Circuit breaker covered by plastic case

Loading (and overloading) of transformers

Canadian Corner
Canadian Corner

As I indicated earlier, it is very encouraging to receive a wide range of questions from the readers.

This article is a compilation of ten questions from the readers (and answers to these questions) on the rating, operation, and location of protective and control devices mandated by Section 14 of the Canadian Electrical Code, Part I (CE Code).

Question 1

What is the difference between a control device and an overcurrent protective device?

Answer to question 1

A control device may include a disconnecting means, an overcurrent device, or a combination of both. A typical general-use switch is disconnecting means, as it manually interrupts a rated current of the circuit at a rated voltage. A typical circuit breaker is a control device, as it combines manual disconnecting means and means to automatically de-energize the circuit.

An overcurrent device may be represented by a fuse or a circuit breaker. An overcurrent device automatically interrupts the current of the circuit if the current reaches a value that will produce a dangerous temperature in the piece of equipment or the conductors supplying this piece of equipment. An overcurrent device also automatically interrupts a fault current in the circuit (ground fault or short circuit fault).

Canadian Electrical Code, Part I (CE Code) offers the following definitions in this regard:

“Circuit breaker — a device designed to open and close a circuit by non-automatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its ratings.”

“General-use switch — a switch intended for use in general distribution and branch circuits and that is rated in amperes and is capable of interrupting its rated current at rated voltage.


Question 2

Which requirements of the CE Code mandate installation of control devices for the disconnection of circuits?

Answer to question 2

Such specific requirements exist in numerous places of the Code, but the following Rules in Section 14 are the most important in this regard:

“14-010 Protective and control devices required

Except as otherwise provided for in this Section or in other Sections dealing with specific equipment, electrical apparatus and ungrounded conductors shall be provided with

a) devices for the purpose of automatically opening the electrical circuit thereto,

i) if the current reaches a value that will produce a dangerous temperature in the apparatus or conductor; and

ii) in the event of a ground fault, in accordance with Rule 14-102;

b) manually operable control devices that will safely disconnect all ungrounded conductors of the circuit at the point of supply simultaneously, except for multi-wire branch circuits that supply only fixed lighting loads or non-split receptacles, and that have each lighting load or receptacle connected to the neutral and one ungrounded conductor; and

c) devices that, when necessary, will open the electrical circuit thereto in the event of failure of voltage in such a circuit.

14-402 Disconnecting means required for fused circuits (see Appendix B)

Circuits protected by fuses shall be equipped with disconnecting means, integral with or adjacent to the fuseholders, whereby all live parts for mounting fuses can be readily and safely made dead; however, such disconnecting means shall be permitted to be omitted in any one of the following cases:

a) instrument and control circuits on switchboards where the voltage does not exceed 250 V;

b) primary circuits of voltage transformers having a primary voltage of 750 V or less, on switchboards;


c) a circuit having only one ungrounded conductor where a plug fuse is used.

14-404 Control devices ahead of overcurrent devices

Control devices used in combination with overcurrent devices or overload devices for the control of circuits or apparatus shall be connected so that the overcurrent or overload devices will be dead when the control device is in the open position, except where this is impracticable.”


Question 3

What is an ampere rating of a circuit breaker or a fused switch?

Answer to question 3

A circuit breaker or a fused disconnect must have a standard voltage rating sufficient for the voltage used in the electrical distribution where such a control device is installed and must have an ampere rating to interrupt the load current manually and overcurrent or fault current automatically.

Rules 14-010 above explain this well, and in addition – Rules 14-012 and 14-400 of the CE Code below state the following:

“14-012 Ratings of protective and control equipment (see Appendix B)

 In circuits of 750 V and less,

a) electrical equipment required to interrupt fault currents shall have ratings sufficient for the voltage employed and for the fault current that is available at the terminals; and

b) electrical equipment required to interrupt current at other than fault levels shall have ratings sufficient for the voltage employed and for the current it must interrupt.

14-400 Rating of control devices

Control devices shall have ratings suitable for the connected load of the circuits that they control and, with the exception of isolating switches, shall be capable of safely establishing and interrupting such loads.”


It means that the ampere rating of each such fused disconnect or circuit breaker is the rating of the largest fuse that can be installed in a fusible disconnecting device or the highest trip setting for which the actual overcurrent device installed in a circuit breaker is rated. If, for example, the frame size of a circuit breaker is rated at 225 A, it means that 225 A is the maximum continuous current that such a circuit breaker can carry.

Of course, this circuit breaker with the 225 A-rated frame may have adjustable trip settings that will allow to select a trip setting up to the frame rating. If a 400 A-rated disconnecting switch is selected for installation, it can safely manually interrupt continuous load up to 400 A.

As such, all overcurrent devices (fuses and circuit breakers) that are required to automatically interrupt fault currents must have interrupting ratings sufficient for the voltage of the circuit and for the fault current that is available at the terminals of the overcurrent device, and all disconnecting means that are required to manually interrupt load currents, must have ratings sufficient for the voltage of the circuit and for the load current that is available at the terminals.

Paragraphs 14-012(a) and (b) of the CE Code, shown above, articulate this requirement for the Code users.


Question 4

What conditions will trigger the automatic operation of an overcurrent device?

Answer to question 4

As was clarified in the answer to question 3, and in Subrule 14-010(1)(a) of the CE Code, shown above, each overcurrent device automatically opens the electrical circuit, which it protects, if the current reaches a value that will produce a dangerous temperature in the apparatus or conductor (overcurrent condition); and in the event of a fault in the circuit (ground fault or short circuit).  The answer to question 5 below clarifies the typical times for operations of overcurrent devices when different types of abnormal conditions occur in the circuit.


Question 5

What is the difference between overcurrent/overload and the fault (ground fault and short circuit), and what are the typical times for operations of overcurrent devices, when different types of abnormal conditions occur in the circuit?

Answer to question 5

The objective of Rule 14-010(a), shown above that an overcurrent device de-energizes a circuit in case of either an overcurrent condition, a short-circuit condition, or a ground fault condition.

An overcurrent condition is interrupted by the thermal protection element of the overcurrent device. The thermal protection element is time dependent. For example, in case of a manual motor controller, it de-energises the circuit in 20 seconds when the current that flows through it exceeds six times its rated tripping current. It means that if a trip setting is at 100 A, and the current through the circuit exceeds 600 A, the circuit will open in 20 seconds.

A short circuit is interrupted by the instantaneous element of the overcurrent device, and such instantaneous protection is also based on the time-independent curve. A typical instantaneous setting of an overcurrent device in a low voltage system is normally 13 to 15 times the nominal rating of the overcurrent protection device. The instantaneous threshold de-energizes the circuit in 25 to 64 ms (1.5 to 4 half cycles). It means that in a 1000 A-rated circuit, instantaneous trip will operate when the short circuit current reaches 13,000 A, and this instantaneous trip will de-energize the circuit in 25 to 64 ms (1.5 to 4 half cycles).

A ground fault on a solidly grounded system is interrupted by the same instantaneous element of the overcurrent device as in the event of a short circuit. Rule 14-102 of the CE Code provides specific requirements for the operation of ground fault protection. The disconnection time must not exceed 200 ms to order to protect a person from the dangerous touch voltage that is present during the time while the ground fault is detected and cleared. That time can be met only by the instantaneous element of the overcurrent protection device or by an auxiliary device such as a ground fault relay.

The CE Code also mandates ground fault protection – to de-energize all normally ungrounded conductors of specific types of electrical equipment with such a ground fault setting that the ground fault protection would allow the normal operation of the protected equipment. (See Rules 26-956; 34-302; 62-116; 62-220; 62-316; 64-064; 64-066; 64-112; 70-122; 78-052 and 84-016 of the CE Code.)


Question 6

What is the maximum permissible height for the installation of control devices?

Answer to question 6

Rule 26-600(2) of the CE Code mandates the maximum height of overcurrent devices handles when such overcurrent devices are located in panelboards installed in dwelling units.

“26-600(2) Panelboards in dwelling units shall be installed as high as possible, with no overcurrent device operating handle positioned more than 1.7 m above the finished floor level.”


However, the National Building Code of Canada (NBC) has the following provisions for the mounting height of control devices that are located in a storey of a building where a barrier-free path of travel is required:

“ Controls

1) Controls described in this Section shall

a) where located in a storey where a barrier-free path of travel is required and unless otherwise stated,

i) be in or adjacent to the barrier-free path of travel,

ii) be mounted 400 mm to 1,200 mm above the floor, and

iii) be adjacent to and centred on either the length or the width of a clear floor space of 1,350 mm by 800 mm.”


Question 7

Are solid-state controls allowed to be used as disconnecting means?

Answer to question 7

No, they are not allowed. The following Rules of the CE Code are applicable to this question:

“14-416 Control devices used only for switching

Except as permitted by other Rules in this Code, control devices that perform only switching functions shall disconnect all ungrounded conductors of the controlled circuit when in the OFF position.

14-700 Restriction of use

Solid-state devices shall not be used as isolating switches or as disconnecting means.”


Question 8

Are disconnecting means allowed to be installed on a platform located 3 m above the floor level?

Answer to question 8

All control devices must be readily accessible, and the terminology “readily accessible” is defined in the CE Code (see below):

“14-406 Location of control devices

1) Control devices, with the exception of isolating switches, shall be readily accessible.“

Readily accessible — capable of being reached quickly for operation, renewal, or inspection, without requiring those to whom ready access is a requisite to climb over or remove obstacles or to resort to portable ladders, chairs, etc.”

So, if access to such platform will require the use of a portable ladder, such location of disconnecting means is not permitted by the Code. However, if there is a permanent ladder or a permanent stair – to access the platform, disconnecting means would be permitted on this platform.


Question 9

What is the ampere rating of overcurrent devices is required by the CE Code for the protection of receptacles?

Answer to Question 9

Rule 14-600 of the CE Code clarifies this question as follows:

14-600 Protection of receptacles

Receptacles shall not be connected to a branch circuit having overcurrent protection rated or set at more than the ampere rating of the receptacle, except as permitted by other Sections of this Code.”


Question 10

Is Rule 14-104 applicable if Rule 14-100 allows omission of overcurrent protection of conductors under relaxation provided by paragraph 14-100(g)?

Answer to Question 10

Rule 14-100(1) mandates overcurrent protection of each ungrounded conductor at the point where it receives its supply of current and at each point where the size of the conductor is decreased, but it allows a few exceptions under the listed conditions in paragraphs (a) to (g).

An Exception to omit overcurrent protection in accordance with paragraph (1)(g) is permitted under the following conditions:

“14-100(1)(g) where the smaller conductor

i) is supplied by a circuit at not more than 750 V;

ii) is supplied from an overhead or underground circuit and is run overhead or underground except where it enters a building;

iii) is installed in accordance with the requirements of Section 6; and\

iv) terminates in service equipment in accordance with Section 6.”

It means that O/C protection of low voltage consumer’s service conductors installed overhead or underground is not required, provided that such service conductors are terminated in a service box located in conformance with Section 6.

Rule 14-104 applies to the correlation between the ampere rating or setting of the overcurrent device, (when such overcurrent protection device is required by Section 14) and the ampacity of conductors protected by this overcurrent device as follows:

14-104 Rating of overcurrent devices (see Appendix B)

1)            The rating or setting of overcurrent devices shall not exceed the allowable ampacity of the conductors that they protect.”


As no O/C protection is required for overhead or underground low voltage consumer’s service conductors, correlation mandated by Rule 14-104 becomes irrelevant in conjunction with relaxation allowed by paragraph 14-100(1)(g).

In fact, Subrule 14-100(2) clarifies that the O/C device located inside the service equipment where such conductors are terminated is allowed to protect upstream service conductors from overload (See below Subrule 14-100(2) and Appendix B Note on this Subrule)

“14-100(2) (2) Notwithstanding Subrule 1), consumer’s service conductors shall be permitted to be protected by an overcurrent device at the service equipment.

Appendix B Note on Rule 14-100(2)

Consumer’s service conductors terminate at service equipment where an overcurrent device is located (See Rule 14-104). This device provides overload protection, but not short-circuit protection, for the conductors on the line side of the service equipment.”

Hopefully, the answers to posted questions will be helpful to the Code users. And as usual, local AHJs should be contacted regarding questions related to each specific installation.

Ark Tsisserev
Ark Tsisserev is president of EFS Engineering Solutions, Ltd., an electrical and fire safety consulting company, and is a registered professional engineer with a master’s degree in Electrical Engineering. Prior to becoming a consultant, Ark was an electrical safety regulator for the city of Vancouver. He is currently the chair of the Technical Committee for the Canadian Electrical Code and represents the CE Code Committee on the CMP-1 of the National Electrical Code. Ark can be reached by e-mail at: His company web site is: