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Branch Circuit Wire and Breaker Pairing Card

Reference ServiceDelivery Updated Oct 28, 2026 ยท Electrical

Why this matters

Most wiring an electrician does in a week is a copper conductor paired with a breaker, and most of the pairing errors an inspector writes up come from reading the right table in the wrong column. A 12 AWG circuit on a 25 A breaker "because the 75 C column says 25" fails inspection and, worse, leaves a conductor that can carry a sustained overload the breaker will never see. This card is the pairing for 14 through 6 AWG copper in NM cable and in THHN, with the reason each line reads the way it does.

The visor card

Values are amperes, copper conductors, NEC 2023 (2020 where the section moved), as adopted by your jurisdiction with local amendments checked. Table 310.16 supplies the ampacity; 240.4 supplies the limits on the breaker.

COPPER BRANCH CIRCUITS - AMPERES - NEC 2023 AS ADOPTED
AWG | NM CABLE    | THHN, BOTH ENDS | LARGEST
    | 60 C COLUMN | RATED 75 C      | BREAKER
----+-------------+-----------------+-------------------
 14 | 15          | 20 -> held to 15| 15  per 240.4(D)
 12 | 20          | 25 -> held to 20| 20  per 240.4(D)
 10 | 30          | 35 -> held to 30| 30  per 240.4(D)
  8 | 40          | 50              | NM 40, THHN 50
  6 | 55          | 65              | NM 60 *, THHN 60 **
----+-------------+-----------------+-------------------
*  55 A has no standard breaker; 60 A is the next size
   up, allowed only under the 240.4(B) conditions
** 60 A plainly; 70 A only under 240.4(B)
90 C column: a starting point for derating, never the
answer. Derating and ambient: Conductor Ampacity Ref.

Read across one line and the card hands you three things: what the cable can carry, what the same copper can carry as single conductors with 75 C terminations, and the largest breaker you may put on it.

Why 14, 12 and 10 AWG stop at 15, 20 and 30 A whatever the table says

Table 310.16 lists 12 AWG copper at 20 A in the 60 C column, 25 A in the 75 C column and 30 A in the 90 C column. A reader who stops there concludes a THHN 12 AWG run with 75 C breakers can go on a 25 A breaker. It cannot. NEC 240.4(D), as adopted, caps the overcurrent protection for small copper conductors at 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG, unless a section listed in 240.4(E) or 240.4(G) specifically permits otherwise.

The cap is a limit on the breaker, not a statement of ampacity. The distinction matters in two places:

  • Derating. When 12 AWG THHN sits in a raceway with other current-carrying conductors or in a hot attic, you start from the 90 C value (30 A), apply the adjustment and correction factors, and then check the result against both the termination column and the 240.4(D) cap. A derated 12 AWG that still clears 20 A stays on a 20 A breaker. One that comes out below 20 A goes down a breaker size or up a wire size. The factors themselves live in 'Conductor Ampacity Reference'; this card only tells you the cap is waiting at the end.
  • Equipment that has its own rules. 240.4(G) lists the applications where the small-conductor cap does not govern, among them motor circuits under Article 430, air-conditioning and refrigerating equipment under Article 440 and welders under Article 630. That is why a condenser nameplate can call for 12 AWG wire and a 30 A maximum breaker and be correct. Reading that nameplate is its own card: see related 'Reading an Appliance Nameplate for Circuit Sizing'. Outside those listed applications, 12 AWG on anything over 20 A is a violation.

The reason the cap exists is worth knowing because customers ask. Small conductors have little thermal mass and the tables assume ideal terminations. Holding the breaker close to the conductor's real duty keeps an overloaded cord-and-plug circuit, the kind a homeowner builds with a space heater and a power strip, from cooking a splice in a wall before anything trips.

Why NM cable reads from the 60 C column

NM-B conductors are rated 90 C, and the 90 C rating is printed on the jacket. NEC 334.80, as adopted, still limits the cable's ampacity to the 60 C column, with the 90 C rating permitted only as the starting point for derating. The reasoning is the cable assembly and its installation: two or three conductors share one jacket, the cable is often packed in insulation, and the terminations at the device end are frequently 60 C.

For 14, 12 and 10 AWG the 60 C column and the 240.4(D) cap land on the same numbers, so nothing changes. For 8 and 6 AWG they part ways, and that is where the field mistakes are:

  • 8 AWG NM is 40 A. Not 50. The 50 A figure is 8 AWG THHN at 75 C, which is a different wiring method.
  • 6 AWG NM is 55 A. There is no 55 A breaker, so 60 A is permitted only under 240.4(B): the conductors are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads, the ampacity does not match a standard rating (55 A does not), and the next standard rating does not exceed 800 A. A 6 AWG NM run to a single range or a single 14-50 receptacle meets that. A 6 AWG NM run feeding two 50 A receptacles for a shop's portable welders does not, and holds at 50 A.

When THHN may use the 75 C column

Single conductors in a raceway can use the 75 C column only when both ends of the circuit are rated 75 C. NEC 110.14(C)(1)(a), as adopted, sets 60 C as the assumption for circuits rated 100 A or less, or marked for 14 through 1 AWG, unless the equipment is listed and identified for 75 C. Most current breakers and many devices are marked 60/75 C, but read the marking on the device you are actually terminating to; an older receptacle or a disconnect without the marking drops the circuit to the 60 C column, which puts 8 AWG THHN back at 40 A and 6 AWG at 55 A. The termination rule is its own card: see related 'The Termination Temperature Rule (110.14(C))'.

Receptacles on the circuit

The breaker pairing has a third partner when the circuit ends in a receptacle. A single receptacle on an individual branch circuit must be rated not less than the circuit (210.21(B)(1)). Where a circuit serves two or more receptacles, Table 210.21(B)(3) pairs them: a 15 A circuit takes 15 A receptacles, a 20 A circuit takes 15 or 20 A receptacles, a 30 A circuit takes 30 A, a 40 A circuit takes 40 or 50 A, a 50 A circuit takes 50 A. The 40 A row is the one that surprises people: a 14-50 receptacle on a 40 A breaker and 8 AWG NM is a legal pairing.

Worked example: a cooktop swap on an existing circuit

A customer replaces a resistive cooktop with an induction unit. The old circuit is 8 AWG copper NM from the panel to a junction box under the counter, on a 2-pole 40 A breaker. The new unit's installation instructions call for a 50 A circuit.

  1. What the existing cable can carry. 8 AWG NM reads from the 60 C column: 40 A. The cable is already at its limit on the 40 A breaker.
  2. The tempting shortcut. Swap the 40 A breaker for a 50 A breaker because "8 gauge is good for 50." That number belongs to 8 AWG THHN with 75 C terminations at both ends. In NM it is 40 A, and a 50 A breaker on 40 A cable is a 125 percent mismatch (50 divided by 40 is 1.25) that no inspector will pass.
  3. Is there a path to keep the cable? Only if the manufacturer's instructions allow a 40 A circuit. Some induction cooktops list both a 40 A and a 50 A option depending on the configuration or a jumper setting; the instructions decide that, not the electrician. This one does not, so the answer is new cable.
  4. New cable. 6 AWG copper NM reads 55 A from the 60 C column. On a 50 A breaker that is a plain pairing, no 240.4(B) step needed because 50 is below 55.
  5. The receptacle question. The cooktop is hardwired to a junction box, so 210.21 does not apply. Had it been cord-connected through a 14-50, the single receptacle on an individual circuit would need a 50 A rating, which a 14-50 has.
  6. Before you pull the old cable, turn the breaker off, lock it out, and verify absence of voltage at the junction box with a CAT III or better meter proven on a known live source before and after the test, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024) and 29 CFR 1910.333(b)(2). The finished circuit: 6 AWG copper NM, 2-pole 50 A breaker, both lugs torqued to the value marked on the breaker and the cooktop's terminal block with a calibrated tool (110.14(D)). With the covers back on, close the breaker standing to the side of the panel and confirm about 240 V across the cooktop terminals before handing it over.

The check that ties it off: 50 A breaker, 55 A cable, 50 A appliance requirement. Breaker at or below conductor ampacity, appliance requirement met, no exception relied on.

Two pairings that come back from inspection

12 AWG on a 25 A breaker for a dedicated 120 V shop circuit. The installer read 25 A from the 75 C column. 240.4(D) holds 12 AWG at 20 A because a general-purpose receptacle circuit is not one of the 240.4(G) applications. The fix is the 20 A breaker, or 10 AWG if the shop really needs more than 20 A of receptacle capacity, in which case it becomes a 30 A circuit with 30 A receptacles under Table 210.21(B)(3).

10 AWG on a 40 A breaker for a water heater. The installer read 40 A from the 90 C column. The 90 C value is a derating start point only, and 240.4(D) holds 10 AWG at 30 A. A water heater is a continuous load sized at 125 percent, which is its own card: see related 'Continuous Load and the 125 Percent Rule'.

What moves a pairing off the card

  • A hot location. An attic or a rooftop raceway pulls the ampacity down through the ambient correction factor; a 12 AWG run that clears 20 A in a wall may not in an attic. Run the numbers in 'Conductor Ampacity Reference' and then come back to the cap.
  • Bundled cables. More than two NM cables through one bored hole that will be sealed with caulk, foam or insulation trigger the 334.80 adjustment, as adopted. See related 'NM Cable Support, Protection, and Clearance Rules'.
  • Aluminum. 240.4(D) carries separate caps for 12 and 10 AWG aluminum, and the ampacities are a different column. This card is copper only.
  • Long runs. Voltage drop is not a code ampacity limit for branch circuits in most adoptions, but a 200 ft run to a well pump or a detached shop may need a size up that this card does not show.

References

  • NFPA 70, National Electrical Code, 2023 edition as adopted by your jurisdiction: 110.14(C), 110.14(D), 210.21(B), 240.4(B), 240.4(D), 240.4(G), Table 310.16, 334.80
  • NFPA 70E, Standard for Electrical Safety in the Workplace, current edition, Article 120; 29 CFR 1910.333(b)(2)
  • See related: Conductor Ampacity Reference
  • See related: Continuous Load and the 125 Percent Rule; Reading an Appliance Nameplate for Circuit Sizing
  • See related: The Termination Temperature Rule (110.14(C))

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