Okay, this is a common and important question when dealing with electrical equipment layouts. Let's break this down and apply the relevant sections of the NEC (National Electrical Code) based on the information you've provided. I need to emphasize that I am an AI and CANNOT give code-compliant advice. You need to consult with a qualified electrical inspector and/or a licensed electrical engineer to make final decisions based on your specific situation.

Key Sections of the NEC to Consider



NEC 110.26 - Spaces About Electrical Equipment

This is the primary section governing working clearances around electrical equipment operating at 600 volts, nominal, or less.

NEC 450.21 - Dry-Type Transformer Installation

This section covers the installation requirements specific to dry-type transformers, including ventilation and clearances.

Understanding the Code and Applying it to Your Situation


NEC 110.26 - Working Clearances



Voltage to Ground

You have a 480V to 240V single-phase transformer. The highest voltage to ground is 240V (from either leg of the 240V side to neutral or ground) or 277V (from either leg of the 480V side). This falls into the 151-600V category for working clearance.

Condition 1, 2, or 3

This is critical. Condition depends on what is located opposite the equipment.

Condition 1

Exposed live parts on one side and grounded parts or concrete/brick/tile wall on the other side. 3 feet (36 inches) clearance.

Condition 2

Exposed live parts on both sides of the working space. 4 feet (48 inches) clearance.

Condition 3

Exposed live parts on one side and bare or insulated parts operating at different potentials on the other side OR exposed live parts on both sides of the working space and the equipment requires access to the back to maintain. 4 feet (48 inches) clearance.

Your Scenario (Based on your Description)

You state there is open space on the opposite side of the transformer. This sounds like it could be Condition 1 if that open space will remain open and not contain anything that falls into Condition 2 or 3 (like another piece of electrical equipment with exposed live parts, or grounded surfaces like walls). If you plan on having the new MCC opposite the transformer, it is Condition 2, or 3 if access is needed for maintenance.2.

Where to Measure From

This is the core of your question.

The 36" (or 48") working space is measured from the exposed live parts or enclosure front of the electrical equipment requiring examination, adjustment, servicing, or maintenance while energized. Transformer and Large Box

The large box on the back of the transformer could contain connections (wiring, fuses, breakers, etc.).

If that box is the access point where a qualified person would need to perform work while the transformer is energized, then the 36" (or 48") clearance is measured from the face of that box. However, if that box is simply a junction box with no energized maintenance, then 110.26 does not apply. New MCC

The working space for the MCC is governed by the same principles of NEC 110.26, but related to its exposed live parts.3.

NEC 450.21 (Ventilation and Clearance - Dry-Type Transformers)



Temperature Rise

Your transformer has an 80°C temperature rise.

Type S

This refers to the sound level rating of the transformer. This does not impact clearance requirements.

Ventilation

This is CRITICAL. You must consult the transformer's manufacturer's specifications (installation manual) for specific ventilation requirements. These requirements often dictate minimum clearances around the transformer to ensure proper airflow for cooling. The nameplate of some transformers will have ventilation requirements; others require you to consult the manufacturer's documentation. This could exceed the working space requirements of 110.26!

Combustible Material

NEC 450.21(A) generally requires dry-type transformers installed indoors to be separated from combustible material by fire-resistant barriers or a minimum clearance of 12 inches. This is a separate requirement from the working clearances.

Steps to Determine the Correct Clearances


Identify Exposed Live Parts

Specifically determine where access is required for examination, adjustment, servicing, or maintenance of the transformer while energized. Is it the large box on the back? Are there other access points?2.

Determine Condition

Based on the position of the new MCC and the presence of the transformer, or other grounded surfaces, what 110.26 condition applies?3.

Consult Transformer Documentation

Absolutely essential. Get the manufacturer's installation instructions for your specific transformer model. This will provide the ventilation requirements. This might dictate a larger clearance around the transformer than 110.26 alone.
4.

Draw a Diagram

Draw a layout of the transformer, the large box, the proposed MCC location, and any walls or other equipment nearby. Show the required clearances from both 110.26 and the manufacturer's ventilation requirements.
5.

Consult with an Electrical Inspector/Engineer

Before making any decisions, have a qualified professional review your layout to ensure code compliance.

Important Considerations



Accessibility

Ensure there is adequate space to open doors/panels on the MCC and the transformer for maintenance.

Future Expansion

Consider future expansion or equipment additions when determining clearances.

Local Codes

Check with your local jurisdiction for any amendments or stricter requirements to the NEC.

In Summary:

You likely need at least 36" or 48" of working space from the face of the large connection box on the back of the transformer to the proposed MCC if that box is an access point for energized work. More importantly, you MUST consult the manufacturer's documentation for the transformer to determine ventilation requirements, which could necessitate even larger clearances. Have a qualified professional review your layout before proceeding with the installation. Good luck! Flag for review