Electrical Systems
Power basics
Ohm's law and power relationships drive most calculation items. Single-phase: P = V × I × PF. Three-phase: P = √3 × V × I × PF, and kVA = √3 × V × I / 1000. Watts equal volt-amps only at unity power factor; motors and transformers make VA larger than W, so never substitute one for the other. Know the service entrance → meter → main disconnect → panelboard → branch circuit hierarchy, plus transformers and their kVA ratings.
Loads and code
NEC load calculations use demand factors and diversity; continuous loads are taken at 125% for conductor and overcurrent sizing, which is why a 20 A circuit serves only 16 A continuously. Voltage drop should stay within about 3% on a branch and 5% total. Conductors require derating for ambient temperature and conduit fill. Grounding, GFCI (wet locations, kitchens, exteriors) and AFCI (dwellings) are frequent items.
Emergency and lighting
Emergency power: generator with automatic transfer switch, or a UPS for ride-through - a generator does not energize instantly. Egress lighting and exit signs, fire alarm initiation/notification, and low-voltage systems round out the domain. Lighting design uses lumens, efficacy (lm/W), footcandles (1 fc ≈ 10.76 lux), coefficient of utilization, and lighting power density (W/sf) limits under the energy code.
Traps: omitting √3 in three-phase math; treating watts as VA; forgetting the 80% breaker rule; confusing footcandles with lumens; assuming a generator carries load without a transfer time.
Rule of thumb: amps = watts ÷ volts at unity PF; lighting power density of 1.0 W/sf means 10 kW for a 10,000 sf building.
Worked example: a 60 kVA three-phase load at 480 V draws I = 60,000 / (1.732 × 480) ≈ 72 A.