NEC 110.14 termination temperature ratings, explained

Conductor ampacity feels like a simple table lookup until NEC 110.14(C) enters the picture. This section is the reason you cannot just read the biggest number in the ampacity table and call it done — the termination temperature rating of the devices at each end of the conductor caps how much current the conductor is allowed to carry. It quietly governs almost every solar circuit.

The core rule

110.14(C) says the ampacity of a conductor is limited by the lowest temperature rating of any connected termination, conductor, or device in the circuit. In practice, the weakest link is usually a lug or breaker terminal, and those are rated 60°C or 75°C — not the 90°C that the conductor's insulation might allow.

So even though a THWN-2 conductor is a 90°C insulation, you cannot use the 90°C ampacity as the final rating if it lands on a 75°C lug. The termination wins.

The three columns and how they're really used

The 310.16 ampacity table has 60°C, 75°C, and 90°C columns. Here is the part that trips people up:

• The 90°C column is the starting point for derating — you apply temperature and conduit-fill correction factors to the 90°C value.
• The 60°C or 75°C column sets the final cap — after derating, the conductor's ampacity still cannot exceed its termination-rated column.
• You take the more restrictive of those two results.

A quick solar example

Say a rooftop conduit run pushes the ambient/rooftop temperature correction down and you have several current-carrying conductors sharing the raceway. You:

1. Start at the 90°C ampacity of the conductor. 2. Apply the temperature correction (hot rooftop) and conduit-fill adjustment (see Chapter 9 conduit fill context). 3. Compare the derated value to the 75°C column cap for the terminations. 4. Use the lower of the two as the conductor's usable ampacity.

The 90°C column earns its keep only in step 1 — it gives you headroom to derate from. The final number that must exceed your circuit current is still bounded by the termination.

Why reviewers hammer on this

Undersized conductors are one of the most common solar corrections, and 110.14(C) is usually the reason: a designer reads the 90°C column, sizes to it, and ignores the 75°C lug. The reviewer checks the equipment termination rating against the conductor ampacity and the OCPD from 240.6. If the termination-capped ampacity falls below the breaker, it comes back.

Always verify the actual temperature rating stamped on the equipment — a device may be dual-rated 60/75°C, and using the 75°C value requires the marking to permit it.

One more nuance: 110.14(C) looks at both ends of the conductor and every device in between. If the conductor lands on a 75°C inverter terminal at one end but a 60°C lug at the other, the 60°C rating governs the whole run. When equipment is not marked with a temperature rating at all, the conservative default is to treat it as 60°C. This is why reading nameplates and lug markings, rather than assuming, is part of a defensible conductor design.

OneLine Studio is a design aid, not a permitting authority. Confirm the enforced NEC edition and any local amendments with your AHJ, and have a licensed electrician or PE review, sign and stamp the conductor design before you file or energize. For the full sizing chain, see the NEC reference hub.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.