformed by the turns ratio, so a 200-ampere secondary fault on a 480-to-240 V transformer appears as only 100 amperes on the primary. Unless the geometry is simple, the primary device cannot reliably protect the secondary conductors.

240.4(F) states this directly:

Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device.

The Two Cases Where Primary Protection Works — 240.21(C)(1)

Only two transformer configurations have a fixed, predictable relationship between primary and secondary current in every fault case:

1. A single-phase transformer with a two-wire (single-voltage) secondary;
2. A three-phase delta-delta connected transformer with a three-wire (single-voltage) secondary.

For these, 240.21(C)(1) permits the secondary conductors to be considered protected by the primary overcurrent device, provided that protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary voltage ratio.

Worked Example — Two-Wire Secondary

A single-phase 480 V to 240 V transformer has a two-wire secondary. The secondary conductors have an ampacity of 100 A. What is the maximum primary overcurrent device that can be said to protect them?

Secondary-to-primary voltage ratio = 240 / 480 = 0.5
Maximum primary OCPD = 100 A x 0.5 = 50 A

A primary device of 50 A or less protects those secondary conductors. Anything larger requires secondary-side protection.

Note that a 208Y/120 V wye secondary never qualifies for 240.21(C)(1), because it is a four-wire, two-voltage secondary. Almost every commercial dry-type transformer you will meet falls outside the exception and needs secondary protection.

The 10-Foot Secondary Conductor Rule — 240.21(C)(2)

Where the length of secondary conductor does not exceed 10 feet (3.0 m) and complies with all of the following:

1. The ampacity of the secondary conductors is:
   • not less than the combined calculated loads on the circuits supplied, and
   • not less than the rating of the device supplied by the secondary conductors or not less than the rating of the overcurrent protective device at the termination of the secondary conductors;
2. The secondary conductors do not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices they supply;
3. The secondary conductors are enclosed in a raceway, extending from the transformer to the enclosure of the equipment supplied;
4. For field installations where the secondary conductors leave the enclosure or vault in which the supply connection is made, the rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary-to-secondary transformer voltage ratio, shall not exceed 10 times the ampacity of the secondary conductor.

Worked Example — 10-Foot Secondary Tap

A 480 V to 240 V single-phase transformer has a 100 A primary overcurrent device. A 9-foot secondary run in a raceway supplies a panelboard. What is the minimum secondary conductor ampacity under condition 4?

Primary-to-secondary voltage ratio = 480 / 240 = 2
Primary OCPD x ratio = 100 A x 2 = 200 A
Minimum secondary conductor ampacity = 200 / 10 = 20 A

20 amperes, subject also to condition 1 — the conductors must still carry the calculated load and match the rating of the device they terminate in.

The 25-Foot Secondary Conductor Rule — 240.21(C)(6)

For industrial installations only, where the length of the secondary conductors does not exceed 25 feet and:

1. The ampacity of the secondary conductors is not less than the secondary current rating of the transformer, and the sum of the ratings of the overcurrent devices does not exceed the ampacity of the secondary conductors;
2. All overcurrent devices are grouped;
3. The secondary conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.

A more commonly applied 25-foot rule appears in 240.21(C)(6) in the general form used by many jurisdictions: the secondary conductors have an ampacity not less than one third of the rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary-to-secondary voltage ratio; terminate in a single circuit breaker or set of fuses limiting the load to that ampacity; and are protected from physical damage.

Because the subsection numbering and conditions for the 25-foot secondary rule have shifted between editions, look this one up in the 2020 book rather than reciting it from memory. Tab 240.21(C) and read the subsections in order on exam day.

Worked Example — 25-Foot Secondary

A 480 V to 208Y/120 V transformer has a 100 A primary device. A 20-foot secondary run supplies a single main breaker panel. Using the one-third rule with the voltage ratio:

Primary-to-secondary voltage ratio = 480 / 208 = 2.31
Primary OCPD x ratio = 100 A x 2.31 = 231 A
Minimum secondary conductor ampacity = 231 / 3 = 77 A

77 amperes.

The Other Subsections of 240.21(C)

• 240.21(C)(3) Industrial Installation Secondary Conductors — not over 25 feet, for industrial installations only, where conditions of maintenance and supervision ensure only qualified persons service the installation.
• 240.21(C)(4) Outside Secondary Conductors — no length limit, mirroring 240.21(B)(5): outside the building except at the point of termination, protected from physical damage, terminating in a single overcurrent device, with the device an integral part of or immediately adjacent to a disconnect at a readily accessible location outside or nearest the point of entrance.
• 240.21(C)(5) Secondary Conductors from a Feeder Tapped Transformer — where the primary conductors are themselves a tap under 240.21(B)(3).

Transformer Overcurrent Protection — Table 450.3(B) Preview

The transformer itself is protected under Table 450.3(B) for transformers 1000 volts nominal or less:

Worked Example — Primary-Only Protection

A 45 kVA, 480 V three-phase to 208Y/120 V transformer has a primary full-load current of 54.1 A. Using primary-only protection under Table 450.3(B):

54.1 A x 1.25 = 67.6 A
67.6 A is not a standard rating; next higher standard from 240.6(A) = 70 A

A 70-ampere primary overcurrent device. Note that this device protects the transformer. Whether it also protects the secondary conductors is answered separately by 240.21(C) — and for a 208Y/120 V four-wire secondary the answer is no, so a secondary overcurrent device is required.

Putting It Together

A complete transformer installation with a 480 V primary and 208Y/120 V secondary typically requires:

1. A primary overcurrent device sized per Table 450.3(B) — up to 250 percent of primary current where secondary protection is also provided;
2. Primary conductors sized for the primary current and protected by that device or by a feeder tap rule;
3. A secondary overcurrent device — usually the main breaker of the panel the transformer feeds — sized at not more than 125 percent of the secondary current;
4. Secondary conductors sized per 240.21(C), most often the 10-foot rule if the panel is close-coupled;
5. A system bonding jumper and grounding electrode conductor for the separately derived system per 250.30, covered in Chapter 9 of this guide.