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

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

1. Fuses on the load side don't protect your voltage transformer. Install overcurrent circuit protection devices (OCPD) on the primary side of any voltage transformer that is indoors or enclosed [450.3(C)].
    
2. OCPD sizing for transformers is confusing. Use Table 450.3(A) to avoid confusion.
    
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. 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.
    
5. Tie circuits are another source of confusion and place of 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. 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 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. The "ground" lug on the transformer needs to be bonded in such a way that you have a path back to the source--not to the earth.
    
8. Not all liquid-filled transformers have the same requirements. Determine what dielectric fluid you have, then apply 450.23, 450.24, 450.25, or 450.26 as 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.