National Electrical Code Top Ten Tips: Article 240 -- Overcurrent Protection

by Mark Lamendola

Based on the 2020 NEC.

Please note, we do quote from copyrighted material. While the NFPA does allow such quotes, it does so only for the purposes of education regarding the National Electrical Code. This article is not a substitute for the NEC.

These are the 10 NEC Article 240 items we deem most important, based on the pervasiveness of confusion and the potential costs of same.

1. General philosophy: Providing overcurrent protection isn't just a matter of selecting a fuse or breaker from a table. When current exceeds the rating of conductors or equipment—due to overload, short circuit, or ground fault—you have overcurrent. This is what you must protect conductors and equipment and equipment against, and to to that you use Overcurrent Protection Devices (OCPDs).
   
2. General background: Article 240, consisting of nine parts, provides the requirements for selecting and installing OCPDs.
   
3. General concept of equipment protection. To protect equipment, an OCPD opens when it detects a short-circuit or ground fault.
   
4. General concept of circuit protection. To protect a circuit, an OCPD opens when current reaches a value that will cause an excessive temperature rise in conductors.
   
5. The Tables in Chapter 3 provide the ampacities you must use for selecting conductors for a given load. Please note the exceptions.
   
6. Motor overload protection and motor circuit protection are not the same thing. See the table at the beginning of Article 430. To size motor branch-circuit conductors, see Table 310.16, 430.22, and Table 430.150. To size motor branch-circuit protection, see 240.6(A), 430.52(C), and Table 430.150.
   
7. Install an OCPD in series with each ungrounded conductor [240.21]. Since the neutral is typically the grounded conductor (at higher voltages, it might not be), you don't install an OCPD there. And you don't install an OCPD in a grounding conductor because it's grounded.
   
8. Location, location, location. Locate OCPDs (and other electrical equipment) to prevent exposure to physical damage [110.27(B)]--but do so in a way that leaves those OCPDs readily accessible [240.24].
   
9. For fuse requirements, see 240.50 through 240.67.
   
10. For breaker requirements, see 240.80 through 240.87.