Electrical Contracting Practice and Theory
Domain 1 - Electrical Contracting Practice and Theory
This domain tests whether you can turn theory into compliant field decisions: branch-circuit and feeder calculations, conductor and overcurrent sizing, grounding and bonding, box fill, voltage drop, and the limited-energy rules that govern low-voltage installations.
Core rules to hold onto
- Ohm's law and the power wheel drive every calculation: E = I × R and P = E × I. For single-phase loads, kVA = volts × amps ÷ 1,000.
- Continuous loads are taken at 125% for branch-circuit and feeder sizing (NEC 210.19 and 215.2). A 16 A continuous load needs a 20 A circuit, not a 16 A one.
- Ampacity comes from the conductor table only after ambient-temperature and conduit-fill corrections; the overcurrent device protects the conductor, not the appliance.
- Voltage drop is usually a design issue rather than a code violation: VD = 2 × K × I × L ÷ CM, with K about 12.9 for copper.
- Low-voltage work is still electrical work. Class 2 and Class 3 circuits under Article 725 are energy-limited but must still be separated, supported, and listed for the environment.
Worked example
A 120 V, 15 A load sits 100 ft from the panel; you plan 12 AWG copper (6,530 circular mils). VD = 2 × 12.9 × 15 × 100 ÷ 6,530 = 38,700 ÷ 6,530 ≈ 5.9 V, about 4.9% of 120 V. That exceeds the commonly applied 3% branch-circuit target, so upsize to 10 AWG (10,380 cmil): VD ≈ 3.7 V, about 3.1%. The reasoning is compute, compare to target, upsize the conductor - never upsize the breaker to fix drop.
Traps
- Using the 90°C column because the wire is THHN; terminations usually limit you to 75°C.
- Forgetting that the neutral counts as a current-carrying conductor on some multiwire branch circuits.
- Treating "low voltage" as "no rules."