You're right to be a bit puzzled, and your QA guy is right to raise a red flag. It sounds like there might be a misunderstanding of how auto-transformers work, or a miscalculation somewhere. Let's break it down

Key Concepts: Auto-Transformers vs. Standard Transformers
Standard Transformer (Isolation Transformer)

Separate primary and secondary windings are electrically isolated. Power is transferred through electromagnetic induction. Amperage in the primary voltage is related to amperage in the secondary voltage based on the turns ratio.

Auto-Transformer

A single winding is used for both the primary and secondary circuits. A portion of the winding is shared. This significantly reduces the amount of copper needed compared to a standard transformer, especially for small voltage changes. The current does not pass through the entire winding.

Understanding the Auto-Transformer's Current Flow

The key to understanding the situation is realizing that the entire 100-amp load current isn't necessarily flowing through all of the #14 wire in the auto-transformer winding. Instead, the autotransformer effectively "injects" or "removes" current to achieve the desired voltage change. Let's look at why:1. Voltage and Current Relationship

You're stepping down the voltage from 115V to 100V. This means the current on the 100V side (output) will be higher than the current on the 115V side (input), assuming the same power is being transferred.2.

Power Conservation (Ideally)

Ignoring losses (which are present in any real transformer), the input power (115V I_in) is approximately equal to the output power (100V I_out). If I_out = 100A, then 115V I_in = 100V 100A I_in = (100V 100A) / 115V ≈ 87A3.

Current in the Common Winding

This is where it gets interesting. The current in the common portion of the winding (the part shared by both the input and output) is the difference between the input and output currents. I_common = I_out - I_in = 100A - 87A = 13A (approximately)

The Crucial Point

Only this 13A of current needs to flow through the common portion of the winding. The rest of the current comes directly from the source. Number 14 wire can easily handle 13 amps, which is what you would expect based on the specifications.

Why Your QA Guy is Right to Question

Even though the common current may be fine with #14 wire, there's still room for mistakes in the design if the total current is 100 amps.Here are some reasons why your QA guy is right to question this setup: He's not seeing the full picture

If he only sees the #14 wire and the 100A requirement, he's missing the critical detail that it's an auto-transformer and the current is divided.

He's right to be concerned about safety factors

Electrical code often requires significant safety margins. Even if #14 wire can handle 13A, depending on the application and the enclosure/environment, a larger gauge might be required for safety, heat dissipation, and longevity.

He's concerned with in-rush current

If the 100A is a continuous current, the #14 might be fine. However, it is not fine at all if there is a startup (in-rush) current when power is applied.

He's concerned with wiring method

Is the #14 wire the transformer wire or a general supply wire? In certain situations, the specifications of the transformer are overridden by wiring requirements.

What You Need to Do1. Double-Check Your Calculations

Make absolutely sure your calculations for input current and common winding current are correct. Confirm the intended load is truly 100A at 100V.2.

Provide a Detailed Schematic

Give your QA guy a complete schematic of the auto-transformer circuit, clearly showing the input voltage, output voltage, load current, tap points, and calculated currents in each section of the winding. Label the wire gauge used in each section.3.

Specify the Transformer's Design

If this is a custom-wound auto-transformer, get the winding design specifications from the manufacturer. They should be able to justify the wire gauge selection based on their calculations and testing.4.

Consider Code and Safety

Review relevant electrical codes (e.g., NEC in the US) and safety standards. These may have requirements for wire gauge based on the application, environment, and load type. Don't skimp on safety!5.

Temperature Rating

Be sure that the temperature rating of the #14 wire is sufficient for the operating temperature of the transformer. Auto-transformers can get hot.6.

Consider a Standard Transformer If it is not too expensive to have an isolation transformer, that will usually allow you to have a far more relaxed design.By providing clear information and addressing his concerns about safety factors, you can demonstrate that the auto-transformer design is safe and appropriate for the application. It's far better to have these questions raised and answered before something goes wrong in the field. Flag for review