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Branch Circuits, Feeders, Conductor Sizing and Overcurrent Protection
This is the largest and most calculation-heavy block on the journeyman exam, and almost every question 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.
Loads, Continuous Loads and the 125 Percent Rule
A continuous load is one expected to run at its maximum current for three hours or more, which describes most commercial lighting, sign circuits and electric heat. Both the conductor and the overcurrent device must be sized at the noncontinuous load plus 125 percent of the continuous load, because standard breakers and terminations are not tested to carry their full rating indefinitely. Noncontinuous loads are added in at 100 percent, so a mixed circuit gets the multiplier applied only to the continuous portion.
Continuous load definition
A load whose maximum current is expected to continue for three hours or more.
2023 NEC Art. 100Branch-circuit conductor sizing
Conductors must have an allowable ampacity of at least the noncontinuous load plus 125 percent of the continuous load.
2023 NEC §210.19(A)Branch-circuit device sizing
The overcurrent device rating must be at least the noncontinuous load plus 125 percent of the continuous load.
2023 NEC §210.20(A)Feeder sizing
Feeder conductors and feeder overcurrent devices follow the same noncontinuous plus 125 percent continuous rule.
2023 NEC §215.2(A)(1)Standard device ratings
Round the calculated value up to a standard rating such as 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 or 110 amperes; values like 55 amperes do not exist.
2023 NEC §240.6(A)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 degrees 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, so a 90 degrees C THHN conductor landing on a 75 degrees C lug is limited to the 75 degrees C column. The higher column may still be used as the starting point for derating calculations, which is the single most misunderstood idea in conductor sizing.
Termination limit, small circuits
Circuits rated 100 amperes or less, or using 14 AWG through 1 AWG conductors, are limited to 60 degrees C ampacities unless the equipment is listed for higher.
2023 NEC §110.14(C)(1)(a)Termination limit, larger circuits
Circuits over 100 amperes or using conductors larger than 1 AWG are limited to 75 degrees C ampacities unless the equipment is listed for higher.
2023 NEC §110.14(C)(1)(b)Small conductor protection
Regardless of table ampacity, overcurrent protection is limited to 15 amperes for 14 AWG, 20 amperes for 12 AWG and 30 amperes for 10 AWG copper.
2023 NEC §240.4(D)Next higher standard device
Where the conductor ampacity does not match a standard rating, the next higher standard device may be used, but only up to 800 amperes.
2023 NEC §240.4(B)NM cable ampacity
Type NM cable ampacity is taken from the 60 degrees C column even though the conductors are rated 90 degrees C.
2023 NEC §334.80Dwelling service allowance
Single-phase dwelling services and main power feeders may be sized from the reduced dwelling conductor table rather than Table 310.16.
2023 NEC §310.12Derating: Ambient Temperature and More Than Three Conductors
Two independent penalties can apply to the same run, and when both apply you multiply them together against the ampacity read from the column matching the conductor insulation. Ambient correction handles hot attics, boiler rooms and rooftops, while the adjustment factor handles the heat that conductors bundled in one raceway impose on each other. After multiplying, the result must still be checked against the termination temperature limit and against the small-conductor protection rules.
Ambient correction
Multiply by the correction factor for the actual ambient temperature; for a 90 degrees C conductor the factor is 0.91 at 36 to 40 degrees C and 0.82 at 46 to 50 degrees C.
2023 NEC §310.15(B)Conductor bundling adjustment
More than three current-carrying conductors in a raceway or cable are adjusted to 80 percent for 4 to 6, 70 percent for 7 to 9, 50 percent for 10 to 20 and 45 percent for 21 to 30.
2023 NEC §310.15(C)(1)Start from the insulation column
Derating begins at the ampacity for the conductor temperature rating, so a THHN conductor derates from the 90 degrees C column even when its terminations are 75 degrees C.
2023 NEC §310.14(A)(2)Counting the neutral
A neutral carrying only the unbalanced current of a wye system is not counted, but a neutral on a circuit whose major portion of load is nonlinear is counted.
2023 NEC §310.15(E)Grounding conductors not counted
Equipment grounding and bonding conductors are never counted as current-carrying conductors.
2023 NEC §310.15(E)(4)Voltage Drop Calculations
Voltage drop is not an enforceable requirement for most branch circuits, but it is a heavily tested calculation because it is the one place the exam asks you to work with conductor resistance directly. The standard approach uses the constant K, roughly 12.9 for copper and 21.2 for aluminum, together with the circular-mil area of the conductor from Chapter 9, Table 8. The recommended targets are 3 percent on a branch circuit and 5 percent for the combined feeder and branch circuit.
Single-phase voltage drop
VD equals 2 times K times I times L divided by circular mils, where L is the one-way length in feet.
2023 NEC Chapter 9, Table 8Three-phase voltage drop
VD equals 1.732 times K times I times L divided by circular mils.
2023 NEC Chapter 9, Table 8Solving for conductor size
Rearrange to circular mils equals 2 times K times I times L divided by the allowable voltage drop, then pick the next larger conductor from the table.
Recommended limits
Three percent on the branch circuit and no more than 5 percent total from the service to the farthest outlet is the widely applied design target.
Dwelling Branch-Circuit Requirements
Residential branch-circuit questions test a small set of memorized numbers rather than calculations, and they show up on every exam. The theme is that certain rooms get dedicated circuits so that a single appliance cannot black out the rest of the house, and that receptacles are spaced so a 6-foot cord always reaches. GFCI and AFCI requirements have expanded steadily and now cover most of a dwelling.
Receptacle spacing
No point along the floor line of any wall space may be more than 6 feet from a receptacle outlet.
2023 NEC §210.52(A)(1)Small-appliance circuits
At least two 20-ampere small-appliance branch circuits must serve the kitchen, pantry, breakfast room and dining room receptacles.
2023 NEC §210.11(C)(1)Laundry and bathroom circuits
A dedicated 20-ampere circuit is required for the laundry receptacles and another for the bathroom receptacles.
2023 NEC §210.11(C)(2)GFCI near sinks
Receptacles within 6 feet of the outside edge of a sink require ground-fault circuit-interrupter protection.
2023 NEC §210.8(A)AFCI coverage
Arc-fault protection is required for 120-volt, 15- and 20-ampere circuits supplying outlets in most habitable rooms, including kitchens and laundry areas.
2023 NEC §210.12(A)Multiwire branch circuits
All ungrounded conductors of a multiwire branch circuit must be disconnected simultaneously at the point where the circuit originates.
2023 NEC §210.4(B)测试你的知识
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