For a standard branch circuit, calculate the minimum conductor ampacity as 125% of the continuous load plus 100% of the noncontinuous load. Use 2020 NEC 210.19(A)(1) for conductors and 210.20(A) for overcurrent protection. A continuous load is one expected to draw its maximum current for at least three hours.
Use this formula
Minimum ampacity = (continuous amps × 1.25) + noncontinuous amps.
Multiply only the continuous portion by 1.25. This is the load-sizing check before correction and adjustment factors. You must also check that the conductor’s ampacity after those factors can carry the load.
| Load | Calculation | Minimum from the load calculation |
|---|---|---|
| 16A continuous | 16 × 1.25 | 20A |
| 12A continuous + 5A noncontinuous | (12 × 1.25) + 5 | 20A |
| 24A continuous | 24 × 1.25 | 30A |
These examples assume ordinary branch-circuit loads, with no special equipment rule or 100%-rated assembly exception. They calculate the required ampacity. Selecting the wire still requires the proper temperature column and installation conditions.
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See CE courses and class datesWatch JCR explain which ampacity column to use
After calculating the load, you still need the correct conductor and terminal-temperature ratings. Victor’s existing NEC 110.14(C) lesson explains that next step. Follow along with your 2020 NEC.
Watch JCR’s ampacity-column lessonExisting JCR video on YouTubeWhy people call it the 80% rule
For an all-continuous load, divide the circuit rating by 1.25. A standard 20A branch circuit supports a 16A continuous load because 20 ÷ 1.25 = 16.
Do not reduce every circuit to 80% regardless of its load. Separate the continuous and noncontinuous portions, then calculate. The 100%-rated assembly exception has specific listing and installation conditions. Our 100%-rated breaker practice question explains that distinction.
Finish the wire-sizing checks
- Calculate continuous and noncontinuous loads.
- Use Table 310.16 and the applicable terminal-temperature limit.
- Apply required ambient-temperature correction and conductor-count adjustment. See our ampacity adjustment guide.
- Check overcurrent protection and any equipment-specific rule.
Practice question
A branch circuit supplies 20A of continuous load and 8A of noncontinuous load. What minimum ampacity results from the load calculation?
33A: (20 × 1.25) + 8 = 33. Multiplying all 28 amps by 1.25 gives 35A, which is not the required calculation. Continue with the wire and overcurrent checks before selecting equipment.
Common questions
Does “continuous” mean the equipment is switched on for three hours?
The definition concerns the expected duration of maximum current. Do not classify equipment solely by how long its power switch stays on. Some equipment articles also require particular loads to be treated as continuous.
Do I multiply a motor’s starting current by 125%?
No. Motor conductors, overload protection and short-circuit protection have different rules. Start with our motor breaker-sizing guide and motor overload example.
Keep practicing
Try our most-missed NEC mock exam questions and 2020 NEC practice questions. For each missed answer, find the section in your code book, read the rule and work the question again.
Reviewed October 25, 2026, for the 2020 NEC. Use the edition required by your licensing exam. Follow the adopted code and local amendments on the job.
