Electrical service and distribution: voltages, wiring, and code requirements

One-line orientation

Building electrical service arrives at high voltage, is stepped down by transformers, and distributed at voltages matched to the load — the exam tests which voltage system serves which building type and which protective devices go where.

Key points

• Ohm’s Law: V = I × R (voltage = current × resistance). For AC circuits, real power in watts also involves the power factor: W = I × V × pf. Apparent power (VA) omits pf.
• Power factor (pf): ratio of real power (W) to apparent power (VA); ideal = 1.0. Low pf means wasted energy, common with inductive loads (motors, transformers).
• Why high-voltage transmission: distributing a million watts at 120V would require impractically large conductors (8,000+ A). High voltage keeps current manageable; substations and transformers step it back down.
• Standard building voltage systems:
  • 120V single-phase — one hot + neutral; residential outlets and lighting.
  • 120/240V single-phase — two hot + neutral; 120V for outlets, 240V for dryers/ovens.
  • 120/208V three-phase — three hot + neutral; commercial buildings, motors, HVAC.
  • 277/480V three-phase — common in large commercial and industrial buildings. Lighting often uses 277V; large motors often use 480V. Small step-down transformers in electrical closets reduce 480V service to 120V for standard outlets.
  • Delta note: a standard 3-wire delta has no neutral and is mainly a motor/industrial service; a 4-wire high-leg delta is the exception that can provide 120/240V plus a high leg.
• Wire gauge and ampacity: higher current → thicker wire (lower AWG number).
  • 14 AWG → 15 A; 12 AWG → 20 A; 10 AWG → 30 A.
• Voltage drop: increases with wire length and load; minimize by using thicker wire or shorter runs.
• Wire types:
  • Romex (NM cable): flexible nonmetallic cable; common in dwelling/light-frame concealed dry runs, but not for exposed, wet, or high-damage locations.
  • Steel conduit: strong protection, commercial/industrial; costly to install.
  • BX (armored cable / Type AC): armor serves as the ground path via an internal aluminum bonding strip; no separate grounding wire.
  • MC (metal-clad): separate grounding wire inside; casing not used for ground.
• Hot (black/red) / Neutral (white) / Ground (green/bare): hot carries current to load; neutral returns it; ground provides fault path to earth.
• Protective outlets:
  • GFCI — monitors current imbalance; required in bathrooms, kitchens, outdoors (wet locations).
  • AFCI — detects arcing; per NEC 2023 §210.12(B), required in nearly all habitable dwelling rooms (bedrooms, living rooms, kitchens, hallways, closets, laundry, dining, sunrooms, and similar).
• Electrical clearances (NEC): 3 ft clearance in front of electrical equipment; 30 in. wide working space; 6 ft 6 in. minimum headroom (NEC 110.26(A)(3)/(D) = 2.0 m).
• Outlet spacing (residential):
  • Walls: Receptacles may be no more than 12 feet apart, and no point may be more than 6 feet from a receptacle.
  • Wall segments: Each segment over 2 feet needs a receptacle.
  • Kitchen counters: No point may be more than 2 feet from a receptacle. Each counter over 1 foot needs a receptacle.

Confusions / comparison

System	Voltage (phase-to-phase / phase-to-neutral)	Typical building type	Typical loads
120V 1-phase	120V / —	Residential	Outlets, lighting
120/240V 1-phase	240V / 120V	Residential	Outlets (120V); dryers, ovens (240V)
120/208V 3-phase	208V / 120V	Commercial	Motors, HVAC, receptacles
277/480V 3-phase	480V / 277V	Large commercial / industrial	Lighting (277V); elevators, motors (480V)

Wire type	Ground method	Best for
Romex (NM)	Separate bare wire	Dwelling/light-frame concealed dry runs
Steel conduit	Conduit itself	Commercial, exposed runs
BX armored (Type AC)	Armor + internal bonding strip = ground	Tight residential/commercial spaces
MC metal-clad	Separate wire inside	Where casing must not carry ground

Electrical service steps voltage down, then matches it to the load

Schematic

A common large-building path distributes 480/277 V and adds a secondary transformer where 120 V is needed.

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Voltage values show a common commercial pattern, not the only service configuration.

Related

→ Systems: Lighting (277V commercial lighting, lamp types) · HVAC (motors, three-phase loads) · Plumbing (dedicated circuits for appliances) · Code: NEC clearance and spacing requirements.