NEC Transformer Overcurrent Protection Calculator

This professional tool helps electrical engineers, electricians, and plan reviewers compute transformer full-load currents and size primary overcurrent protective devices (OCPDs) per NEC best practices. It supports single- and three-phase systems, NEC-based sizing factors from 125% to 250%, and rounds to the next standard device size when permitted.

Data Source and Methodology

Primary Authority: National Electrical Code (NEC), NFPA 70, 2023 Edition — Articles 450.3(A) (Transformers 600 V and less) and 240.6(A) (Standard Ampere Ratings of Overcurrent Devices). Access via NFPA (free account required): NFPA 70 (2023). Selected public summaries: ECM: Transformer Calculations and the NEC.

All calculations strictly follow engineering formulas for full-load current and apply NEC-driven multipliers and standard device ratings to assist in selecting compliant OCPDs.

Important: NEC-specific limits and exceptions can vary by device type, protection scheme, current magnitude, and local amendments. Always verify with the adopted Code edition and your AHJ (Authority Having Jurisdiction).

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The Formula Explained

Full-Load Current (FLC):
Single\text{-}phase:\; I_1 = \dfrac{S_{kVA}\times 1000}{V_1}
Three\text{-}phase:\; I_1 = \dfrac{S_{kVA}\times 1000}{\sqrt{3}\, V_1}
Secondary Current (optional):
Single\text{-}phase:\; I_2 = \dfrac{S_{kVA}\times 1000}{V_2}
Three\text{-}phase:\; I_2 = \dfrac{S_{kVA}\times 1000}{\sqrt{3}\, V_2}
OCPD Sizing:
I_{OCPD} = \left(\dfrac{\text{Factor}}{100}\right)\times I_1
\text{Standard Size} = \min\{ s \in \mathcal{S} \mid s \ge I_{OCPD} \}, where \mathcal{S} is the NEC 240.6(A) set of standard ratings.

Glossary of Variables

    - S_kVA: Transformer apparent power rating in kilovolt-amperes (kVA).
    - V1: Primary voltage (V).
    - V2: Secondary voltage (V, optional).
    - I1: Primary full-load current (A).
    - I2: Secondary full-load current (A).
    - Factor: NEC-based multiplier (e.g., 125%, 167%, 200%, 225%, 250%).
    - I_OCPD: Calculated OCPD amperes prior to selecting a standard device size.
    - Standard Size: The nearest higher rating from NEC 240.6(A), when permitted.

How It Works: A Step-by-Step Example

Scenario: 45 kVA, 480 V to 208Y/120 V, three-phase transformer. Both primary and secondary OCPD provided. Choose 125% factor.

  1. Compute primary FLC:
    I_1 = \dfrac{45\times 1000}{\sqrt{3}\times 480} \approx \dfrac{45000}{831.36} \approx 54.1\ \text{A}
  2. Apply factor:
    I_{OCPD} = 1.25 \times 54.1 \approx 67.6\ \text{A}
  3. Select standard size (NEC 240.6(A)):
    \text{Next standard} = 70\ \text{A}
  4. Secondary current for reference:
    I_2 = \dfrac{45\times 1000}{\sqrt{3}\times 208} \approx \dfrac{45000}{360.26} \approx 124.9\ \text{A}

Result: A 70 A primary OCPD is indicated for this scenario, subject to conductor ampacity, tap rules, small-current exceptions, and local amendments.

Frequently Asked Questions (FAQ)

Do I always use 125% for transformer primary protection?

No. 125% is common when both primary and secondary protection are provided. Primary-only protection can allow higher factors (e.g., up to 250% with inverse-time CBs). Confirm with NEC 450.3(A) and your AHJ.

What standard OCPD sizes does the calculator use?

It uses the NEC 240.6(A) list for common ratings (e.g., 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A, and larger like 225, 250, 300, 400 A, etc.).

When can I select the next higher standard size?

NEC permits next standard ratings in specific conditions, often for devices 800 A or less. The calculator can show the next higher size; you must verify if the rule applies to your case.

Does breaker 80% vs 100% rating affect results?

Yes, equipment and listing conditions matter. The calculator provides NEC-based current sizing, but device application must follow product listing and manufacturer instructions.

How do tap rules affect OCPD selection?

Tap conductors and feeder configurations (NEC 240.21) can impose additional constraints. This tool focuses on OCPD sizing from transformer FLC; always verify conductor sizing/routing rules separately.

Are there special rules for very small primary currents?

Yes. Small-current exceptions exist in NEC 450.3(A). Review the applicable notes and exceptions for your transformer and confirm with your AHJ.

Does this tool replace engineering judgement?

No. It streamlines calculations but does not substitute for a licensed professional’s judgement or the adopted Code. Always verify with the latest NEC and local amendments.

Tool developed by Ugo Candido.
Content verified by IEI Labs Engineering Editorial Team.
Last reviewed for accuracy on: .