National Electrical Code Top Ten Tips:
Article 310 -- Conductors for General Wiring
by Mark Lamendola
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 conductoróthe
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
- NEC Article 310.4 explains the requirements for running conductors
- 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.
- 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. Itís the largest table in the
- 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. Itís 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.
- 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 dropóthe 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 donít 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.
- 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.
- NEC Table 310.17 applies to situations where you have
single-insulated conductors (that doesnít mean a single conductoróit
means the conductor isnít insulated twice, as would be the case if
itís 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
- NEC Tables in Article 311 all have temperature correction factors.
- 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
- 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.
- 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. Itís usually worthwhile to read through
the descriptions to see if one of these applies to your situation.