12.3 Electrical Systems, Service, and NEC Basics
Key Takeaways
- Ohm's law (V = I x R) and Power = Volts x Amps underpin all sizing; commercial uses three-phase 120/208V or 277/480V.
- Conductors are sized by ampacity from NEC Table 310.16, then protected by a matching breaker or fuse.
- Continuous loads (3 hours or more) must be sized at 125 percent, so a 40A load needs a 50A circuit.
- Grounding ties the system to earth; bonding ties metal parts together so fault current trips the breaker; GFCI/AFCI add protection.
- NEC 110.26 requires 36 inches clear depth, 30 inches width, and 6.5 feet headroom of working space that cannot be blocked.
Electrical Systems, Service, and NEC Basics
Electrical work follows the National Electrical Code (NEC), NFPA 70. As the general contractor (GC) you license the electrical subcontractor and coordinate rough-in, but the exam expects you to know service sizing logic, conductor protection, and grounding. The fundamental relationship is Ohm's law: Volts = Amps x Ohms (V = I x R), and Power (watts) = Volts x Amps for resistive loads. Commercial buildings typically receive three-phase power; common service voltages are 120/208V wye and 277/480V wye.
Service and Distribution
Power enters at the service: the utility transformer feeds the meter, then the main service disconnect and service panel (panelboard), which distributes to branch circuits. Service size is given in amperes (A) — a small commercial building might have 400A or 800A service. The NEC requires the service disconnect to be readily accessible and limited to no more than six switches/breakers grouped together (the historic 'six-throw' rule, tightened to a single main in recent NEC editions for many occupancies).
Conductor Sizing and Overcurrent
Conductors are sized by ampacity from NEC Table 310.16, then protected by a breaker or fuse no larger than that ampacity (with rounding-up allowances). Copper, 75 degrees Celsius column examples:
| Copper conductor (AWG/kcmil) | Approx. ampacity (75C) |
|---|---|
| 12 AWG | 25 A (protected at 20 A) |
| 10 AWG | 35 A (protected at 30 A) |
| 8 AWG | 50 A |
| 4 AWG | 85 A |
| 1/0 | 150 A |
A core trap: continuous loads (on 3 hours or more) must be sized at 125 percent of the load. A 40A continuous load needs a circuit rated 40 x 1.25 = 50A. Voltage drop should be held to about 3 percent on branch circuits, 5 percent total, which drives upsizing on long runs.
Grounding, Bonding, and Protection
Grounding connects the system to earth; bonding ties metal parts together so fault current returns and trips the breaker. The grounding electrode conductor and equipment grounding conductor (EGC) are separate concepts the exam likes to test. Protection devices:
- Ground-fault circuit interrupter (GFCI) — required for receptacles in wet/damp locations (bathrooms, kitchens, rooftops, outdoors).
- Arc-fault circuit interrupter (AFCI) — required for many habitable-space circuits.
- Overcurrent protection — breakers/fuses against overload and short circuit.
Working-space clearance per NEC 110.26 is a frequent answer: at least 3 feet (36 inches) of clear depth in front of equipment up to 150V to ground, more at higher voltages, and a minimum 30-inch width and 6.5-foot headroom.
Boxes, Conduit Fill, and Box Fill
The NEC limits how many conductors fit in a box and a conduit. Conduit fill is capped at 40 percent of cross-sectional area for three or more conductors (53 percent for one, 31 percent for two) per NEC Chapter 9 tables. Box fill counts each conductor by volume — for example, a 14 AWG conductor counts as 2.0 cubic inches and a 12 AWG as 2.25 cubic inches, with allowances for devices, clamps, and grounds. Overfilling causes overheating and fails inspection. Worked example: a box with four 12 AWG conductors plus a device (counts as two more conductors) needs at least 6 x 2.25 = 13.5 cubic inches of box volume.
Coordination and Exam Traps
The GC sequences electrical: temporary power first, then rough-in (boxes, conduit, home runs) before insulation and drywall, then trim (devices, fixtures, panel labeling) and final inspection. Watch the OSHA 29 CFR 1926 Subpart K electrical-safety rules on jobsites — lockout/tagout and ground-fault protection (GFCI) on construction sites are GC responsibilities.
A classic trap mixes up grounding versus bonding, or forgets the 125 percent continuous-load multiplier. Another: stacking storage or equipment in the NEC 110.26 working space, which fails inspection. Confirm panel directories are filled in and that the service grounding electrode (ground rods or Ufer/concrete-encased electrode) is installed before the slab covers it.
A commercial lighting circuit carries a continuous load of 40 amps. What minimum circuit rating does the NEC require?
Per NEC 110.26, what is the minimum clear working depth in front of electrical equipment operating at up to 150 volts to ground?
Ohm's Law, Power, and Service Sizing
The NEC math you must know: Ohm's Law V = I × R and Power P = V × I (watts). Worked example: a 120 V circuit drawing 10 A = 1,200 W. Service/feeder sizing uses amps: a typical dwelling service is 200 A at 240 V. Conductors are sized by ampacity (NEC Table 310.16) and protected by a breaker rated at or below the wire's ampacity — a 20 A circuit uses 12 AWG copper; 15 A uses 14 AWG.
Common Exam Traps
- Trap: Use 14 AWG on a 20 A breaker. 20 A requires 12 AWG; 14 AWG is only 15 A.
- Trap: The breaker can exceed the wire ampacity. The breaker protects the wire — it must not exceed its ampacity.
- Trap: GFCI and AFCI are the same. GFCI protects people from shock (baths, kitchens, exterior); AFCI prevents arc-fault fires (bedrooms/living areas).
- Trap: Grounding and bonding are interchangeable terms — they are distinct NEC concepts.
A general-purpose 20-amp, 120-volt branch circuit is being wired with copper conductors. What is the minimum conductor size?
Receptacle, GFCI, and Working-Clearance Rules
The NEC sets layout rules the inspector checks: dwelling wall receptacles so no point along a wall is more than 6 ft from one (the "6-12 rule"), GFCI protection in bathrooms, kitchens, garages, outdoors, and within 6 ft of a sink, and AFCI in living areas. Panels need working clearance — generally 30 in wide, 36 in deep, and 6.5 ft high of clear space in front — never blocked by storage. These access and protection rules are common exam items even in an overview chapter.