The Termination Temperature Rule (110.14(C))

Why this matters

THHN is a 90 C conductor, and the 90 C column of NEC Table 310.16 is the most flattering column in the book. A tech who sizes from it lands one size small on a lot of circuits, and the install looks fine at rough-in and at final. The failure shows up years later as a breaker that runs hot, insulation browned an inch back from the lug, and nuisance trips on a thermal-magnetic breaker that is reacting to its own terminal heat. NEC 110.14(C) (NEC 2023, 2020 where the section moved, as adopted, local amendments checked) is the rule that stops it: the conductor is sized from the column its weakest termination allows, whatever the insulation is rated.

What the rule actually says

In substance (check the current edition for the wording):

• Equipment for circuits rated 100 A or less, or marked for 14 AWG through 1 AWG conductors (110.14(C)(1)(a)): the terminations are treated as 60 C unless the equipment is listed and identified for higher. A conductor with a higher insulation rating may be used, but its ampacity is taken from the 60 C column for the size used. Motors marked design B, C or D are the exception: 75 C conductors may be used at their 75 C ampacity.
• Equipment for circuits over 100 A, or marked for conductors larger than 1 AWG (110.14(C)(1)(b)): treated as 75 C unless listed and identified for higher.
• Equipment marked 75 C at its terminals lets you use the 75 C column, provided the conductor is also rated at least 75 C and every other termination on that conductor allows it.
• Separately installed pressure connectors (a split bolt, a power distribution block) are used at no more than their own listed temperature rating (110.14(C)(2)).

The 90 C column still has a job. It is the starting point for adjustment and correction when you derate for ambient or for bundling, and the result is then capped at the termination column. That arithmetic belongs to 'What Derating Is, and Why a Conductor Rating Is Conditional'; this card is only the cap.

Why the lug cares

A terminal is tested by its listing standard with a conductor of a particular size attached, and that conductor is part of how the lug sheds heat. A 60 C rated lug was proven running at its rated current with a conductor sized from the 60 C column. Land a conductor one size smaller, sized from the 90 C column, and the same current now flows through less copper with less mass and surface to pull heat out of the joint. The lug runs hotter than it was ever tested at, the conductor insulation nearest the lug runs hottest of all, and the breaker's thermal element, which sits right behind the terminal, reads the extra heat as overload.

That is why the rule is about the termination and not the wire. The conductor's own insulation would survive 90 C. The joint and the device it is bolted to would not.

The weakest end governs the whole conductor

A conductor has two ends and sometimes a splice in the middle. The column you size from is the lowest rating among:

• the breaker or fuse terminal at the panel,
• the terminal at the load: the equipment's lug, the device's screw, the disconnect's terminal,
• any separately installed connector along the run,
• the conductor's own insulation rating,
• the cable's own limit, where the wiring method sets one. NM is held to the 60 C column regardless of its 90 C conductors (334.80), and so is UF (340.80).

So a 75 C breaker feeding equipment with unmarked terminals is a 60 C circuit. A breaker and equipment both marked 75 C, fed with NM cable, is still a 60 C circuit. The 75 C column is available only when everything on that conductor says 75 C or better.

Reading the marking

The marking is on the device. A common breaker label reads like this:

     2-POLE CIRCUIT BREAKER   (illustrative label)
  +---------------------------------------------+
  |  50 A     120/240 V AC                      |
  |  10,000 A INTERRUPTING RATING               |
  |  USE CU OR AL WIRE                          |
  |  60/75 C                                    |
  |  WIRE RANGE AND TORQUE: SEE PANEL LABEL     |
  +---------------------------------------------+

  READ:  60/75 C = terminal is listed for use at
         the 75 C column with 75 C or better wire;
         with 60 C wire, use the 60 C column.
  NEXT:  find the LOAD end's marking before you
         pick a column. Unmarked = 60 C.

What the markings mean:

• "60/75 C" means the terminal is suitable for either: it may be used at the 75 C column with conductors rated 75 C or better, or with 60 C conductors at the 60 C column. It does not by itself mean the circuit may use the 75 C column.
• "75 C" alone means 75 C or better conductors at the 75 C column.
• "60 C" or no marking on equipment for circuits of 100 A or less: the 60 C column.
• Device terminals (receptacles, switches, small disconnects) are often marked on the back or in the instructions. Look before you assume, and if you cannot find a marking, the rule assumes 60 C.

Torque goes with the same label. Terminations are torqued to the value marked on the device or in its instructions with a calibrated tool (110.14(D)); a correctly sized conductor in an under-torqued lug fails the same way an undersized one does.

Worked example: a 50 A cooktop circuit in EMT

The job. A 240 V electric cooktop in a remodel. The office has already worked the nameplate and set the branch circuit at a 50 A breaker (that derivation belongs to 'Reading an Appliance Nameplate for Circuit Sizing'). The run is 40 ft of 3/4 in EMT, so individual THHN/THWN-2 conductors, 90 C insulation. Copper. Ambient is ordinary interior air and there are three current-carrying conductors at most in the raceway, so no derating applies.

The candidate conductor sizes, copper, from Table 310.16:

Step 1, the tempting answer. 8 AWG THHN at 90 C is 55 A, comfortably over 50 A. A tech who stops here pulls 8 AWG.

Step 2, the panel end. The 50 A breaker is marked 60/75 C. The panel end allows the 75 C column with 75 C or better wire. THHN qualifies.

Step 3, the load end. The cooktop's junction box connections are made to its supply leads with listed connectors, and the instructions call for conductors rated at least 75 C and give the minimum size. Say the instructions read: "Supply conductors: copper, 75 C minimum, 8 AWG." The load end allows the 75 C column.

Step 4, the weakest end. Breaker 75 C, load 75 C, conductor 90 C, no cable limit because this is a raceway with individual conductors. The governing column is 75 C. 8 AWG at 75 C is 50 A, equal to the 50 A breaker. 8 AWG passes, with no margin.

Step 5, the same job with an unmarked load end. Change one thing: the cooktop instructions say nothing about conductor temperature. The equipment is for a circuit of 100 A or less, so it is treated as 60 C. 8 AWG at 60 C is 40 A, which does not support a 50 A breaker. The answer moves to 6 AWG: 55 A at 60 C, over 50 A, and it also meets the 75 C requirement if one appears later.

Step 6, the same job in NM. Change a different thing: the run is 6-3 NM with ground instead of EMT. NM is held to the 60 C column whatever the terminals say. 6 AWG at 60 C is 55 A, so 6-3 NM supports the 50 A breaker; 8-3 NM, at 40 A, would not.

What the example shows. The same 50 A cooktop needs 8 AWG in one install and 6 AWG in the other two, and nothing about the cooktop's current changed. What changed was a line in the instructions and the wiring method. The 90 C column never decided anything.

The small-conductor cap sits on top

For 14, 12 and 10 AWG copper, 240.4(D) caps the overcurrent device at 15 A, 20 A and 30 A respectively, regardless of what any column says. 10 AWG THHN is 40 A at 90 C and 35 A at 75 C, and it still goes on a 30 A breaker at most, outside the tap rules of 240.4(E) and the specific applications 240.4(G) lists (motors, air-conditioning equipment and the like). So for the small sizes the termination rule rarely bites; 240.4(D) bites first. It starts deciding things at 8 AWG and up, which is exactly where ranges, cooktops, water heaters, subfeeds and EV circuits live.

What going one size small looks like in the field

The tell is heat at the termination, not anywhere along the run:

• Insulation darkened or stiff within an inch or two of the breaker lug, while the rest of the conductor looks new.
• A breaker that trips on a long load (a self-clean cycle, a long EV charge) and resets fine. The load did not change; the terminal heat did.
• The breaker body warm to the back of a hand with the panel cover on, compared with its neighbors on the same bus.

Check it de-energized. Before reaching into the panel, de-energize, lock out and verify absence of voltage with a CAT III or better meter proven on a known live source before and after (NFPA 70E Article 120, current edition; 29 CFR 1910.333(b)(2)). The fix is the conductor, not a re-torque: a breaker whose lug has run hot cannot be trusted, so replace the breaker along with the conductor and re-torque the new terminations to their marked value. When the circuit is restored, stand to the side as the breaker closes and confirm the load runs with the breaker cool after its first long cycle.

References

• NFPA 70, National Electrical Code, 110.14(C) and (D), Table 310.16, 240.4(D), 334.80, 340.80 (NEC 2023, 2020 where the section moved), as adopted by your jurisdiction.
• Equipment and device installation instructions and labels for terminal temperature ratings and torque.
• NFPA 70E, Article 120 (current edition); 29 CFR 1910.333(b)(2).
• See related: What Derating Is, and Why a Conductor Rating Is Conditional; Branch Circuit Wire and Breaker Pairing Card; Reading an Appliance Nameplate for Circuit Sizing.