Technical Article

National Electrical Code Basics: Overcurrent Protection Part 4

September 12, 2022 by Lorenzo Mari

Learn about rules permitting the absence of overcurrent protection for the conductors connected to the transformer secondary.

To catch up on Lorenzo Mari's series on Overcurrent Protection, please visit:

The NEC requires protecting the transformer windings and the conductors connected to the primary and secondary. Yet, there are some conditions listed in Sections 240.21(C)(1) through 240.21(C)(6), permitting the absence of overcurrent protection for the conductors connected to the transformer secondary.

Image used courtesy of IAEI

Overcurrent Protection of Conductors Connected to the Transformer Secondary

NEC Section 240.21(C) permits conductors connected to the transformer secondary without overcurrent protection with a set of prerequisites. This section does not allow using the next larger standard-size overcurrent protective device to protect the transformer secondary conductors (See Section 240.4(B)).

Six Situations that Allow the Absence of Overcurrent Protection of Conductors at the Transformer’s Secondary

The rulings for the protection of the conductors connected to the transformer’s secondary and the feeder tap rules of Section 240.21(B) are similar.

1. Protection by primary overcurrent device. The primary overcurrent device protects the conductors in the secondary if supplied by the secondary side of a single-phase transformer with a 2-wire (single-voltage) secondary or a three-phase, delta-delta connected transformer having a 3-wire (single voltage) secondary.

The protection by the primary side is permitted for these two transformer connections only. Section 240.4(F) asserts the same limitation – circuit analysis shows that the per unit (PU) fault current at the primary and secondary sides of the transformer is the same, and the primary overcurrent protection “sees” a secondary fault entirely.

Other transformer connections may not provide sensitive fault protection. For example, a secondary line-to-ground fault in a delta/grounded-wye connection will produce 0.58 PU line current on the delta side.

The primary protection of the transformer must be per Section 450.3 and not surpass the value found by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

Example 1. A 2 kVA, single-phase, 120/240 V, 2-wire transformer supplies power to a 2 kVA load.

a. Compute the secondary current.

Is = 2 kVA/120 V = 16.67 A

b. Compute the primary current.

Ip = 2 kVA/240 V = 8.33 A, or

16.67 A x 120 V/240 V = 8.33 A

c. Compute the maximum rating of a fuse to protect the transformer primary.

NEC Table 450.3(B) sets the protective device at 167% of the rated primary current for less than 9 A, primary-only protection.

167% x 8.33 A = 13.9 A

Use a fuse rated 10 A

d. Compute the minimum size secondary conductor required to be protected by the primary fuse.

10 A x 240 V/120 V = 20 A

NEC Table 310.16 shows a conductor size N° 12 AWG TW Cu with 20 A ampacity under the 60°C column.

Use a conductor size N° 12 AWG TW Cu

2. Transformer secondary conductors not over 3 m long. The secondary conductors must comply with the following conditions:

• No longer than 3 m.

• Ampacity, not less than

○ The calculated load on the circuits supplied.

○ The rating of the equipment supplied or the rating of the overcurrent protective device where the secondary conductor terminates.

○ 1/10 of the rating of the overcurrent device protecting the primary of the transformer multiplied by the primary to secondary transformer voltage ratio if the secondary conductors leave the enclosure or vault where the supply connection is made.

• Do not extend beyond the equipment they supply.

• Enclosed in a raceway.

Example 2. Figure 1 shows a 9 kVA, 3-phase, delta-delta transformer field installation with one set of secondary conductors terminating at a 20 A overcurrent protective device.

Figure 1. Layout for example 2. Image used courtesy of Lorenzo Mari

a. Compute the rated primary current.

Ip = 9 kVA/(√3 x 480 V) = 10.83 A

b. Compute the primary protection

NEC Table 450.3(B) sets the protective device at 125% of the rated primary current for primary-only protection – currents of 9 A or more.

125% x 10.83 A = 13.54 A

Note 1 to NEC Table 450.3(B) permits the next higher standard.

Use a circuit breaker rated 15 A

c. Compute the rated secondary current.

Ip = 9 kVA/(√3 x 240 V) = 21.66 A, or

10.83 A x 480 V/240 V = 21.66 A

d. Compute the minimum size for the set of secondary conductors without secondary protection.

Not less than 15 A x 1/10 x 480 V/240 V = 3 A

But, the ampacity of the secondary conductor cannot be less than the rating of the overcurrent protective device where the secondary conductor terminates.

Not less than 20 A

NEC Table 310.16 shows an ampacity of 20 A for conductor size N° 12 AWG TW Cu under the 60°C column.

20 A > 3 A

20 A = 20 A

Select a conductor size N° 12 AWG TW Cu.

e. Compute the maximum power this set of secondary conductors can supply.

The power cannot exceed the conductor’s ampacity.

Apparent power = √3 x 240 V x 20 A = 8.31 kVA

The load cannot go above the circuit breaker rating – 20 A – as it will trip.

Example 3. Figure 2 shows the example 2 hookup, but one set of secondary conductors terminates at a group of fuses rated 3 A.

Note. The secondary terminals of a transformer may supply one or more sets of secondary conductors, following different rules and treated individually. This example ponders only one set of secondary conductors supplying a fraction of the transformer’s rated power. Additional groups of secondary conductors may handle the remaining available capacity.

Figure 2. Layout for example 3. Image used courtesy of Lorenzo Mari

a. Compute the minimum size for the set of secondary conductors without secondary protection.

Not less than 15 A x 1/10 x 480 V/240 V = 3 A

NEC Table 310.16 shows an ampacity of 15 A for conductor size N° 14 AWG TW Cu under the 60°C column.

15 A > 3 A

Select a conductor size N° 14 AWG TW Cu.

It is common practice in industrial applications – electric power – to use conductor size No. 12 AWG as a minimum, even with a slight current. But this is not required by the NEC.

b. Compute the fraction of the transformer’s rated power supplied by this set of secondary conductors.

You cannot exceed the fuse rating of 3 A.

Apparent power = √3 x 240 V x 3 A = 1.25 kVA

1.25 kVA/9kVA x 100 = 13.89%

3. Industrial installation secondary conductors not over 7.5 m long. Only to supply switchgear or switchboards in industrial facilities. The secondary conductors must comply with the following conditions:

No longer than 7.5 m.

System serviced by qualified persons only.

Ampacity is not less than the secondary current rating of the transformer.

The sum of the ratings of the overcurrent devices does not exceed the ampacity.

Grouped overcurrent devices.

Installed in a raceway or other approved means.

Example 4. Figure 3 shows a typical transformer arrangement supplying a switchboard with an 800 A main circuit breaker and a noncontinuous load of 660 A.

Figure 3. Transformer supplying a switchboard. Image used courtesy of Lorenzo Mari

a. To handle the load, compute the nearest rating of a standard 3-phase, 480/240 V, delta-delta transformer.

Apparent power = √3 x 240 V x 660 A = 274.35 kVA

Use a 300 kVA standard transformer.

b. Compute the secondary current rating of the transformer.

Is = 300 kVA/(√3 x 240 V) = 721.71 A

c. Compute the minimum ampacity of the secondary conductors.

The conductor ampacity depends on the secondary current rating of the transformer and the rating of the overcurrent devices in the switchboard.

800 A > 721 A

800 A = 800 A

The minimum ampacity is 800 A.

d. What’s the effect of the primary overcurrent protection on the ampacity of secondary conductors?

None. There is no rule connecting these two parts.

e. Size the secondary circuit conductors.

NEC Table 310.16 shows an ampacity of 420 A for conductor size N° 600 kcmil XHHW Cu under the 75°C column.

Use 2 x 600 kcmil XHHW Cu per phase leg.

4. Outside secondary conductors. For secondary conductors located outside of a building or structure and complying with the following:

Protected from physical damage in an approved manner.
Terminate in a single circuit breaker or set of fuses that limit the load to the ampacity of the conductors. The single overcurrent protective device may supply any number of additional overcurrent devices on its load side.
The overcurrent protective device is an integral part of a disconnecting means or is located immediately adjacent.
Disconnecting means installed at a readily accessible location either inside or outside a building or structure.

5. Secondary conductors from a feeder-tapped transformer.

Follow Section 240.21(B)(3) (Taps supplying a transformer within 7.5 m or less.)

6. Secondary conductors not over 7.5 m long. The secondary conductors must comply with the following conditions:

No longer than 7.5 m.
An ampacity not less than Vprimary/Vsecondary x 1/3 x rating of the overcurrent device protecting the transformer primary.
Terminate in a single circuit breaker or set of fuses that limit the load to the ampacity of the conductors.
Installed in a raceway or other approved means.

Use this set of rules when the length of the secondary conductors exceeds 3 m but not 7.5 m.

Example 5. A 3-phase, 480/240 V, delta-delta transformer supplies power to a panelboard with a 225 A main circuit breaker at a distance of 7 m. The overcurrent protection on the primary side is a 125 A circuit breaker. Compute the minimum size for the secondary side conductor.

Minimum ampacity = 480 V/240 V x 1/3 x 125 A = 83.33 A

NEC Table 310.16 shows an ampacity of 230 A for conductor size N° 4/0 AWG XHHW Cu under the 75°C column.

230 A > 83.33 A

230 A > 225 A

Use conductor size N° 4/0 AWG XHHW Cu.

Example 6. Assume that the transformer in example 5 supplies three fused disconnects with 70 A, 80 A, and 90 A fuses. Compute the minimum size for each set of secondary conductors.

From example 6, the minimum required ampacity is 83.33 A

NEC Table 310.16 shows an ampacity of 85 A for conductor size N° 4 AWG XHHW Cu under the 75°C column.

85 A > 83.33 A

85 A > 70 A

85 A > 80 A

Use conductor size N° 4 AWG XHHW Cu to supply the 70 A and 80 A fused disconnects.

NEC Table 310.16 shows an ampacity of 100 A for conductor size N° 3 AWG XHHW Cu under the 75°C column.

100 A > 83.33 A

100 A > 90 A

Use conductor size N° 3 AWG XHHW Cu to supply the 90 A fused disconnect.

Rules Permitting Absence of Overcurrent Protection

Figure 4 shows a summary of the rules permitting the absence of overcurrent protection on the conductors connected to the transformer secondary.

Figure 4. Summary of rules for secondary conductors. Image used courtesy of Lorenzo Mari

Featured image used courtesy of Adobe Stock