Short answer: A welder does not draw full current all the time, so Article 630 sizes its supply conductors on a reduced current. For an individual arc welder, use the effective primary current on the nameplate. If that is not given, multiply the rated primary current by the duty cycle multiplier from Table 630.11(A). A group of welders is sized with a step-down percentage for each welder after the two largest (NEC 630.11(B)). The welder’s overcurrent protection can be up to 200% of its rated primary current (NEC 630.12(A)), and Article 630 sets a separate limit for the supply conductors based on their ampacity. Check both.
PSI lists welders in the Special Occupancies, Equipment and Conditions subject area of the Texas Journeyman exam: 6 scored NEC Knowledge questions and 3 scored Calculations questions. PSI does not say how many cover welders alone. The Texas Journeyman Practice Pack has 1,000 original questions and 10 timed mock exams in the real two-part format, with step-by-step solutions for every calculation. Try free practice questions without signing up. Get the full pack for US$39 with one payment.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.
Why welders get their own rules
An arc welder strikes an arc, welds for a while and then stops while the welder moves, chips slag or changes rods. The duty cycle is the share of a 10-minute period the welder can run at its rated output. A 40% duty cycle means about 4 minutes on and 6 minutes off. Because the welder runs on and off, its supply conductors heat up less than conductors carrying the same current all the time, and Article 630 sizes them on a duty cycle basis.
Calculation 1: one arc welder
Use the first value that is available:
- Effective primary current (I1eff) from the nameplate. Use it directly.
- Rated primary current (I1max) × the duty cycle multiplier from Table 630.11(A). The table has separate columns for transformer and dc rectifier welders and for motor-generator welders.
Worked example. A transformer arc welder has a rated primary current of 50 A at a 40% duty cycle and no effective current on the nameplate. The Table 630.11(A) multiplier for a 40% duty cycle, transformer type, is 0.63.
- Minimum conductor ampacity: 50 × 0.63 = 31.5 A.
- Choose a conductor with at least 31.5 A ampacity in Table 310.16, using the correct temperature column and any adjustment factors.
Calculation 2: a group of welders
When several welders share one feeder, Article 630 applies step-down percentages to the group. The conductor ampacity is the sum of these values, each welder’s current found as in Calculation 1 (NEC 630.11(B)):
| Welder, largest first | Share of its current |
|---|---|
| Two largest | 100% |
| Third largest | 85% |
| Fourth largest | 70% |
| All others | 60% |
Worked example. Three transformer welders on one feeder, with currents of 31.5 A, 25 A and 20 A after the duty cycle step.
- Two largest at 100%: 31.5 + 25 = 56.5 A.
- Third at 85%: 0.85 × 20 = 17 A.
- Minimum feeder ampacity: 56.5 + 17 = 73.5 A.
Calculation 3: overcurrent protection
Each arc welder must have overcurrent protection rated or set at not more than 200% of I1max, or of the rated primary current where I1max is not given (NEC 630.12(A)). Where that value is not a standard size, check your NEC for the next standard rating it allows.
For the 50 A welder above, the welder limit is 50 × 2 = 100 A. That is not automatically the breaker you install.
The welder limit is figured from the rated primary current, not the reduced current used for the conductor. Article 630 also limits the protection of the supply conductors, based on their ampacity. Look up both limits in your NEC; the lower one governs the device you choose.
Resistance welders
Resistance welders, such as spot and seam welders, use a different duty cycle method and different multipliers in Part III of Article 630. Do not apply the arc welder multipliers or limits to them. Read Part III in your NEC before you size a resistance welder circuit.
Check yourself
- What does a 60% duty cycle mean? The welder can run at rated output for about 6 minutes in each 10-minute period.
- The nameplate gives an effective primary current of 28 A. What do you size the conductor for? 28 A. Use I1eff directly.
- A welder has a rated primary current of 40 A. What is the welder overcurrent limit? 40 × 2 = 80 A. Then check the supply conductor limit before you choose the device.
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Find Article 630 faster on exam day
NFPA 70, National Electrical Code, 2026 Edition (softcover). Highlight Table 630.11(A), the group of welders percentages and the 200% overcurrent limit, and practice going from the table to the multiplication quickly. The Texas exam is open book. PSI allows a soft-bound NEC with highlighting, underlining and notes made in advance. Loose-leaf, spiral-bound and ring-bound copies are not allowed, and only permanent index tabs made or supplied by the NEC publisher are permitted; homemade and third-party tabs are not.
FAQ
How do you size wire for a welder?
Use the effective primary current from the nameplate. If it is not given, multiply the rated primary current by the duty cycle multiplier from Table 630.11(A).
What size breaker can a welder have?
The welder limit is 200% of the rated primary current. Article 630 also limits the protection of the supply conductors based on their ampacity, so check both and use the lower one.
How do you size a feeder for several welders?
Add 100% of the two largest welder currents, 85% of the third, 70% of the fourth and 60% of the rest.
Are welders on the Texas Journeyman exam?
PSI lists welders in Special Occupancies, Equipment and Conditions: 6 scored NEC Knowledge questions and 3 scored Calculations questions.
Sources checked October 2026
- PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70, National Electrical Code, 2026 Edition, Article 630 and Table 630.11(A)
