Guide to the Canadian Electrical Code, Part 1[i] – A Road Map: Section 42 — Electric Welders

September 15, 2025

by William (Bill) Burr[i]

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The Code is a comprehensive document. Sometimes it can seem quite daunting to quickly find the information you need. This series of articles provides a guide to help users find their way through this critical document. This is not intended to replace the notes in Appendix B or the explanations of individual requirements contained in the CEC Handbook but will hopefully provide some help in navigating the Code. These notes are based on the 26^th edition.

Section 42 — Electric Welders

Rule 42-000 Scope – states that Section 42 is a supplementary or amendatory section of the code and applies to the installation of electric welders. There are two main types of welders – arc and resistance welders. Arc welders use a sacrificial rod to weld the metal target material together. Resistance welders fuse the metal target material together. Arc welders are divided further into transformer and motor-generator types.

Rule 42-002 Special terminology – contains definitions for special terms that apply to Section 42 that are not defined in Section 0 such as: actual primary current, duty cycle, and rated primary current. These terms will be important when calculating the ampacity of the supply conductors for welders.

Rule 42-004 Receptacles and attachment plugs – along with Appendix B notes, outlines that

• where receptacles are installed for the plug and cord-connection of a portable welder, the ampacity rating of the overcurrent protection for the circuit can be more than the rating of the receptacle, but the receptacle must have a rating equal to or greater than the ampacity of the supply cable required for the primary current of the cord-connected welder. This is important because the high instantaneous inrush start-up current of the welder will exceed the branch circuit conductor ampacity. This prevents nuisance tripping.
• where a welder receptacle is installed, it must be labeled to indicate:
  • the receptacle is for a welder connection only,
  • the AWG size of the insulated conductors,
  • whether the insulated supply conductor is Cu/Al,
  • the conductor insulation designation and temperature rating, and
  • the ampere rating of the overcurrent device protecting the receptacle insulated supply conductors based on the duty rating (80% or 100%)

Transformer arc welders and inverter welders

Rule 42-006 Supply conductors – provides the factors in determining the ampacity of insulated supply conductors for transformer type arc or inverter welders. The ampacity depends on the duty cycle of a single welder, as in sub-rule (1) and the demand factors where a group of welders are supplied by one circuit as in sub-rule (2).  Table 42A shows the correlation between duty cycle and demand factor and determination of the demand factor for multiple welder installations.

Rule 42-006: Supply Conductors

• Individual Welders – For each transformer arc welder or inverter welder installed individually, the ampacity of the insulated supply conductors must be at least equal to the rated primary current of the welder (in amperes) multiplied by the corresponding factor listed in Table 42A.
• Groups of Welders -When supplying a group of transformer arc welders or inverter welders from a single circuit, the ampacity of the supply conductors must be at least the sum of the current values determined for each welder using the method above, with the following demand factors applied:
  • 100% of the two largest calculated currents
  • 85% of the third largest calculated current
  • 70% of the fourth largest calculated current
  • 60% of the calculated currents for all remaining welders in the group
• Reduced Values for Low Duty Cycles -Lower ampacity values than those specified above are allowed if the nature of the work makes a high operating duty cycle for individual welders impossible.

Rule 42-008: Overcurrent Protection for Transformer Arc Welders and Inverter Welders – specifies the factors to be used in calculating the rating of circuit overcurrent protection:

• Overcurrent protection for each transformer arc welder and inverter welder must be rated or set at no more than 200% of the welder’s rated primary current, unless the overcurrent device protecting the insulated supply conductors meets this requirement.
• Every ungrounded conductor supplying a transformer arc welder or inverter welder must be protected by an overcurrent device rated or set at no more than 200% of the ampacity of the insulated conductor, as specified in Table 1, 2, 3, or 4. The next higher rating or setting may be used if:
  • The nearest standard rating of the overcurrent device is less than the required rating or setting, or
• The required rating or setting would cause the overcurrent device to open too frequently.
• For feeders supplying a group of transformer arc welders or inverter welders, the maximum rating or setting of the overcurrent protection must not exceed a value determined by:
  • Calculating the maximum rating or setting permitted for the welder with the highest overcurrent protection, and
  • Adding the sum of ampacities (as calculated by Rule 42-006 (1)) for all other welders in the group.

Rule 42-010 Disconnecting means – requires that where there is no integral disconnect on the welder, a switch or disconnect must be provided to disconnect the supply conductors and must have sufficient current rating capacity to accommodate the overcurrent protection as specified in Rule 42-008.

Motor-generator arc welders

Rule 42-012 Conductors, protection, and control of motor-generator arc welders – specifies that the requirements for insulated conductors, protection, and control of motor-generator arc welders are primarily governed by Sections 4 and 28. However, there are a few notable exceptions:

• Motors must be marked in amperes only.
  • If a controller is built-in as an integral part of the motor-generator set, it does not need to be separately marked, if all necessary data are included on the motor nameplate.
• For an individual motor-generator arc welder, the ampacity of its insulated supply conductors must be at least the rated primary current (in amperes) of the welder multiplied by the applicable factor found in Table 42B.
• When supplying a group of motor-generator arc welders, the ampacity of the insulated supply conductors must be not less than the sum of the calculated currents for each welder (using the method above), multiplied by the following demand factors:
  • 100% of the two highest calculated currents in the group
  • 85% of the third largest calculated current
  • 70% of the fourth largest calculated current
  • 60% of the calculated currents for all remaining welders in the group
• In situations where it is impossible for individual welders to operate at a high duty cycle due to the nature of the work, lower values than those specified above may be used.

Resistance welders

Rule 42-014 Supply conductors for resistance welders – providesthe factors in determining the ampacity of insulated supply conductors serving resistance welders according to the various operating conditions of the equipment:

• Individual seam or automatically fed resistance welders – the insulated supply conductors must have an ampacity of at least 70% of the welder’s rated primary current when operated at varying values of primary current or duty cycle.
• Manually operated, non-automatic resistance welders – for welders operated at different times with different primary current or duty cycle values, supply conductors must have an ampacity of at least 50% of the rated primary current.
• Welders with known and constant primary current and duty cycle – the ampacity of supply conductors should not be less than the value obtained by multiplying the actual primary current by the appropriate factor found in Table 42C.
• Conductors supplying multiple resistance welders – the ampacity required is at least the sum of the following:
  • The ampacity for the welder with the largest calculated supply conductor requirement (using the factors described above).
  • 60% of the conductor ampacities calculated for all other welders in the group.

42-016 Overcurrent Protection – specifies the factors in determining the rating of the overcurrent protection:

• Each resistance welder must have overcurrent protection rated or set at no more than 300% of its rated primary current, unless the device protecting supply conductors offers equivalent protection.
• Each ungrounded conductor of a resistance welder must have overcurrent protection rated or set at no more than 300% of the conductor’s ampacity (as per Tables 1–4). The next higher standard rating is allowed if:
  • the nearest standard rating is less than required; or
  • the required rating causes nuisance tripping.
• For feeders supplying multiple welders, the maximum overcurrent device rating is the highest permitted for any single welder plus the total ampacities calculated for the rest as per Rule 42-014.

Rule 42-018 Control of resistance welders – requires that a resistance welder have a disconnecting switch or circuit breaker rated at not less than the capacity of the insulated supply conductors, as per Rule 42-014 to isolate the welder from the supply circuit.

Rule 42-020 Nameplate data for resistance welders – requires data specific to the resistance welder to be provided on the nameplate including rated KVA at 50% duty cycle, maximum and minimum open-circuit secondary voltage, short circuit current and the throat and gap setting.

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[i] Source: CSA C22.1:24, Canadian Electrical Code, Part 1 – Safety Standard for Electrical Installations. © 2024 Canadian Standards Association. Please visit store.csagroup.org. With the permission of CSA Group, material is reproduced from CSA Group standard CSA C22.1:24, Canadian Electrical Code, Part 1 – Safety Standard for Electrical Installations. This material is not the complete and official position of CSA Group on the referenced subject, which is represented solely by the Standard in its entirety. While use of the material has been authorized, CSA Group is not responsible for the manner in which the data are presented, nor for any representations and interpretations. No further reproduction is permitted. For more information or to purchase standard(s) from CSA Group, please visit store.csagroup.org or call 1-800-463-6727.

[i] William (Bill) Burr is an associate member of the Canadian Electrical Code, Part 1, Technical Committee and formerly Chair of the Canadian Advisory Council on Electrical Safety (CACES), Chief Electrical and Elevator Inspector for the Province of BC & the Northwest Territories, Director of Electrical and Gas Standards Development and Director of Conformity Assessment at CSA Group. Bill can be reached at Burr and Associates Consulting billburr@gmail.com.