Almost every question in this chapter comes down to the same chain of reasoning: find the load, apply the continuous-load multiplier, pick a conductor from the ampacity table, adjust it for heat, then choose an overcurrent device. Master that sequence and you will answer branch-circuit, feeder, and voltage-drop questions with the same handful of moves. Learn the chain, not a hundred separate facts.

Loads, continuous loads, and the 125 percent rule

A continuous load is one whose maximum current is expected to continue for three hours or more (2023 NEC Art. 100). That describes most commercial lighting, sign circuits, and electric heat. The rule that flows from it is the single most-tested idea on the exam: both the conductor and the overcurrent device must be sized for the noncontinuous load plus 125 percent of the continuous load. Standard breakers and the terminations they land on are not tested to carry their full rating indefinitely, so the code builds in a 25 percent cushion on the part of the load that never lets up.

The multiplier applies only to the continuous portion. Noncontinuous load is added in at 100 percent. So a mixed circuit is: noncontinuous amps + (1.25 × continuous amps). Conductor minimum ampacity uses this rule (2023 NEC §210.19(A) for branch circuits, §215.2(A)(1) for feeders), and the overcurrent device minimum uses the same rule (§210.20(A) for branch circuits, §215.3 for feeders).

Once you have the calculated minimum device size, you round up to a standard rating. The standard ampere ratings in 2023 NEC §240.6(A) are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, and up. A value like 55 amperes is not a standard rating and does not exist as a breaker — a favorite trap.

Reading the ampacity table and respecting termination temperature

Table 310.16 gives the allowable ampacity of insulated conductors in a raceway, cable, or earth at a 30 °C ambient with not more than three current-carrying conductors. The column you finish in is set by the temperature rating of the terminations, not by the insulation printed on the wire. A 90 °C THHN conductor that lands on a 75 °C lug is limited to the 75 °C column. The higher column may still be used as the starting point for derating — this split (finish at the termination column, start derating from the insulation column) is the most misunderstood idea in all of conductor sizing.

Two termination limits control most jobs (2023 NEC §110.14(C)(1)):

Circuits 100 A or less, or using 14 AWG through 1 AWG, are limited to the 60 °C ampacities unless the equipment is listed for higher.
Circuits over 100 A, or using conductors larger than 1 AWG, are limited to the 75 °C ampacities unless listed for higher.

Two more rules protect small conductors and let you round devices up:

Small-conductor protection (§240.4(D)): regardless of table ampacity, overcurrent protection is limited to 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG copper. This overrides the table for these three sizes.
Next higher standard device (§240.4(B)): where a conductor's ampacity does not land on a standard rating, you may protect it with the next higher standard device — but only up to 800 A.

Type NM cable ampacity is always taken from the 60 °C column even though its conductors are rated 90 °C (§334.80). Single-phase dwelling services and main power feeders may be sized from the reduced conductor table in §310.12 rather than Table 310.16.

Derating: ambient temperature and more than three conductors

Two independent penalties can stack on the same run, and when both apply you multiply them together against the ampacity read from the column that matches the conductor's insulation (not its termination):

Ambient correction (§310.15(B)) handles hot attics, boiler rooms, and rooftops. For a 90 °C conductor the factor is 0.91 at 36–40 °C and 0.82 at 46–50 °C (read the actual factor from the table for the actual ambient).
Conductor bundling adjustment (§310.15(C)(1)) handles the heat bundled conductors impose on each other: 80% for 4–6 current-carrying conductors, 70% for 7–9, 50% for 10–20, 45% for 21–30.

Start from the insulation column (§310.14(A)(1); the "lowest value applies" rule that reconciles this with the 75 °C terminations is §110.14(C)): a THHN conductor derates from the 90 °C value even when its terminations are 75 °C. After multiplying, the result must still be checked against the termination-temperature limit and the small-conductor protection rule. Two counting rules keep the "number of current-carrying conductors" honest: a neutral that carries only the unbalanced current of a wye system is not counted, but a neutral on a circuit whose major portion of load is nonlinear (electric-discharge lighting, electronics) is counted (§310.15(E)); equipment grounding and bonding conductors are never counted (§310.15(F)).

Voltage drop

Voltage drop is not an enforceable requirement for most branch circuits — the NEC states recommended maximums in informational notes, not mandatory rules. But it is heavily tested, because it is the one place the exam makes you work with conductor resistance directly. The standard method uses the constant K (about 12.9 for copper, 21.2 for aluminum) and the conductor's circular-mil area from Chapter 9, Table 8.

Single-phase: VD = (2 × K × I × L) ÷ circular mils, where L is the one-way length in feet.
Three-phase: VD = (1.732 × K × I × L) ÷ circular mils.
Solving for size: circular mils = (2 × K × I × L) ÷ allowable VD, then pick the next larger conductor from the table.

The widely applied design target is 3% on a branch circuit and 5% total from the service to the farthest outlet.

Dwelling branch-circuit requirements

Residential questions test a small set of memorized numbers rather than calculations, and they appear on every exam:

Receptacle spacing (§210.52(A)(1)): no point along the floor line of any wall space may be more than 6 feet from a receptacle (so a 6-foot cord always reaches).
Small-appliance circuits (§210.11(C)(1)): at least two 20-A small-appliance branch circuits serve the kitchen, pantry, breakfast room, and dining room receptacles.
Laundry and bathroom (§210.11(C)(2), (3)): a dedicated 20-A circuit for laundry receptacles and another 20-A circuit for bathroom receptacles.
GFCI near sinks (§210.8(A)): receptacles within 6 feet of the outside edge of a sink require GFCI protection (the 2020 and 2023 editions expanded GFCI coverage — confirm your edition).
AFCI coverage (§210.12(A)): arc-fault protection for 120-V, 15- and 20-A circuits supplying outlets in most habitable rooms, including kitchens and laundry areas.
Multiwire branch circuits (§210.4(B)): all ungrounded conductors must be disconnected simultaneously at the point where the circuit originates.

Key numbers & facts — Chapter 1 (2023 NEC; confirm your adopted edition) - Continuous load = maximum current for 3 hours or more — Art. 100 - Sizing rule (conductor and device): noncontinuous + 125% × continuous — §210.19(A), §210.20(A), §215.2(A)(1), §215.3 - Standard device ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110… — §240.6(A); 55 A is NOT standard - Next higher standard device allowed up to 800 A — §240.4(B) - Small-conductor protection: 14 AWG → 15 A, 12 AWG → 20 A, 10 AWG → 30 A (copper) — §240.4(D) - Termination limits: ≤100 A or 14–1 AWG → 60 °C; >100 A or larger than 1 AWG → 75 °C — §110.14(C)(1) - Table 310.16 basis: 30 °C ambient, ≤3 current-carrying conductors - NM cable uses the 60 °C column — §334.80 - Bundling adjustment: 4–6 → 80%, 7–9 → 70%, 10–20 → 50%, 21–30 → 45% — §310.15(C)(1) - Voltage drop: single-phase 2·K·I·L / CM; three-phase 1.732·K·I·L / CM; K ≈ 12.9 Cu / 21.2 Al; targets 3% branch, 5% total - Dwelling: receptacle within 6 ft; two 20-A small-appliance circuits; 20-A laundry and bath; GFCI within 6 ft of a sink

Worked example — the full sizing chain

A 240-volt, single-phase electric heater draws 40 amperes and runs continuously. It is wired with copper THWN to 75 °C terminations, in a raceway with no more than three conductors at 30 °C. Size the conductor and the branch-circuit breaker.

Continuous load, so apply 125%: 40 A × 1.25 = 50 A minimum for both the conductor ampacity and the device.
Conductor: in the 75 °C column of Table 310.16, 8 AWG copper is 50 A — exactly meets the 50 A minimum. (6 AWG at 65 A would also work but is oversized for the minimum asked.)
Device: 50 A is itself a standard rating (§240.6(A)), so the breaker is 50 A. No rounding needed.
Check small-conductor rule: 8 AWG is above the 14/12/10 AWG limits, so §240.4(D) does not cap it. Done: 8 AWG Cu on a 50 A breaker.

Notice the order never changes: multiply the continuous load, read the termination column, then land on a standard device.

Named exam traps

"55-ampere breaker." There is no 55 A standard rating. If your calculation lands between standards, round up to the next one in §240.6(A) (subject to the §240.4(B) 800 A ceiling). Watch also for 90 A looking "close enough" when the answer is a real standard size.
Wrong column. THHN is a 90 °C wire, but if it lands on a 75 °C lug you size from the 75 °C column. The 90 °C column is only the starting point for derating, never the finish for a termination.
Applying 125% to the whole load. The multiplier hits only the continuous part. On a mixed circuit, add the noncontinuous load at 100%.
Forgetting small-conductor protection. Even if a derated 12 AWG "calculates" to 25 A, §240.4(D) still caps its breaker at 20 A.
Counting the wrong conductors when derating. Don't count a wye neutral that carries only unbalanced current, and never count the EGC — but do count the neutral when the load is majority nonlinear.
Voltage-drop length. Use the one-way run length L in the formula; the factor of 2 (single-phase) already accounts for the return conductor.

Branch Circuits, Feeders, Conductor Sizing and Overcurrent Protection