Wiring & Protection

Learning Objectives

By the end of this chapter, you should be able to:

4.Identify the scope and structure of NEC Chapter 2, including which installations are exempt.

5.Apply the general requirements for grounding and bonding, including system grounding, enclosure grounding, and equipment grounding conductors.

6.Determine the correct sizing of branch circuits, feeders, and service conductors based on ampacity and voltage drop.

7.Select the proper overcurrent protection device (OCPD) for conductors and equipment, including the "next size up" rule.

8.Calculate feeder and service loads using standard and optional calculation methods.

9.Navigate to the correct NEC tables for conductor ampacity, correction factors, and adjustment factors.

10.Recognize common field errors and exam traps related to Chapter 2 requirements.

1.1 Scope and Structure of Chapter 2

Article 200 through 285 of the NEC comprise Chapter 2, titled "Wiring and Protection." This chapter is the backbone of any electrical installation because it governs the safety envelope of the system: how electricity is grounded, how conductors are sized, and how they are protected from overcurrent.

The chapter is organized into logical groups:

Article 200 – Use and identification of grounded conductors (the neutral).

Article 210 – Branch circuits (general requirements, ratings, and outlets).

Article 215 – Feeders.

Article 220 – Branch-circuit, feeder, and service load calculations.

Article 225 – Outside branch circuits and feeders.

Article 230 – Services (service entrance conductors, disconnects, and masts).

Article 240 – Overcurrent protection (fuses and breakers).

Article 250 – Grounding and bonding (the largest article in the Code).

Article 285 – Surge-protective devices (SPDs).

Exam Trap: Many candidates confuse Article 210 (branch circuits) with Article 215 (feeders). Remember: a branch circuit is the wiring between the final overcurrent device and the outlet(s). A feeder is the wiring between the service equipment and the branch-circuit OCPD.

1.2 Branch Circuits (Article 210)

A branch circuit is defined as the conductors between the final overcurrent device protecting the circuit and the outlet(s). The NEC sets minimum ratings for branch circuits based on the connected load.

1.2.1 General Requirements (210.19, 210.20)

Branch-circuit conductors must have an ampacity of not less than the maximum load they serve.

The rating of the branch-circuit OCPD must not exceed the ampacity of the conductor, except for the specific "next size up" allowance (see Section 1.6).

For circuits supplying continuous loads (loads expected to operate for 3 hours or more), the OCPD and conductor ampacity must be sized at 125% of the continuous load, plus 100% of the noncontinuous load.

Field Point: When installing a 20 A circuit for a continuous load like a heater, you must use a conductor rated for at least 25 A (20 A × 1.25). That means 12 AWG copper (rated 20 A at 60 °C) is not sufficient; you would need 10 AWG or a conductor with a higher temperature rating if terminations permit.

1.2.2 Required Outlets (210.52)

For dwelling units, the NEC mandates specific outlet locations:

Receptacle outlets in every habitable room must be spaced so that no point along the floor line is more than 6 ft from a receptacle (measured horizontally). This effectively requires a receptacle every 12 ft.

Kitchen counters require at least two small-appliance branch circuits (20 A) for countertop receptacles.

Bathrooms require at least one 20 A branch circuit dedicated to receptacle outlets, and all receptacles must be GFCI-protected.

Laundry areas require at least one 20 A branch circuit for the receptacle.

1.3 Feeders (Article 215)

Feeders carry power from the service equipment or a downstream panelboard to a branch-circuit panelboard or a specific load. The minimum feeder ampacity is the sum of the noncontinuous loads plus 125% of the continuous loads supplied by the feeder.

Exam Trap: The 125% factor for continuous loads applies to both the conductor ampacity and the OCPD rating. Do not apply it twice to the same load.

1.4 Services (Article 230)

Service conductors connect the utility supply to the service disconnecting means. Key requirements:

Service disconnects must be grouped and clearly labeled. Each disconnect must be rated for the available fault current.

Service conductors must have an ampacity of not less than the calculated load, and must not be smaller than 8 AWG copper or 6 AWG aluminum for residential services (unless the load calculation permits smaller, which is rare).

Service masts used as supports must be of adequate strength and must be installed per 230.28.

Field Point: The service entrance conductor ampacity is based on the calculated load, not the sum of the branch-circuit OCPDs. A 200 A service does not require 200 A of conductor if the calculated load is only 150 A, but in practice, most AHJs require the conductor to match the service rating.

1.5 Load Calculations (Article 220)

Load calculations determine the minimum service or feeder size. The NEC provides two methods: the standard method (220.40 through 220.61) and the optional method (220.82 through 220.85) for dwelling units and certain other occupancies.

1.5.1 Standard Method – Dwelling Unit (220.42, 220.52)

General lighting and receptacle loads: 3 VA per sq ft of living area.

Small-appliance circuits: 1,500 VA each (minimum two).

Laundry circuit: 1,500 VA.

Appliances: Nameplate ratings for ranges, ovens, dryers, water heaters, etc.

Demand factors from Table 220.42 apply to the general lighting load (e.g., the first 3,000 VA at 100%, the next 117,000 VA at 35%, etc.).

1.5.2 Optional Method – Dwelling Unit (220.82)

This method is simpler and often yields a smaller service. It uses the total connected load (all loads at 100%) and applies a single demand factor based on the first 10 kVA at 100%, the next 10 kVA at 40%, and the remainder at 30%.

Exam Trap: The optional method is only permitted for dwelling units with a total connected load served by a single 3-wire, 120/240 V or similar system. It cannot be used for commercial or industrial occupancies.

1.6 Overcurrent Protection (Article 240)

Overcurrent protection devices (fuses and circuit breakers) protect conductors and equipment from excessive current. The OCPD must:

Have an ampere rating not less than the load.

Have an ampere rating not greater than the conductor ampacity, unless the "next size up" rule applies.

1.6.1 The "Next Size Up" Rule (240.4(B))

If the conductor ampacity does not correspond to a standard OCPD rating (15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000 A), you may use the next higher standard size, provided:

The OCPD does not exceed 800 A.

The conductor is not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.

Example: A 10 AWG copper conductor rated 35 A at 75 °C can be protected by a 40 A breaker, because 35 A is not a standard size and 40 A is the next standard size up.

Field Point: This rule does not apply to branch circuits supplying receptacles. A 15 A receptacle circuit must use a 15 A breaker, even if the wire is 12 AWG.

1.7 Grounding and Bonding (Article 250)

Article 250 is the most heavily tested article in the NEC. The core concepts are:

Grounding: Connecting to the earth (via a grounding electrode) to limit voltage surges and stabilize the system.

Bonding: Connecting metallic parts to establish electrical continuity and ensure fault current has a low-impedance path back to the source.

1.7.1 System Grounding (250.20, 250.24)

A grounded system (e.g., 120/240 V single-phase) must have the grounded conductor (neutral) connected to the grounding electrode at the service.

The grounding electrode conductor (GEC) connects the grounded conductor to the grounding electrode (e.g., ground rod, concrete-encased electrode, water pipe).

The GEC must be sized per Table 250.66 based on the size of the largest ungrounded service conductor.

1.7.2 Equipment Grounding Conductors (EGC) (250.118, 250.122)

The EGC bonds non-current-carrying metal parts (enclosures, raceways, equipment) to the system grounded conductor.

The EGC is sized per Table 250.122 based on the rating of the OCPD protecting the circuit.

Field Point: A 20 A circuit requires a minimum 12 AWG copper EGC (or 10 AWG aluminum). Do not confuse the EGC with the GEC; they are sized differently.

1.7.3 Bonding Jumpers (250.102)

The main bonding jumper connects the grounded conductor to the equipment grounding conductor at the service.

Equipment bonding jumpers are used around flexible metal conduit or other non-continuous raceways.

Exam Trap: The grounded conductor (neutral) is not permitted to be used as an EGC on the load side of the service disconnecting means. This is a common violation and a frequent exam question.

1.8 Conductor Ampacity and Tables

Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. The NEC provides ampacity tables in Article 310 (specifically Table 310.16 for copper and aluminum at 60 °C, 75 °C, and 90 °C).

1.8.1 Table 310.16 – Key Rows

14 AWG copper: 15 A (60 °C), 15 A (75 °C), 20 A (90 °C)

12 AWG copper: 20 A (60 °C), 20 A (75 °C), 25 A (90 °C)

10 AWG copper: 30 A (60 °C), 35 A (75 °C), 40 A (90 °C)

8 AWG copper: 40 A (60 °C), 50 A (75 °C), 55 A (90 °C)

Critical Rule: The ampacity of a conductor is limited by the lowest temperature rating of any termination, device, or conductor in the circuit. Most standard breakers and lugs are rated 60 °C or 75 °C. Therefore, you cannot use the 90 °C column for ampacity unless all terminations are rated 90 °C (rare).

1.8.2 Correction and Adjustment Factors

Ambient temperature correction (Table 310.15(B)(1)): If the ambient temperature exceeds 30 °C (86 °F), the ampacity must be reduced.

Adjustment factors (Table 310.15(C)(1)): If more than three current-carrying conductors are bundled together, the ampacity must be reduced (e.g., 4–6 conductors: 80%; 7–9: 70%; 10–20: 50%).

Exam Trap: Apply the adjustment factor first, then the correction factor, to the base ampacity from Table 310.16. Do not combine them into a single percentage.

1.9 Voltage Drop (Informational Only)

The NEC does not mandate a specific voltage drop percentage, but Informational Note No. 2 to 210.19(A) recommends limiting voltage drop to 3% for branch circuits and 5% for feeders plus branch circuits (total). Many local jurisdictions adopt these as enforceable limits.

Field Point: For long runs, calculate voltage drop using the formula: VD = (2 × K × I × L) / CM, where K is the conductor resistivity (12.9 for copper, 21.2 for aluminum), I is the current, L is the one-way length in feet, and CM is the circular mil area of the conductor.

1.10 Code Navigation: Where to Find It

Concept	NEC Location
Branch circuit definitions and requirements	Article 210, Sections 210.1–210.52
Feeder requirements	Article 215, Sections 215.1–215.3
Load calculations (standard)	Article 220, Sections 220.40–220.61
Load calculations (optional)	Article 220, Sections 220.82–220.85
Service entrance requirements	Article 230, Sections 230.1–230.95
Overcurrent protection devices	Article 240, Sections 240.1–240.100
Grounding and bonding	Article 250, Sections 250.1–250.122
Grounding electrode conductor sizing	Table 250.66
Equipment grounding conductor sizing	Table 250.122
Conductor ampacity tables	Table 310.16
Ambient temperature correction	Table 310.15(B)(1)
Adjustment factors (bundling)	Table 310.15(C)(1)
Standard OCPD ratings	Section 240.6(A)

1.11 Common Exam Traps and Field Errors

112.Using 90 °C ampacity for terminations: Always check the termination temperature rating. Most breakers are 60/75 °C.

113.Forgetting the 125% continuous load factor: This applies to both conductor and OCPD sizing.

114.Sizing the EGC from Table 250.66 instead of 250.122: The GEC is for grounding electrodes; the EGC is for equipment bonding.

115.Applying the "next size up" rule to receptacle branch circuits: Not permitted.

116.Neutral as an EGC on the load side: Illegal. The neutral must be isolated from the equipment grounding system beyond the service disconnect.

117.Ignoring adjustment factors for more than 3 conductors: A common cause of undersized conductors in conduit.

118.Using the optional method for non-dwelling occupancies: Only applies to dwelling units under 220.82.

1.12 Practical Field Application

On the job, a journeyman electrician uses Chapter 2 every day:

When pulling wire for a new branch circuit, you must verify the conductor ampacity matches the breaker and the load.

When bonding a metal water pipe, you must ensure the bonding jumper is sized per Table 250.102(C)(1).

When installing a subpanel, you must isolate the neutral from the ground bus (no bonding screw) to avoid parallel neutral paths.

When calculating a service upgrade, you must perform a load calculation per Article 220 and present it to the AHJ.

Final Tip: In the open-book exam, do not memorize every table. Instead, memorize the structure of Chapter 2 and the location of each table. When you see a question about grounding electrode conductor size, your hand should go immediately to Table 250.66. When you see a question about equipment grounding, your hand goes to Table 250.122. Speed and accuracy in code navigation are your greatest assets.

Wiring & Protection