You're right to be skeptical. The answer of 3' doesn't automatically come directly from NEC 450.3 or 110.32 in a universally applicable way. Here's a breakdown of why 3' might be the answer and what else you need to consider

Why 3' Might Be the Answer (and likely the intended rationale):
NEC 110.26(A)(1) - Working Space

This is the MOST LIKELY place the 3' comes from. While it's not specific to transformers, it is for "electrical equipment" and it addresses safety concerns for personnel working on or near the equipment. Here's how it applies:

Voltage to Ground

The clearance is dependent on the voltage to ground of the equipment.

Condition 1, 2, and 3

110.26(A)(1) lays out different clearance requirements based on "Conditions" (described below), and the voltage to ground. A transformer typically will have voltages that can easily require 3' clear.

Conditions of 110.26(A)(1)



Condition 1

Exposed live parts on one side and grounded parts on the other side. 3' might be appropriate here.

Condition 2

Exposed live parts on one side and grounded parts on the other side and requiring additional insulation.

Condition 3

Exposed live parts on both sides of the workspace (personnel could be between two live parts). This will require the most clearance.

Assumptions Made in the Question

To justify 3' as the minimum, the question is implicitly assuming several things: The transformer's voltage to ground is low enough that 3' is sufficient under Condition 1. This is generally up to 150V to ground. However, many transformers would exceed this threshold. The 'concrete wall' is considered a "grounded" part for Condition 1. (This is a reasonable assumption). There are no exposed live parts on the wall side of the transformer that would push the installation into a more stringent condition.

Why the 3' Answer is Potentially Wrong (or at least incomplete)



Voltage is Key

The 3' minimum in 110.26(A)(1) absolutely depends on the voltage to ground. For higher voltages (above 150V to ground), the required working space increases. The question doesn't provide the voltage, so assuming 3' is dangerous.

Specific Transformer Type

The type and size of the transformer can influence working space. Large, high-voltage transformers might have manufacturer-specified clearances that exceed the general NEC minimums.

Accessibility

Is the 3' required for the front of the transformer for access, or the side? Working space is generally required for the front or the equipment to allow personnel to work on the equipment safely.

Local Amendments

Local jurisdictions can amend the NEC. Some might have stricter clearance requirements for transformers near walls.

AHJ (Authority Having Jurisdiction)

The AHJ always has the final say and can interpret the code differently or require additional measures.

Manufactures Specs

The transformer manufacture could have recommendations or requirements.

How to Answer the Question Correctly (in a real-world scenario)


Determine the Voltage to Ground

This is absolutely the first step. You cannot determine the minimum clearance without knowing this.2.

Determine the NEC Condition

Given the wall, is it Condition 1, 2, or 3?3.

Consult NEC 110.26(A)(1) Table

Look up the required working space based on the voltage and condition.4.

Check Transformer Manufacturer's Specifications

See if they have any specific clearance requirements.5.

Check Local Codes

Are there any local amendments to the NEC that might affect clearance?6.

Consider Accessibility

Ensure adequate space for maintenance and operation.

In conclusion:

The 3' answer is likely based on a simplified application of NEC 110.26(A)(1) for a low-voltage transformer in Condition 1. However, it's incomplete without knowing the voltage and other site-specific factors. The fact that you recognized this ambiguity shows you're thinking critically, which is exactly what's needed in your master's studies and as an electrical professional. If this is just a test or study question, it is poorly worded.Good luck with your studies! Flag for review