Violations of the National Electrical Code (NEC) regarding transformers are often encountered while performing electrical plan reviews. There are far too many NEC requirements to explain in just one short article, but three common types of violations are: (1) transformer secondary conductors (wiring) having an ampacity less than the OCPD rating at the termination of the secondary conductors; (2) lack of overcurrent protection for transformer secondary conductors; and (3) lack of information regarding system bonding jumpers.
For clarification, the types of transformers discussed in this article do not include electric utility transformers or autotransformers. Rather, the types of transformers that are part of the discussion are those having primary windings that are isolated from the transformer’s secondary windings. These transformers are commonly installed at non-residential buildings where there will be electrical equipment that requires connection to a system having a different voltage than that of the electrical service for the building.
Ampacity of transformer secondary conductors
The first common violation for our discussion is when the transformer’s secondary conductors are not sized properly and have an ampacity less than the current rating of the overcurrent protection device (OCPD) located at the termination (end) of the secondary conductors. Designers sometimes make the mistake of thinking that the provisions of Sec. 240.4(B) can be used for transformer secondary conductors. This section of the Code essentially allows the use of the next standard OCPD rating over the ampacity rating of the conductor (if certain conditions are met). However, it’s very clear under Sec. 240.21(C) that the provisions of Sec. 240.4(B) are not permitted for transformer secondary conductors.
The ampacity of the secondary conductors, as calculated per Art. 310, cannot be less than the current rating of the OCPD at the terminations of the conductors. An example of this situation could be a system with a 112.5kVA rated transformer with a 400A overcurrent protection device at the termination of the transformer’s secondary conductors, and the plans show secondary conductors only having an allowable ampacity of 380A. Again, this would be a violation of Sec. 240.21(C).
Overcurrent protection for transformer secondary conductors
The second common violation is when the electrical plans don’t show any OCPD at the termination of transformer secondary conductors. Section 240.21(C)(1) allows limited situations where the transformer’s primary OCPD can be used to protect the transformer’s secondary conductors, but only single-phase transformers having a 2-wire (single-voltage) secondary or a 3-phase delta-delta transformer having a 3-wire (single-voltage) secondary are allowed in that case. Unfortunately, those types of transformers are not common, and the applicable requirements of Sec. 240.21(C)(2) through (6) typically apply to transformers that are common for electrical systems.
Typical transformers for electrical systems are usually the 3-wire secondary type for single-phase systems and the 4-wire secondary type for 3-phase systems. So, the provisions of Sec. 240.21(C)(1) cannot be used for these types of transformers. Per any of the situations noted in Sec. 240.21(C)(3) through (C)(6), an overcurrent protection device (rated not more than the ampacity of the secondary conductors) must be provided at the termination of the secondary conductors.
Also, it must be pointed out that Sec. 408.36 requires overcurrent protection for panelboards and the OCPD must have a current rating not exceeding the rating of the panelboard. In addition, when a transformer is supplying a panelboard, the OCPD must be located on the secondary side of the transformer. The exception is if the transformer and its primary OCPD meet the provisions of Sec. 240.21(C)(1), but it was discussed earlier that being able to use those provisions is rare. Essentially, situations when a transformer supplies a panelboard will always require an OCPD on the secondary side of the transformer and the OCPD must be rated not more than the panelboard.
But what about Sec. 240.21(C)(2) for secondary conductors not more than 10 ft in length? Determining compliance with this section of the Code can sometimes be tricky, depending on one’s interpretation of the requirements and what equipment the secondary conductors will feed.
The second half of Sec. 240.21(C)(2)(1)(b) allows the option of having an OCPD at the termination of the transformer’s secondary conductors, like what’s noted under Sec. 240.21(C)(3) through (6). This is a simple method for providing overcurrent protection for secondary conductors, but this is not the tricky part of Sec. 240.21(C)(2)(1)(b). Rather, the more difficult provisions to determine compliance for is when designers choose to apply the first part of Sec. 240.21(C)(2)(1)(b), which essentially says that the ampacity of the secondary conductors must not be less than the current rating of the equipment the secondary conductors supply, and there must be “an overcurrent device(s)” contained within the equipment. This poses the question, what type of “equipment” is being installed?
What if the equipment is a switchboard or switchgear, for example? Depending on an AHJ’s interpretation of the first part of Sec. 240.21(C)(2)(1)(b), a switchboard or switchgear not having an overall main OCPD (and no OCPD on the secondary side of the transformer) may potentially be used if the switchboard or switchgear has one or more OCPDs in it. However, this introduces the potential for overloading the secondary conductors and the equipment itself.
Some individuals are quick to point out that Sec. 240.21(C)(2)(1)(a) requires the ampacity of the secondary conductors to be not less than the calculated load(s) that the conductors serve. Could ensuring that the calculated loads do not exceed the ampacity of the transformer’s secondary conductors be considered a form of overload protection for the conductors? Some individuals think so. However, a similar requirement regarding calculated loads does not exist in Art. 408 for switchboards and switchgear. In other words, the NEC does not currently specify that switchboards and switchgear must be rated not less than the calculated load(s) that the switchboard or switchgear serves. Several Public Inputs for the 2026 edition of the NEC were submitted with hopes of adding overcurrent protection requirements for switchboards and switchgear, but so far, those efforts have failed.
It’s this author’s opinion that unless the provisions of Sec. 240.21(C)(1) are allowed, it’s recommended that an OCPD be provided at the termination of the transformer’s secondary conductors and such OCPD be rated not more than the ampacity of the secondary conductors. However, the OCPD is always required when the transformer is supplying a panelboard.
Transformer system bonding jumper
The last common NEC violation for transformers is when electrical plans do not show the required size and location of the system bonding jumper (for systems required to be grounded, per Sec. 250.20) at the secondary side of a transformer. The system bonding jumper is necessary to bond the grounded conductor to the equipment grounding conductor(s) and grounding electrode conductor(s) of the system.
Section 250.30(A)(1) requires that a system bonding jumper be provided at either the transformer or at the first disconnecting means or overcurrent protection device (OCPD) enclosure and requires compliance per Sec. 250.28 for the system bonding jumper.
Section 250.28 has provisions for the required type of material, construction, and attachment of the system bonding jumper. Section 250.28(D) also has provisions for the required size of the system bonding jumper and references Table 250.102(C)(1) for determining the required minimum size of the system bonding jumper based on the size of the transformer’s secondary conductors. The plans should specify the required size of the system bonding jumper to be installed and clarify the location where it will be installed.
Summary
Lots of requirements exist in the NEC regarding transformers — again, too many to cover in this article. However, the three situations presented here are indeed common violations encountered while performing electrical plan reviews. Having a good understanding of requirements concerning transformers is an essential part of performing detailed and thorough reviews.
The information provided in this article is based on the author’s understanding of the requirements explained, and any opinions shared are his own. The author understands that others may have different interpretations and opinions regarding the NEC requirements explained herein.
Inspector Intel articles are provided by the Independent Alliance of the Electrical Industry (IAEI), www.iaei.org, a membership-driven, non-profit association headquartered in Richardson, Texas, that promotes electrical safety throughout the industry by providing education, certification of inspectors, advocacy, partnerships, and expert leadership in electrical codes and standards.
