During a recent electrical training program, questions arose regarding how to apply the National Electrical Code (NEC) to three-phase transformer installations. More specifically, how dry-type transformers (rated at less than 1,000V) are supposed to be protected to meet minimum NEC requirements.

Related questions during the same class were about the overcurrent protection for conductors supplying such transformers. These great questions created an opportunity for this article.

Start at the beginning

A good first step is to be sure the appropriate Code rules are referenced and applied. Section 90.3 clearly indicates that NEC chapters 1–4 have general application. A review of the table of contents reveals that transformer requirements reside in Chapter 4, Equipment for General Use, and more specifically, Article 450 covers transformers.

As an example application, consider a 15-kVA transformer that has a three-phase, 480V delta primary, and a 208Y/120, three-phase, 4-wire secondary. It is important to stay focused on the protection for the transformer windings. The conductor protection is addressed in Article 240.

First, Section 450.3(B) indicates that overcurrent protection for this transformer shall be provided in accordance with Table 450.3(B). Code rules in tabular form are often more user friendly, in my opinion. Remember that notes to tables are applicable rules. They are not like informational notes, which are unenforceable. Based on the table, for this 15-kVA transformer, there is a 125% limit on the rating of the overcurrent protective device (OCPD). Per Note 1 for this table, if the value 125% of the primary current of the transformer does not correspond directly to a standard rated overcurrent device, a higher rating that does not exceed the next-higher standard size shall be permitted.

Remember, the focus is protecting the transformer windings, and in this case, primary protection only is sought.

Selecting properly sized conductors

Properly sized conductor(s) for the primary should be selected in accordance with the minimum sizes provided in Table 310.16, and the overcurrent protection must meet the provisions in sections 240.4(B) and 240.6, which address standard ratings of OCPDs. Knowing the transformer’s ratings, a primary current value can be used for an equation, or if the primary current is not given on a nameplate, it can be calculated.

To determine the primary current, multiply the transformer’s kVA rating (15 kVA) by 1,000 and then divide that resulting value (15,000 kVA) by the primary voltage (480V) x 1.73. The value 15,000 divided by 830 results in a primary current of 18.07A. 

Table 450.3(B) says 125% of the primary current is the allowance; therefore, 18.07A x 125% results in 22.58A. With that value known, the next standard size rating for an OCPD (in this case, an inverse time circuit breaker) is 25A based on Table 240.6(A). That said, a three-pole, 25A circuit breaker would be required for protection of the primary windings of this transformer. 

Before sizing the conductors for the primary of this transformer, a visit to Informational Note No. 1 for Section 450.3 is in order. This informational note provides relative clarity regarding conductor protection, which must be in accordance with sections 240.4 and 240.21. (Sections 245.26 and 245.27 apply to installations over 1,000V, so they don’t apply to this example.)

For this specific example, Type TW copper conductors will be installed. A reference to Table 310.16 reveals that a 10 AWG copper TW conductor is good for 30A. Notice that this example only reviewed the minimum requirements for transformer protection, using primary-only protection. No mention of protection for the secondary conductors is provided here.

Transformer secondary conductors

Know that transformer secondary conductors in this example would require Section 240.21(C), not the rules in Article 450. Looking at Informational Note No. 1 to Section 240.21(C), one can see the correlation between the two sets of rules. One focuses on protecting the transformer and the other addresses installation and protection of transformer secondary conductors. 

A primary overcurrent device for a transformer can provide overcurrent protection for secondary conductors, but this is limited to installations of single-phase, two-wire (single voltage) secondary, or three-phase, delta-delta connected having a three-wire (single voltage) secondary. This method of providing overcurrent protection using the primary OCPD meets the minimum Code requirements, provided the values in 450.3 do not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

Code-compliant transformer overcurrent protection must meet Section 450.3, and for transformers rated at less than 1,000V, Section and Table 450.3(B). Overcurrent protection for the conductors is provided in sections 240.4 and 240.6.

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