Guide
NEC Sizing Disclosure and Limits
Screening tools; AHJ governs.
These tools apply common NEC sizing rules such as 125 percent continuous loading, motor rules in Article 430, and raceway fill in Chapter 9. They do not replace an engineered review.
Formulas
Continuous sizing
\[I_{sized} = 1.25 \times I_{load}\]
Continuous means 3 hours or more; protection steps up to the next standard size.
Multi-motor feeder
\[I_{feeder} = 1.25 \times I_{largest} + \sum I_{other}\]
NEC 430.24 pattern for several motors plus other continuous load at 125 percent.
Worked example
Reading a breaker result
Inputs: 16A continuous load.
Calculation: Sized load is 1.25 x 16 = 20A, so the next standard size is a 20A device.
Lesson:
Frequently asked questions
Is a calculator output an approval?
No. It is a screening number, not a permit, quote, or guarantee. The AHJ and the adopted code govern.
Which code edition applies?
The edition adopted locally with its amendments. Confirm it before ordering equipment.
Sources and further verification
- NFPA 70, National Electrical Code National Fire Protection Association
- Understanding the NEC NFPA
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