National Electrical Code Top Ten Tips: Article 450 -- Transformers and Transformer Vaults

by Mark Lamendola

Based on the 2202 NEC

Please note, we do quote from copyrighted material. While the NFPA does allow such quotes, it does so only for the purposes of education regarding the National Electrical Code. This article is not a substitute for the NEC.

These are the 10 Article 450 items we deem most important, based on the pervasiveness of confusion and the potential costs of same.

  1. Article 450 provides the requirements for transformers, but there are eight exceptions! Those are listed in the Exception notes of 450.1. Remember, the x.1 is (as in this case) normally about the scope. So this means that, for example, transformers for signs and lighting are exempted from Article 450 if they comply with Article 600 [Exception No. 5].
  2. OCPD sizing for transformers is confusing. To avoid confusion, use Table 450.3(A) if over 100V or Table 450.3(B) if 1000V or less.
  3. Autotransformers frequently are installed with improper overcurrent protection and improper grounding. If installing an autotransformer, read 450.4 and 450.5 carefully. Do not install an OCPD in series with the shunt windings.
  4. 450.5 discusses the grounding of autotransformers. The NEC in this instance means bonding, not grounding. Review the definitions of each in Article 100. If you drive a separate ground rod for an autotransformer instead of bonding it, you create a difference of potential and have an unsafe system. For a transformer that serves as a separately derived source, you do ground the transformer. The rule is you ground sources but bond loads.
  5. Tie circuits are another source of confusion and error. Draw these out on paper (or in CAD), so you have a clear idea of what is going on. Decide where to locate the OCPDs and then review the design by walking through 450.6, one provision at a time.
  6. Pay attention to transformer ventilation [450.9]. Many transformers are installed with their vents facing walls, columns, corners, or the vents of other transformers (think about how stupid that is). A transformer in a confined space is also problematic, ventilation-wise. Consider adding ductwork, flues, or vents to the confined space so heat doesn't build up; these can be passive (no fan) or active (fan) and also thermostatically controlled (vent vane/damper position, fan speed). Don't put vents on doors (doing so exposes personnel to blast hazards).
  7. If you see a transformer that's "grounded" by a ground rod driven next to it, you might think it complies with 450.10 but it maybe it doesn't. That's because 450.10 tells you to ground it in accordance with Article 250. Read through Part V of Article 250. Read the definitions of grounding and bonding in Article 100. If your transformer is a step-down transformer in a building distribution system, it's not a source in the sense a service transformer is. The "ground" lug on that transformer needs to be bonded in such a way that you have a path back to the source (e.g., the service)--not to the earth. A rule of thumb is if the transformer supplies a branch circuit (as in a lighting panel transformer) or is merely used for isolation, it's bonded (to the EGC) not grounded (to the earth). Another rules of thumb is if it supplies a feeder panel, you probably need to ground it (connect to earth). To arrive at the correct answer, draw a one-line diagram. And in all cases, the enclosures are bonded to the EGC. In most cases, you don't make an electrical connection between the primary and secondary side (that's another whole discussion).
  8. Not all liquid-filled transformers have the same requirements. Determine what dielectric fluid you have, then apply 450.23, 450.24, 450.25, 450.26, or 450.27as appropriate.
  9. When configuring the containment aspects of a transformer vault, doorways and their sills [450.43] become a focal point. The NEC provides minimal guidance. You must ensure the vault can contain all transformer fluid in the event of a leak. There are people who specialize in this kind of thing, so ask if you are not sure. Even if a transformer isn't in a vault, be thinking in terms of spill containment while designing the installation.
  10. A floor drain isn't required unless a vault contains more than 100kVA of transformer capacity [450.46]. But who wants to have people standing around mopping up transformer fluid and filling drums with materials that require EPA reports? With a floor drain, the oil is collected at a central location and disposal requires far less absorbent material. Discuss the pros and cons thoroughly, if you are the designer working on a client's project. If you are at the client end (or doing the work inhouse), consider the impact on operations if equipment is idled during the cleanup. Also, account for the buildup of oil fumes (a fire hazard) if a spill simply sits there.