Technical Article

What is Transformer Overcurrent Protection?

July 15, 2021 by Alex Roderick

Most electrical devices require overcurrent protection and transformers are no exception. Transformer overcurrent protection is required to protect the primary windings from short circuits and overloads and the secondary windings from overloads. NEC® Section 450.3 contains the requirements for overcurrent protection of transformers.

Overcurrent protection requirements depend upon several factors that will be outlined in this article.

The first factor is the voltage at which the transformer operates. Section 450.3(A) contains the rules for transformers rated over 600 V, nominal. Section 450.3(B) contains the rules for transformers operating at 600 V, nominal or less. The second factor is the location of the overcurrent device. Requirements vary depending on whether only the primary winding is protected or both the primary and secondary windings are protected.

The last factor applies to transformers rated over 600 V only. If conditions of maintenance ensure that only qualified personnel will work on the transformers, NEC® Table 450.3(A) permits primary protection only for these supervised installations.

The rules of these sections are intended to provide protection for the transformers only. Conductors on both the primary and secondary sides will still need to be protected in accordance with the provisions of NEC® Sections 240.4 (F) and 240.21 (C).

Transformer Overcurrent Protection Over 600 V, Nominal

The requirements for transformer overcurrent protection for transformers rated over 600 V depend upon whether the installation is non-supervised or supervised. A supervised installation is an electrical installation in which the conditions of maintenance are such that only qualified persons monitor or service the electrical equipment. See Note 3 to NEC® Table 450.3(A).

Primary and Secondary

For non-supervised transformer installations, each transformer shall have primary and secondary OCPDs in accordance with NEC® Table 450.3(A). The Table is organized around the permissible percentages for fuses and circuit breaker ratings on both the primary and secondary sides, the location, and the impedance of the transformer. For the purposes of this section of the NEC®, electronically actuated fuses are rated in accordance with the circuit breaker percentages. Per NEC® Table 450.3(A), Note 1, the next higher standard rating of a fuse or circuit breaker is permitted when the calculated value does not correspond to a standard rating of a fuse or circuit breaker. See the following example and Figure 1.

What size primary OCPD, using a fuse, is needed to protect the non-supervised transformer?

$$Current=\frac{kVA\times 1000}{\sqrt{3}\times V}=\frac{50\times 1000}{1.73\times 4160}=6.94A$$

Table 450.3(A): 6.94 × 300% = 20.82 A

Table 450.3(A), Note 1; 240.6(A): Next higher standard size: 20.82 = 25 A

Primary OCPD: 25 A fuse

Supervised Installations

For supervised transformer installations, there are two options for providing the required overcurrent protection. The first option is based on primary protection only. In these installations, each transformer shall be protected with a primary overcurrent device that does not exceed 250% for fuses or 300% for circuit breakers of the rated primary current of the transformer. If the calculated value does not correspond to a standard rating of a fuse or circuit breaker, the next higher standard size shall be permitted. See the following example and Figure 2. As with transformer installations of 600 V or less, individual overcurrent protection is not required at the transformer if the primary circuit OCPD provides the necessary protection.

What size primary OCPD, using an adjustable trip CB, is needed to protect the supervised transformer?

$$Current=\frac{kVA\times 1000}{\sqrt{3}\times V}=\frac{25\times 1000}{1.73\times 2400}=6.01A$$

Table 450.3(A): 6.01 × 300% = 18.03 A

Table 450.3(A), Note 1; 240.6(A): Next higher standard size: 18.03 = 20 A

Primary OCPD: 20 A ATCB

Figure 2. Supervised transformers over 600 V shall be provided with primary OCPDs not exceeding 250% for fuses and 300% for circuit breakers.

The second option for transformers rated over 600 V is to provide both primary and secondary protection. NEC® Table 450.3(A) provides the permissible percentages for fuses and circuit breaker ratings on both the primary and secondary sides based upon the impedance of the transformer. For transformers with an impedance of not more than 6%, the maximum rating or setting of the primary feeder OCPD can range from 300% to 600%, depending on the voltage of the primary and the secondary and the type of OCPD used, either a fuse or circuit breaker (see the following example and Figure 3).

What size primary and secondary OCPDs, using CBs, are needed to protect the supervised transformer?

Primary:

$$Current=\frac{kVA\times 1000}{\sqrt{3}\times V}=\frac{25\times 1000}{1.73\times 4160}=3.47A$$

Table 450.3(A): 3.47 × 600% = 20.82 A

Table 450.3(A), 240.6(A): Next lower standard size: 20.82 = 20 A

Primary OCPD: 20 A CB

Secondary:

$$Current=\frac{kVA\times 1000}{\sqrt{3}\times V}=\frac{25\times 1000}{1.73\times 2400}=6.01A$$

Table 450.3(A): 6.01 × 300% = 18.03 A

Table 450.3(A), Note 1; 240.6(A): Next lower standard size: 18.03 = 15 A

Secondary OCPD: 15 A CB

Transformer Overcurrent Protection at 600 V, Nominal, or Less

The requirements for overcurrent protection for transformers rated at 600 V, nominal, or less are given in NEC® Table 450.3(B) and accompanying notes. The overcurrent protection is necessary to protect the windings of the transformer and is independent of the overcurrent protection required for conductors. The secondary overcurrent device is permitted, as it is with services, to consist of not more than six fuses or circuit breakers grouped in one location. The total rating of the six OCPDs cannot exceed that of the required value for a single OCPD. In installations where the six OCPDs consist of both fuses and circuit breakers, the rating shall not exceed the value permitted for fuses.

Primary

The general rule for transformers rated 600 V, nominal, or less is to protect the primary windings of the transformer at not more than 125% of the rated primary current of the transformer. Where this calculated value does not correspond to a standard rating for a fuse or a circuit breaker, per NEC® Section 240.6, and the rated primary current of the transformer is 9 A or more, NEC® Table 450.3(B), Note 1 permits the next higher standard rating to be used (see Figure 4).

 What size primary OCPD, using a fuse, is needed to protect the transformer? $$Current=\frac{kVA\times 1000}{\sqrt{3}\times V}=\frac{15\times 1000}{1.73\times 208}=41.67A$$ Table 450.3(B): 41.67 × 125% = 52.09 A Table 450.3(B), Note 1; 240.6(A): Next higher standard size: 52.09 = 60 A Primary OCPD: 60A fuse 9A Or More

 What size primary OCPD, using a fuse, is needed to protect the transformer? $$Current=\frac{kVA\times 1000}{\sqrt{3}\times V}=\frac{6\times 1000}{480\times 1.73}=7.22A$$ Table 450.3(B): 7.22 × 167% = 12.06 A Table 450.3(B); 240.6(A): Next lower standard size: 12.06 = 10 A Primary OCPD: 10A fuse Less Than 9A

 What size primary OCPD, using a fuse, is needed to protect the transformer? $$Current=\frac{kVA\times 1000}{\sqrt{3}\times V}=\frac{0.250\times 1000}{ 240}=1.04A$$ Table 450.3(B): 1.04 × 300% = 3.12A Table 450.3(B); 240.6(A): Next lower standard size: 3.12 = 3A Primary OCPD: 3A fuse Less Than 2A
Figure 4. Transformers at 600 V or less shall be provided with primary OCPDs at not more than 125% of the rated primary current, with exceptions.

If the rated primary current of the transformer is less than 9 A, the rating of the OCPD is permitted to be set at not more than 167% of the rated primary current. For transformer installations where the rated primary current is less than 2 A, the rating of the OCPD shall be permitted to be set at not more than 300% of the primary current. With the lower primary current values, the impedance of the transformer will act to limit potential fault current, and the rating of the primary OCPD can be increased.

NEC® Table 450.3(B) establishes maximum settings for transformer overcurrent protection as a percentage of transformer-rated current. A separate fused disconnecting means could be provided ahead of the transformer that complies with the maximum percentages of NEC® Table 450.3(B). If, however, the circuit overcurrent device is sized so that it also provides the necessary overcurrent protection for the transformer, a separate fused disconnecting means may not be required (see Figure 5).

Figure 5. Transformers less than 600 V are permitted to be installed without individual primary OCPDs when the circuit OCPDs provide this protection.

Motor control circuit transformers installed per NEC® Section 430.72(C) (1–5) are permitted to be installed without primary overcurrent protection. These transformers are generally under 50 VA and have a low or limited primary current rating.

Primary and Secondary

Another option in providing overcurrent protection for transformers rated 600 V, nominal, or less is to protect both the primary and the secondary of the transformer. In these cases, NEC® Table 450.3(B) permits the primary feeder OCPD, set at not more than 250% of the rated primary current of the transformer, to protect the transformer primary, provided the secondary OCPD is set at a value which does not exceed 125% of the rated secondary current of the transformer. As with the primary-only protection requirements, if the rated secondary current of the transformer is 9 A or more and the calculated value does not correspond to a standard rating of a fuse or circuit breaker from NEC® Section 240.6, the next higher standard rated OCPD is permitted.

Unlike the requirements for primary-only protection, there is no permission to increase the rating of the primary OCPD when it does not correspond to a standard rating of a fuse or circuit breaker. If the calculated value at 250% does not correspond to a standard rating, the next lower standard rating shall be used. If the rated secondary is less than 9A, the rating of the secondary OCPD is permitted to be increased to a maximum value of 167% of the rated secondary current of the transformer. This rule is useful for applications in which several transformers are supplied from a single primary OCPD. Sizing at 250% prevents nuisance tripping due to the transformer current inrush characteristics.

Arresters

A transformer, with all its metal hardware, can be damaged by lightning strikes. If lightning strikes a feeder and enters the primary, the transformer can be damaged or destroyed. A large voltage spike can destroy the dielectric quality of the insulation, resulting in a failure. An arrester is a device used to shunt a voltage spike to the ground or from line to line. See Figure 6. Arresters are also used to control transient overvoltage situations that might affect the input to the transformer. A transient overvoltage or overcurrent is the result of sudden changes or surges on the line due to a variety of causes.

Figure 6. Arresters are used to protect transformers from voltage surges. Image courtesy of IMMR.com

The arresters used today are constructed from a stack of metal oxide varistors (MOVs). A metal oxide varistor (MOV) is a nonlinear resistor that can change resistance with changes in voltage. At low voltage, an MOV has very high resistance, allowing very little current to flow. At high voltage, such as during a voltage spike, an MOV has very low resistance, allowing a transient current to flow through the MOV to the ground.