Branch Circuit Conductor and Breaker Check
Check continuous-load sizing, adjusted conductor ampacity, and a selected breaker for a general branch circuit.
Load and protection details
Enter the portion expected to run at maximum current for 3 hours or more.
NM-B cable ampacity is limited to the 60°C column.
Conductor and installation conditions
Use equipment markings. NM-B remains limited to its 60°C ampacity.
Count conductors under the applicable adjustment rules.
The conductor ampacity includes the entered ambient and conductor-count adjustments and the applicable temperature limit.
About this calculator
Use this worksheet to compare a general nonmotor branch-circuit load with a selected conductor and breaker. Enter continuous and noncontinuous load current separately, then enter conductor size and installation conditions.
The result is a screening comparison. It does not approve an installation or replace equipment-specific sizing rules.
Method and assumptions
The calculator counts noncontinuous load at 100% and continuous load at 125% to determine the calculated load requirement. It finds the smallest supported standard breaker rating at or above that requirement for comparison; the user-selected breaker remains the rating being checked.
Selected conductor ampacity uses the existing Table 310.16 lookup with the entered insulation rating, ambient temperature, current-carrying conductor count, and equipment terminal rating. NM-B cable is capped at the 60°C ampacity column. The calculator checks the adjusted conductor ampacity against the calculated load and reports the small-conductor overcurrent limit where applicable.
If the selected breaker exceeds the calculated conductor ampacity, the result calls for review of any permitted conductor-protection exception. It does not decide whether an exception applies or automatically use the next standard breaker size.
FAQs
What counts as a continuous load?
Enter load current expected to run at maximum current for 3 hours or more as continuous. Enter the remaining load separately as noncontinuous.
Why does the calculator use 125% of continuous load?
The general branch-circuit conductor and overcurrent sizing rules apply a 125% factor to continuous load, with noncontinuous load counted at 100%. Special equipment and listed 100%-rated assemblies can follow different rules and are outside this screening tool.
Does the calculator choose the breaker for me?
No. It shows the smallest listed standard rating at or above the calculated load and checks the breaker rating you select. Equipment ratings, conductor protection rules, and installation exceptions still need review.
Can I use this for a motor, HVAC unit, EVSE, or welder?
No. Those loads have additional or different sizing rules. Use the relevant equipment data and a dedicated calculation method.
Sources
- NFPA 70, National Electrical Code (NEC), 2026 edition, §§210.19(A), 210.20(A), and 240.4(B), 240.4(D), with Table 310.16.
- Confirm local adoption, equipment markings, listed assembly ratings, wiring method rules, and current amendments.