User: msilva94, Location: PA, USA
Asked on: Monday, July 7, 2025 at 10:34 am EDT
Answered on: Monday, July 7, 2025 at 10:34 am EDT
Question: A coworker said it’s illegal to mount a transformer high up on a wall since it’s not directly next to its panel or disconnect and isn’t easily accessible for maintenance. I disagreed, since I’ve seen and designed plenty of setups where a transformer is wall-mounted, with the panel or disconnect nearby and accessible.Example setup:480/277V Panel -> 150kVA Wall-Mounted Transformer (offset from panels, hosted 6'+ AFF to reduce space consumption) -> 208/120V PanelI reviewed NEC 450.13, which restricts transformers over 112.5 kVA from being installed in ceilings, floors, or hollow spaces in combustible buildings unless:-It’s rated 600V or less-It’s in a vault per 450.41-It meets ventilation requirements in 450.9and450.14 Disconnecting Means. Transformers, other than Class2 or Class 3 transformers, shall have a disconnecting means located either in sight of the transformer or in a remote location.Where located in a remote location, the disconnecting means shall be lockable in accordance with 110.25, and its location shall be field marked on the transformer.In this case, the transformer is under 600V and in an open area and as long as the disconnect is accessible and clearly associated, I believe that satisfies code.A side note: I assume the same exceptions would apply if this equipment/setup was mounted on any structure (i.e. - uni-strut)?Thoughts?
You are correct in your understanding of the NEC and your assessment of the scenario. Your coworker is mistaken.Here's a breakdown of why your reasoning is sound: NEC 450.13 (Location): This section primarily addresses the installation of large transformers within concealed spaces. Your 150kVA transformer is not within a ceiling, floor, or hollow space. It's mounted on a wall in an open area. The height above finished floor (AFF) is a space-saving measure, not a concealment issue. Therefore, 450.13 doesn't prohibit the installation. The key point is ensuring proper ventilation and that the building materials meet fire-resistance ratings as required by code. NEC 450.14 (Disconnecting Means): This is the crucial section for your scenario. You are already planning to have a disconnecting means either in sight of the transformer or in a remote location that is lockable and clearly marked. This fulfills the requirements. The distance between the panel and the transformer is irrelevant, provided the disconnect is in sight or the remote disconnect is properly labeled and lockable. Accessibility for Maintenance
While not explicitly defined by the NEC as it relates to transformer placement (other than vault access in 450.43), general principles of electrical safety and good design dictate that the transformer should be accessible for maintenance and inspection. Mounting it 6'+ AFF does not automatically make it inaccessible. Qualified personnel can use appropriate equipment (ladders, lifts) to reach it safely. It's important to consider this when designing the installation.
Mounting on Structures (Uni-strut or Similar)
Your assumption about mounting on a structure is correct. The NEC does not prohibit mounting electrical equipment (including transformers) on appropriately rated and installed structures like uni-strut. As long as the structure provides adequate support and meets any applicable seismic or other structural requirements, it's an acceptable practice. Again, ensure accessibility for maintenance and consider working clearance requirements.
In Summary
Your proposed installation is code-compliant as long as you meet the following conditions:1.
Disconnecting Means
A disconnecting means is located either in sight of the transformer or in a remote location that is lockable per 110.25 and field-marked on the transformer.
2.
Accessibility
The transformer is accessible for qualified personnel to perform maintenance and inspections using appropriate equipment.3.
Ventilation
The transformer is adequately ventilated per NEC 450.9 and the manufacturer's instructions.
4.
Structural Support
The mounting structure (wall or uni-strut) is adequately sized and rated to support the weight of the transformer and any associated equipment.5.
Clearances
Ensure appropriate working clearances are maintained around the transformer according to NEC 110.26.
6.
Local Amendments
Always check for any local amendments to the NEC that might affect transformer installations in your specific jurisdiction.
Recommendation:
To avoid future confusion, it's a good idea to cite the specific NEC sections you are relying on when discussing the installation with your coworker. Showing the code language can often resolve misunderstandings. You might also want to consult with a qualified electrical inspector or engineer to confirm your design meets all applicable requirements. Flag for review