You've hit on a common point of confusion when dealing with transformer primary protection. Here's the breakdown of why your calculation seems off and how the code works

Understanding the Code Logic

The NEC allows for higher overcurrent protection (OCPD) settings for transformer primaries due to the inrush current transformers experience when energized. This inrush can be many times the normal full-load current (FLA) for a very short duration. The code recognizes this and provides leeway to avoid nuisance tripping.However, the conductor sizing rules are different. The goal is to protect the conductor from sustained overcurrents, not just inrush.

Here's how the code works, step by step


Transformer Primary FLA

You've established this as 120A.2.

Maximum Primary OCPD (Fuse) Size (NEC 450.3(A))

250% of 120A = 300A (This is the maximum allowed, not necessarily the required size).3.

Conductor Sizing (NEC 240.4(B) - Typically, but depends on application)



General Rule

The conductor must have an ampacity of at least 125% of the transformer's primary FLA. 125% of 120A = 150A.

Standard Ampere Ratings (NEC 240.4(B))

If 150A doesn't exactly match a standard conductor ampacity, you're allowed to use the next higher standard size, provided certain conditions are met (the next higher standard size does not exceed 800 amps, the conductors being protected are part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads, or the ampacity of the conductor has been derated, if applicable).

Result

You would need to select a conductor with an ampacity of at least 150A. For example, using the 75°C column of Table 310.16, you might choose a #1/0 AWG copper conductor which is rated at 150 Amps.
4.

OCPD Coordination

This is where the key lies! While you can use a 300A fuse, you don't have to. You need to consider the conductor's ampacity. You should strive to select an OCPD that is closer to the conductor's ampacity, while still allowing for the transformer's inrush current. You could also utilize Inverse Time Circuit Breakers which allow for short duration inrush current and still protect the conductors against damage in the event of a fault.

Why the Conductor Isn't Sized at 300%

Sizing the conductor at 300% of the transformer's FLA would be very expensive and generally unnecessary. The NEC's goal isn't to protect the transformer with the primary OCPD. The transformer has its own internal protection (often thermal overloads). The primary OCPD's main job is to protect the primary conductors from sustained overcurrents and short circuits between the source and the transformer.

What You're Missing Coordination

The key is to coordinate the conductor size with the OCPD. The 300A fuse is a maximum, not a requirement. You can (and often should) use a smaller fuse. The OCPD protects the conductors.

Conductor Protection

The 125% rule is intended to protect the conductor from overload conditions. A sustained overload on the transformer's primary will eventually cause the conductor to overheat if it's not properly sized and protected.

Example Scenario

Let's say you choose #1/0 AWG copper conductors (150A ampacity) for your 120A primary. The maximum fuse you could use is 300A. However, a more appropriate fuse might be in the 175A to 200A range. This would provide better protection for the conductors while still accommodating the transformer inrush. You might need to consult the transformer manufacturer's inrush current data to make a more precise selection.

In Summary The transformer primary protection rules allow for oversized OCPDs to accommodate inrush current. Conductor sizing is based on 125% of the transformer's FLA (or the next standard size), to protect against sustained overcurrents. You must coordinate the conductor size and OCPD selection to ensure adequate protection. Don't automatically use the maximum allowed OCPD size. Strive to use an OCPD closer to the conductor's ampacity while still preventing nuisance tripping.

Important Considerations



Specific Installation

The correct conductor size and OCPD will depend on the specific installation conditions (e.g., ambient temperature, number of conductors in a raceway, type of insulation). Derating factors (NEC 310.15) must be considered.

Transformer Manufacturer's Data

Consult the transformer manufacturer's specifications for recommended fuse sizes and inrush current characteristics. This is crucial for optimal protection and performance.

Local Codes and Regulations

Always consult your local electrical codes and regulations, as they may have additional requirements or restrictions.*

Engineering Judgment: In complex situations, it's always best to consult with a qualified electrical engineer to ensure proper design and compliance.I hope this clarifies the issue! Let me know if you have any more questions. Flag for review