Short answer: Article 660 covers X-ray equipment used for industrial and other nonmedical purposes (NEC 660.1); X-ray equipment in health care facilities follows Article 517. X-ray units draw a very large current for a fraction of a second (the momentary rating) or a smaller current for minutes (the long-time rating). For one unit, the branch circuit conductors and the disconnecting means are sized for at least 50 percent of the momentary rating or 100 percent of the long-time rating, whichever is greater. For two or more units on one feeder, the feeder is sized for 100 percent of the momentary rating of the two largest units plus 20 percent of the momentary ratings of the other units. Control and operating circuits can use 18 AWG or 16 AWG fixture wire or cord when protected at not more than 20 amperes.
PSI lists Special Occupancies, Equipment and Conditions (6 scored NEC Knowledge and 3 scored Calculations questions) as a subject area of the Texas Journeyman exam. PSI does not name Article 660 or say how many questions cover X-ray equipment. 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.
Where this fits
Article 660 sits with the other special equipment articles. It is short, and its exam value is one calculation: sizing a circuit for a load that is huge for an instant and small the rest of the time. Industrial X-ray shows up in Texas pipeline weld inspection shops, foundries and security screening. Medical X-ray rooms are in NEC 517 health care facilities.
Two ratings, two numbers
| Rating | What it means | Typical size |
|---|---|---|
| Momentary rating | The current the unit draws during an exposure of a few seconds or less | Very large |
| Long-time rating | The current the unit draws when operated for longer periods, such as 5 minutes or more | Much smaller |
In a practice problem, find the two ratings you are given and work out which one produces the larger minimum ampacity.
The sizing rules
- One unit: branch circuit and disconnect. The ampacity of the branch circuit conductors and the rating of the disconnecting means are not less than 50 percent of the momentary rating or 100 percent of the long-time rating, whichever is greater.
- Two or more units: feeder. The feeder ampacity and its overcurrent protection are based on 100 percent of the momentary rating of the two largest units plus 20 percent of the momentary ratings of all the other units.
- Overcurrent protection. The same section that sets conductor ampacity also covers the overcurrent devices, using the same branch circuit and feeder calculations as the minimum. Final device selection also follows the equipment instructions.
- Disconnect. A disconnecting means is provided in the supply circuit, within sight of the control and readily accessible. A plug and receptacle can serve for portable equipment of small rating where the article allows it.
- Minimum conductor size. Fixture wires and flexible cords of 18 AWG or 16 AWG are permitted for the control and operating circuits of X-ray and auxiliary equipment when protected by an overcurrent device of not more than 20 amperes. This does not set the size of branch circuit or feeder conductors.
Worked example: one industrial X-ray unit
A unit has a momentary rating of 120 A and a long-time rating of 40 A.
- 50 percent of momentary. 120 × 0.50 = 60 A.
- 100 percent of long-time. 40 A.
- Greater value. 60 A. This is the calculated minimum ampacity for the branch circuit conductors and the minimum rating of the disconnect, 60 A. Real equipment selection also checks voltage drop and the manufacturer’s instructions.
Worked example: a feeder for four units
Four X-ray units on one feeder have momentary ratings of 100 A, 80 A, 60 A and 50 A.
- Two largest at 100 percent. 100 + 80 = 180 A.
- The others at 20 percent. (60 + 50) × 0.20 = 110 × 0.20 = 22 A.
- Feeder. 180 + 22 = 202 A minimum calculated feeder ampacity. This is the Article 660 result, not a standard conductor or breaker size.
Check yourself
- Which rating is the large current drawn for a few seconds? The momentary rating.
- For one unit, what is the branch circuit based on? The greater of 50 percent of the momentary rating or 100 percent of the long-time rating.
- For a feeder, how are the two largest units counted? 100 percent of their momentary ratings.
- Where are X-ray rules for hospitals? Article 517.
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Write the two X-ray numbers on one line
NFPA 70, National Electrical Code, 2026 Edition (softcover). At the top of Article 660, write “1 unit: 50% momentary or 100% long-time, larger. Feeder: 2 largest 100% + rest 20%.” so the calculation is one glance on exam day. 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
Does Article 660 cover medical X-ray machines?
No. Article 660 covers industrial and other nonmedical X-ray equipment. X-ray equipment in health care facilities follows Article 517.
How is the branch circuit for one X-ray unit sized?
At least 50 percent of the momentary rating or 100 percent of the long-time rating, whichever is greater.
How is a feeder for several X-ray units sized?
100 percent of the momentary ratings of the two largest units plus 20 percent of the momentary ratings of the other units.
What is the momentary rating?
The current the unit draws during an exposure lasting a few seconds or less.
What is the long-time rating?
The current the unit draws when it is operated for a longer period, such as several minutes.
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, Articles 517 and 660
