How to correctly apply NEC Table 310.16 ampacity rules

Conductor ampacity is the ability for the conductor to carry a maximum current, continuously, under certain conditions of use without exceeding the temperature rating of the conductor.

Understanding how to correctly use and apply Table 310.16 is crucial for the electrician to master. The ampacities of conductors in Table 310.16 are subject to certain conditions of use to apply. This month’s column will address and identify some general requirements and identify other requirements for specific conductors in other Articles of the Code.

Compliance with 240.4(D) for small conductor ampacities is required as the asterisk (*) identifies those for copper conductors sized 10 AWG down to 14 AWG. Compliance with 110.14(C)(1)(a) and (b) is also required which basically puts guard rails on conductor ampacities when terminating on equipment rated at 100 amps or less to be sized from the 60°C column of Table 310.16 and if the equipment is over 100 amps then conductors may be selected from the 75°C column. This concept could take another entire column to explain in greater detail.

210.19 you will see requirements for sizing branch circuits and 215.2(A) for feeders conductors. It requires the conductor to be not less than the larger options:

• The loads plus 125% of the continuous loads
• Adjustments and corrections in accordance with 310.15

Adjustments and corrections are the conditions of use. Corrections are for ambient temperature and adjustments are for the number of current carrying conductors greater than 3 installed in the same raceway. The rules of 210.19 and 215.2 have some exceptions when terminations are listed at 90°C but that is another discussion for later. The ampacities of Table 310.16 will only work if the wires are installed within raceways that contain no more than three current carrying conductors and if the ambient temperature does not exceed 86°F (30°C). Both conditions of use must be considered and applied.

Ampacities of flexible cords and cables are located in Article 400 at section 400.5 using Tables 400.5(A)(1) for cables with up to three current carrying conductors and ambient temperatures of 86°F. Table 400.5(A)(2) is for flexible stage and lighting power cables or portable power cables over 2000 volts. This table is also limited to 86°F with columns for one, two or three current carrying conductors. For adjustments of both ampacity Tables use Table 400.5(A)(3) for adjustment factors when the cable contains more than three current carrying conductors.

Table 400.4 describes the intended use of the flexible cable or cord with voltage ratings, wire sizes available, and insulation types. It also shows if listed as a wet or dry use conductor or cord set. A new Part IV was added to the 2023 NEC that has requirements for Portable Power Feeder Cables over 2000 volts nominally. These cables must comply with ampacities located in Table 400.51(A)(1) adjustment factors in Table 400.51(A)(2). Interestingly, there you will find another Table 400.51(A)(3) to make corrections when these cords and cables are wound on a reel with not more than four layers.

Fixture wires are found in Article 402. Table 402.3 has a list of name, letter type, maximum operating temperature and voltage limitations. Fixture wire ampacities are found in Article 402 in section 402.5 and Table 402.5. The maximum number of fixture wires permitted in a raceway or conduit must comply with Table 1 of Chapter 9. I cannot find a table to make adjustments. However, fixture wires are not permitted to be used to serve branch circuits.

For other cable assemblies such as Service Entrance Cables, Nonmetallic-Sheathed Cable (Romex), or Metal-Clad Cable Type MC, to name a few, you must reference ***.80 of the specific Articles to find requirements on how to determine the ampacities of those assemblies. For example, Romex, Type NM-B is manufactured and listed with 90°C insulation. However, the ampacity of this cable assembly must be that of a 60°C. The 90°C is permitted for adjustments and/or corrections. Using Table 310.16 without understanding the specific rules in ***.80 could result in a misapplication with conductors overheating under the conditions of use.

Conductors installed in cable trays are subject to the requirements of 392.22 for the maximum number of conductors and 392.80 for the maximum ampacity of multiconductor cables and single-conductor cables.

Tap conductors are conductors that are conductors other than service conductors that are protected by overcurrent protection ahead of its point of supply that exceeds the values permitted for similar conductors that are protected in accordance with 240.4. Some examples of tap conductors would be:

• Fixture Wires 240.5(B)(2)
• Ranges and Cooking Equipment for Dwellings 210.19
• Feeder and Transformer Secondary Taps 240.21
• Busway Taps 368.17

When it comes down to selecting and calculating for the ampacities of conductors with conditions of use. Most electricians will make adjustments and corrections on a selected conductor by multiplying the ampacity by the adjustment factor or correction factor. This could result in two or more calculations.

20 amps X .82 (Temp) X .80 (CCC) = 13.12 amps

25 X .82 X .80 = 16.4 amps

30 X .82 X .80 = 19.68

35 X .82 X .80 = 22.96 – FINALLY!

This 35 amp rated conductor will carry the 20 amp load under the conditions of use, 8 current carrying conductors in a room that is 115°F.

As you can see, you could calculate again and again with another size or temperature rating insulation until you have calculated to a compliant conductor.

I like to just divide when I do these calculations. The only caveat is to correct 90°C wire using the 90°C column in Table 310.15(B)(1)(1). This will result in a one and done math problem. For example, you have a 35 amp load installed with 8 current carrying conductors in the raceway and an ambient temperature of 110°F. The adjustment factor is 70% and the correction factor for a 90°C wire is 87%. The math looks like this:

Load ÷ CCC% ÷ Temp% = New Load

35 ÷ .7 ÷ .87 = 57.47 amps.

Find a 90°C conductor that can carry 57 amps in Table 310.16, which would be a 6 AWG, and you are done!  I know this can seem complicated but with some practice you can get it right. The National Fire Protection Association (NFPA) published the ampacity flow chart back in 2021  but I cannot seem to find it on their website However, if you send me an email and request it, I will send you the PDF, and then you too can select the correct size conductor for the conditions of use.