National Electrical Code Top Ten Tips:
Article 310, Conductors for General Wiring

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 310 items we deem most important, based on the pervasiveness of confusion and the potential costs of same.

Article 310 is arguably the most misunderstood and misapplied area of the National Electrical Code. The same claim can be made for Article 250 (grounding) and Article 430 (motors). Our intent here is to help clear up some of the mystery, but a true understanding requires dedicated study. Do keep in mind ampacity is the amperage capacity of a conductor—the lower the ampacity, the larger the conductor must be to handle a given current (though this is in a step fashion, as conductors come in standard sizes).

1. NEC Article 310.4 explains the requirements for running conductors in parallel.
    
2. NEC Table 310.5 gives the minimum size for current-carrying conductors, with 10 exceptions noted. This table is based on voltage ranges, only. You can expect ampacity issues to require a larger conductor than the size shown.
    
3. NEC Article 310.8 lists which conductor insulation types you may use for locations other than dry locations. When using the conductor tables for ampacity-based conductor sizing, you need to limit your selections per this listing. You must also consider NEC Article 310.9 and NEC Article 310.10 when using those tables. NEC Table 310.13 also contains the information of these three articles, in more detail and in a tabular format. It’s the largest table in the NEC.
    
4. NEC Table 310.13 is a treasure-trove of information. You must use this table to select the cable type(s) appropriate for your particular installation. It’s typical to make a "short list" of those types that meet the installation needs and then select based on price, workability of the insulation, and availability of the wire in sufficient quantities for your project. The circular mils information allows you to calculate wire fill, which is another consideration to add to the previous one. A thinner jacket allows you to put more wires in a smaller space, and this aspect may produce a significant cost variance in wireway, supports, enclosures, and labor.
    
5. NEC Article 310.15 addresses ampacity in great detail. You must cross-reference this information to NEC Table 310.5, as NEC Article 310.15 does not account for voltage. Nor does NEC Article 310.15 account for voltage drop—the wire size the NEC recommends might be too small for efficient operation. Also, you can get more than one ampacity from the tables or your calculations. In such cases, (A)(2) instructs you to use the lower of two or the lowest of more than two. That is, you must assume less ampacity than the more favorable calculation(s) show. You must adjust (derate) based on the number of current-carrying conductors in a given wireway, as well. NEC Article 310.15(B)(2) explains this and the exceptions. NEC Table 310.15(B)(2)(a) shows these in a very clear format. Another important point is you don’t count a grounding or bonding conductor when using this table. NEC Table 310.15(B)(6) shows conductor types and sizes for dwelling services and feeders.
    
6. NEC Table 310.16 applies to situations where you have three or less current-carrying conductors in a single wireway. This is typical for services and feeders, but not very typical for branch circuits. You must select from the column that shows the cable (identified by the insulating material letter designation) you intend to use, and choose between copper and aluminum.
    
7. NEC Table 310.17 applies to situations where you have single-insulated conductors (that doesn’t mean a single conductor—it means the conductor isn’t insulated twice, as would be the case if it’s in an insulated sheath with other conductors) in free air. This is typical for branch circuits. You must select from the column that shows the cable (identified by the insulating material letter designation) you intend to use, and choose between copper and aluminum.
    
8. NEC Tables in Article 311 all have temperature correction factors.
    
9. NEC Article 311 Tables are a great source of confusion, but need not be. What really helps is understanding their arrangement and purposes. These tables are arranged in logical order, assuming you are using typical wiring methods for each application.

• NEC Table 310.16 is typically for service entrances.
• NEC Table 310.17 is typically for branch circuits.
• NEW Table 310.16 and NEC Table 310.17 are your basic ampacity tables.
• NEC Table 310.18 is NEC Table 310.16, but for higher temperatures.
• NEC Table 310.19 is NEC Table 310.17, but for higher temperatures.
• NEC Table 320 is Table NEC Table 310.17, but for cables supported on a messenger.

10. NEC Tables 310.21 through 310.86 are special circumstance tables. They typically apply to services and generally allow for more ampacity in a given cable. It’s usually worthwhile to read through the descriptions to see if one of these applies to your situation.