Let's break down whether reverse-feeding an autotransformer is a common and code-compliant practice, considering your specific scenario.

Understanding Autotransformers



Single Winding

Unlike a standard transformer with separate primary and secondary windings, an autotransformer uses a single winding for both primary and secondary circuits. A tap or taps along this winding create the different voltage levels.

Isolation

A key difference is that autotransformers do not provide isolation between the input and output circuits. The primary and secondary are electrically connected.

Efficiency and Size

Autotransformers are typically smaller, lighter, and more efficient than traditional isolation transformers for similar VA ratings, especially when the voltage ratio is close to 1:1.

Analyzing Your Scenario



Autotransformer Specs

240x480 input, 120/240 output. This implies it's intended to step down either 240V to 120/240V or 480V to 120/240V. The 240x480 input suggests the higher voltage tap has a higher ampacity than the 240 volt tap to allow for the full rating of the autotransformer.

Reverse Feed

You're feeding 240V into what's labeled as the secondary (120/240V side) and taking 480V out of the labeled primary (240x480V side).

Is This Common and Code-Compliant? Common? Sometimes, but with caveats. It is possible to reverse-feed an autotransformer, effectively using it as a step-up transformer. However, it's not generally recommended or considered best practice, and it must be done with careful consideration. NEC Requirements

NEC 210.9 addresses the voltage between ungrounded conductors. As you feed voltage backwards, you are doubling the voltage.

Code Compliance - The Critical Issues

The following points are essential for code compliance and safe operation when reverse-feeding an autotransformer:1.

Nameplate Markings and VA Rating

The autotransformer must be suitable for the applied voltage and the output power required. The autotransformer is designed to output a maximum VA at a specific input voltage/current based on where the taps are placed in the winding.Reverse feeding may be acceptable, but the autotransformer still has the same VA rating at a higher voltage. You must be sure the output current and voltage will be sufficient for the 480V circuit.If you are feeding the autotransformer 240V at a specific ampacity and outputting 480V from the higher voltage side, the current output from the 480V side must be adjusted to stay within the VA rating of the autotransformer.2.

Overcurrent Protection

Overcurrent protection is critical! You must protect the input and output circuits appropriately, considering the VA rating of the autotransformer and the conductor ampacity.You must make sure the input side conductors can handle the full voltage of the 480V circuit while maintaining the original VA rating of the autotransformer.Reverse feeding typically increases the current on the input side to achieve the desired 480 volt output, therefore the correct wire ampacity is essential to maintain NEC code compliance.3.

Taps and Voltage Ratios

Double-check the voltage ratios when reverse-feeding. Make sure the output voltage you're getting is what you expect based on the input voltage and the autotransformer's taps. Minor variations might exist, but significant deviations indicate a problem.4.

Suitability

The autotransformer needs to be UL-listed (or equivalent) for the intended application, or have the testing to show the suitability for this unconventional application.Even if the autotransformer is marked as "Suitable for reverse-feeding", there are still NEC requirements to adhere to.5.

Grounding and Bonding

Ensure proper grounding and bonding connections are in place. Because autotransformers do not provide isolation, grounding and bonding need extra attention.6.

Labeling

Clearly label the autotransformer to indicate the reverse-feeding configuration and the new input/output voltage and current ratings. This is crucial for safety and future maintenance.7.

Local Codes and Inspections

Always consult with local electrical codes and inspectors. They may have specific requirements or restrictions regarding reverse-feeding autotransformers.

Why It's Not Recommended (General Concerns)



Increased Fault Current

Without isolation, a fault on the 480V side can directly affect the 240V side, potentially leading to higher fault currents.

Overvoltage Issues

Voltage regulation can be more sensitive when reverse-feeding.

Safety

Lack of isolation means voltage is being stepped-up (or stepped down) along a common winding. Personnel need to be aware of the increased potential hazards.

In Summary

While technically possible, reverse-feeding an autotransformer is not a simple plug-and-play solution. It requires careful analysis, proper overcurrent protection, grounding, and labeling, and it must comply with all applicable codes. It's generally better to use a correctly sized step-up transformer that is designed for the specific voltage transformation, has the correct isolation, and has the correct VA rating.

Recommendation:

Before proceeding with reverse-feeding the autotransformer, consult with a qualified electrical engineer or a licensed electrician familiar with autotransformers and local codes. They can assess the situation, perform the necessary calculations, and ensure that the installation is safe and code-compliant. If the existing autotransformer is not explicitly designed and rated for reverse-feeding, it's best to avoid this practice and use a proper step-up transformer instead. Flag for review