Low Voltage Transformer Application

Name: Low Voltage Transformer Application
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Transformer protection fundamentals with a 75kVA and 500kVA transformer as examples

Created byThomas Domitrovich

Last updated 12/2024

English

What you'll learn

Calculate necessary parameters of low voltage transformers
Selection line-side and Secondary-side overcurrent protective devices
Select Line and Load conductors
Navigate the National Electrical Code for transformer and conductor protection
Safety by design tips and tricks to address risk for both electrical equipment damage and electrical worker safety

Course content

7 sections • 18 lectures • 2h 33m total length

Introduction to the instructor2:46
This is an introduction from the instructor. Thomas Domitrovich is a professional engineer with 34 years of experience in various areas of power systems engineering. Mr. Domitrovich has global codes and standards responsibilities for the electrical sector and industrial sector of Eaton, assists customers in the proper application of Eaton solutions to manufacturer instructions and codes and standards. Thomas began his career in 1990 with Gilbert Commonwealth in Reading PA as an Electrical Engineer working in Industrial Power Systems, Fossil and Nuclear power generation power distribution systems analysis and design. Thomas joined Eaton Corporation in 1996 and has held various roles within Eaton including Power Quality and Residential Products working with customers on system solutions for construction of industrial, commercial, and residential power systems. Thomas manages engineers focused on overcurrent and over voltage protection, safety, codes, and standards. Thomas is an active member of various organizations including the Institute of Electrical and Electronics Engineers (IEEE), International Association of Electrical Inspectors (IAEI), National Fire Protection Association (NFPA), National Electrical Manufacturers Association (NEMA) and others.
I am a technical committee member on the following committees:
NFPA 70, National Electrical Code CMP 2
NFPA 70, National Electrical Code CMP 10
NFPA 78, Guide on Electrical Inspections
NFPA 1078, Standard for Electrical Inspector Professional Qualifications
NFPA 110, Standard for Emergency and Standby Power Systems
NFPA 111, Standard on Stored Electrical Energy Emergency and Standby Power Systems
NFPA 73, Standard for Electrical Inspections for Existing Dwellings
NFPA 70B, Standard for Electrical Equipment Maintenance (2023 Edition)
IEC TC 64 Technical Advisor (United States)
IEC TC 32B Technical Advisor (United States)
Introduction and review of our goals and objectives for this class3:38
This class will focus on establishing all that you need to know to properly select primary and secondary overcurrent protection for a transformer as well as the primary and secondary conductors. This module will review some standard definitions and the standards that apply to transformers. This program will use a 75 kVA transformer and a 500 kVA transformer to apply what we have learned to practical applications.
Product Standards7:50
This session dives into the critical product standards that govern both medium- and low-voltage transformers, equipping electrical professionals with the tools to ensure proper installation and application. We'll begin by clarifying the relationship between Article 450 of the National Electrical Code (NEC) and transformer listing requirements, emphasizing where additional NEC sections may come into play. Attendees will explore key standards for medium-voltage transformers, including IEEE C57.12.00 and C57.12.01, and learn how these documents shape the design and performance requirements. On the low-voltage side, we’ll walk step-by-step through UL’s Product IQ tool—an invaluable, free resource for identifying UL product standards, such as UL 1561 for dry-type transformers. By leveraging this tool, participants will gain practical knowledge about transformer terminations, markings, and other critical details required for code compliance. This interactive lecture will leave you with actionable insights to navigate transformer standards confidently and effectively.
Navigating the NEC8:17
This lecture helps the student understand that Article 450 only applies to the transformer and that for proper application the student must consider other areas of the National Electrical Code that applies. This module will review some of the major areas of the NEC that must be considered including the following: (Note, most of what we will cover will apply to previous editions as well. The instructor will identify when requirements are specific to the 2023 edition.)

Article 450 Transformers and Transformer Vaults (Including Secondary Ties)
This Article provides the requirements for transformers and transformers only. The student will understand how to apply these rules accurately for selection of overcurrent protection.
Article 110 General Requirements for Electrical Installations
This Article includes some general requirements that may impact transformer applications including arc flash labeling and available fault current markings.
Article 215 Feeders
This article covers the installation requirements, overcurrent protection requirements, minimum size, and ampacity of conductors for feeders not over 1000 volts ac or 1500 volts dc, nominal.
Article 235 Branch Circuits, Feeders, and Services Over 1000 Volts ac, 1500 Volts dc, Nominal
This article provides the general requirements for branch circuits, feeders, and services over 1000 volts ac or 1500 volts dc, nominal.
Article 240 Overcurrent Protection.
Parts I through VII of this article provide the general requirements for overcurrent protection and overcurrent protective devices not more than 1000 volts, nominal. Part VIII covers overcurrent protection for those portions of supervised industrial installations operating at voltages of not more than 1000 volts, nominal.
Article 408 Switchboards, Switchgear, and Panelboards
This article covers switchboards, switchgear, and panelboards. It does not apply to equipment operating at over 1000 volts, except as specifically referenced elsewhere in the Code.
We will navigate the Code to provide the attendee with a foundation of knowledge that can be applied to many different transformers.
450.3 Overcurrent Protection14:45
This module will drill into Section 450.3 titled "Overcurrent Protection" and specifically review the two important tables that will be leveraged when selecting primary and secondary overcurrent protective devices.

There are two tables that are important to align with when selecting the primary and secondary overcurrent protective devices. Section 450.3 Overcurrent Protection states that overcurrent protection of transformers must comply with 450.3(A), (B), or (C). As used in this section, the word transformer shall mean a transformer or polyphase bank of two or more single-phase transformers operating as a unit. 450.3(A), (B), and (C) are as follows:
(A) Transformers Over 1000 Volts, Nominal.
(B) Transformers 1000 Volts, Nominal, or Less.
(C) Voltage (Potential) Transformers.
Introduction Knowledge Check

Overview of key parameters15:34
This session provides an overview to the key parameters that are needed when determining protection of a transformer. There are some very fundamental parameters that will need to be calculated but many are available right off of the nameplate or from manufacturer literature.
Calculating full load amperes of a transformer (single phase and three phase)8:15
The full load amps of a transformer for both the primary and secondary is one of the most fundamental calculations pertaining to a transformer that you will perform. You cannot practice enough performing this calculation as with this value you can determine the largest OCPD permitted by code and determine the maximum available fault current that could be seen on the secondary.
Reflecting currents through a transformer9:34
This may seem like a trick of the trade but knowing how to properly reflect currents from the secondary of a transformer to the primary of a transformer is needed to get some work done. This session will look at a three phase bank of transformers and discuss the relationship between secondary and primary of the transformer not only under normal conditions but also under faulted conditions. You can use this tool to check your answers or in practical application to determine incident energy and even selective coordination.
Transformer Percent Impedance - A Per Unit introduction21:56
Percent impedance can sometimes seem like a mystery. This session will help you understand how percent impedance of a transformer is determined and what the tolerances are based on product standards. It's important to understand percent impedance because you will need it to determine available fault current in a power distribution system. Two parameters (FLA and %Z) facilitate the calculation of the highest amount of fault current that could ever be seen on the secondary of a transformer on the fly. You'll look like a rock star with this information and ensure proper application of electrical equipment.
The transformer damage curve7:21
Everything that carries current has a damage curve. Transformers are no different. This session will introduce the transformer damage curve and get you started. Most of what we will discuss is based on NEC requirements but providing this additional look into transformers help us understand the purpose of both primary and secondary overcurrent protective devices.
Transformer Fundamental Parameters Knowledge Check

Introduction and overview of the requirements found in Article 450 of the NEC8:23
Transformer primary overcurrent protection has the function of providing short-circuit protection of the transformer. The NEC includes requirements that dictate the maximum size primary OCPD that is permitted. This session will review those requirements and discuss the foundational items needed to select the proper OCPD for your applications.
Select the primary protective device13:31
This session pulls together alot of the materials discussed to this point. This session will select the primary OCPD for a 75kVA and 500kVA transformer. We will determine the maximum size primary OCPD for these transformers and talk about the minimum size OCPD possible. This session will give some tips as well on how to specify primary protection before the transformer is purchased to help reduce the likelihood of modifications after the design is complete.
Transformer primar overcurrent protective device

Introduction and overview of NEC requirements for secondary OCPD3:06
The secondary OCPD is a critical part of transformer protection. This session will help you avoid the pitfalls that result in a red tag or worst case, a transformer that is not properly protected. Emphasis here is that the secondary OCPD of a transformer is selected first to protect the transformer and nothing else. This is a part of the process. First you select the OCPDs for proper transformer protection and then you select the conductors.
Select the secondary protective device5:23
Selecting the secondary OCPD isn't as hard as you would think and this video sheds some light on this topic for both the 75kVA and 500kVA transformers. We'll use what we have learned in this discussion specifically focusing on the two subject transformer sizes of 75kVA and 500kVA.
Secondary OCPD Selection

Fundamentals and navigating the NEC9:03
This session is the introduction to selecting the conductors for primary and secondary of the 75kVA and 500kVA transformer examples. Transformer secondary conductors are not protected at their source for many transformer applications we deal with. It is important to realize this fact and the NEC does not disappoint when it comes to providing requirements to ensure proper protection of transformer and conductors. This session will reference some key areas of the NEC including the following:

Article 215 Feeders: Conductors on primary and secondary of the transformer are feeders and the requirements of Article 215 apply
Article 240 Overcurrent Protection: Includes important requirements that directly apply to transformer secondary conductors
Article 408 Switchboards, Switchgear, and Panelboards: Will be important when supplying this equipment directly from a transformer
Selecting the primary and secondary conductors6:12
In this session the rubber meets the road where we implement what we just learned in the previous lecture and select the primary and secondary conductors for the 75kVA and 500kVA transformers.
Transformer secondary conductor selection

Secondary maximum available fault current3:51
Icing on the cake is a follow-up review of calculating the maximum available fault current on the secondary of our 75kVA and 500kVA transformers. We'll refresh our memories on the pertinent equations and determine the maximum available fault current that can be seen on the secondary. We'll employ the infinite bus equation to make our determination.
Maximum available fault current

Requirements

General knowledge of what a transformer is and what it is used for

Description

This course will give the student a foundation to handle many transformer applications regardless of the size of the transformer. Two transformers will be used as our example including a 75kVA and 500kVA transformer. The student will understand when Article 450 applies and when it does not. A review of various areas of the NEC will give the students skills necessary for many different applications that require navigating NEC requirements.

NEC Areas that will be explored and understood include the following:

Article 215 Feeders: Conductors on primary and secondary of the transformer are feeders and the requirements of Article 215 apply. This article will help determine the minimum conductor ampacity but essentially points the user to Article 240 for proper conductor protection. the section discussed here includes 215.3 but as you'll see, we do provide a short-cut and the reason why the short-cut is appropriate to a transformer application.

215.3 Overcurrent Protection.

Article 240 Overcurrent Protection: Includes important requirements that directly apply to transformer secondary conductors. A transformer secondary presents a unique situation in that the overcurrent protection for that conductor is not located at the source of the secondary conductor. This program will help you navigate the NEC to ensure proper protection of not only the transformer but also the transformer conductors.

240.4 Protection of Conductors.
(F) Transformer Secondary Conductors.

Article 408 Switchboards, Switchgear, and Panelboards: Will be important when supplying this equipment directly from a transformer. The requirements here especially for panel boards is often missed by design engineers and installers. We'll understand why in many applications, we will be required to place a secondary OCPD on the transformer and the reason may not be what you expect.

408.36 Overcurrent Protection.
(B) Supplied Through a Transformer.

Article 450 Transformers and Transformer Vaults (Including Secondary Ties): Critical Article for all things transformers but for this curriculum we will focus on the overcurrent protection for the transformer.

450.3 Overcurrent Protection.
(A) Transformers Over 1000 Volts, Nominal.
(B) Transformers 1000 Volts, Nominal, or Less.

This course will provide a foundation for proper transformer protection and give the attendee a foundation to understand how to navigate the National Electrical Code. Proper transformer protection begins during the design phase. Throughout this course, the instructor will provide tricks of the trade and advice on how to ensure a solid design that will have minimal changes once equipment is purchased and installed. This course will also provide a foundation to address existing installations to ensure the continued protection of installed transformers and related equipment.

Who this course is for:

Electricians, Electrical Engineers, Electrical Contractors, Electrical Inspectors

Low Voltage Transformer Application

What you'll learn

Explore related topics

Course content

Introduction5 lectures • 37min

Transformer Fundamental Parameters5 lectures • 1hr 3min

Selecting the transformer primary overcurrent protective device2 lectures • 22min

Selecting the transformer secondary overcurrent protective device2 lectures • 8min

Selecting transformer conductors (Primary and Secondary)2 lectures • 15min

Transformer secondary fault current1 lecture • 4min

Summary and Closing remarks1 lecture • 4min

Requirements

Description

Who this course is for: