DC Electrician cheat sheet

Ohm's law: E = I × R; power in watts: P = E × I = I²R = E²/R. Single-phase power in watts equals volts times amps times power factor.

Voltage drop (single phase, copper or aluminum): VD = 2 × K × I × L ÷ CM, where K = 12.9 ohms-cmil/ft for copper and 21.2 for aluminum; L in feet, CM = circular mils of the conductor.

Three-phase formulas: VD = 1.732 × K × I × L ÷ CM; three-phase power P = 1.732 × E × I × power factor.

Ampacity adjustment: conductors in a raceway or cable are derated per Table 310.15(C)(1) - 4 to 6 current-carrying conductors = 80%, 7 to 9 = 70%, 10 to 20 = 50%.

Continuous loads are calculated at 125% of the load for branch-circuit and feeder sizing; the neutral need not be counted as a current-carrying conductor when it carries only unbalanced linear load.

Standard overcurrent device ratings: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A, then in 10 A steps to 400 A, then in 50 A steps to 1200 A.

Receptacle branch circuits: 15 A and 20 A, 125 V receptacles require 20 A circuits for small-appliance, laundry, and bathroom branch circuits; bathroom receptacles require GFCI protection.

Box fill uses Table 314.16(B) volume allowances: 14 AWG = 2.00 cu in, 12 AWG = 2.25 cu in, 10 AWG = 2.50 cu in; each device yoke counts two conductor volumes, all equipment grounds count as one.

NM-B cable (Romex) is permitted in one- and two-family dwellings and multifamily dwellings not exceeding three floors, but is prohibited in hazardous locations, wet locations, and most commercial structures.

Support intervals: NM cable every 4.5 ft and within 12 in of every box; rigid metal conduit every 10 ft; EMT within 3 ft of a box and every 10 ft; PVC Schedule 40 every 3 ft.

Conduit fill: 1 conductor = 53%, 2 conductors = 31%, over 2 conductors = 40% of the conduit's internal cross-sectional area.

Bend radius for nonmetallic-sheathed cable must be at least 5 times the cable diameter; for MC cable it is 7 times the cable diameter.

Boxes must be accessible; junction boxes cannot be buried in walls or ceilings. A box must be large enough for the conductors, devices, and fittings per Article 314.

Conductors in a raceway must be de-rated for ambient temperature per Table 310.15(B)(1) and for the number of current-carrying conductors.

Recessed luminaires require minimum clearances from thermal insulation unless listed for direct contact (IC-rated); junction boxes must remain accessible.

Grounding electrode system: a single rod, pipe, or plate requires 25 ohms or less, or a second electrode must be added; the grounding electrode conductor is sized per Table 250.66.

Receptacle configurations: 5-15R is 15 A/125 V; 5-20R is 20 A/125 V; 6-15R is 15 A/250 V; 6-20R is 20 A/250 V; 14-30R is 30 A/125/250 V; 14-50R is 50 A/125/250 V.

AFCI protection is required for 15 A and 20 A branch circuits supplying outlets in dwelling unit bedrooms, living rooms, dining rooms, kitchens, laundry, and similar areas; GFCI is required for bathrooms, garages, outdoors, and within 6 ft of sinks.

Tamper-resistant receptacles are required in dwelling units for 15 A and 20 A, 125 V receptacles, with limited exceptions such as dedicated equipment outlets.

Switch loops, three-way switches, and four-way switches: three-way switches have three terminals (common plus two travelers); four-way switches have four terminals and are placed between two three-way switches.

Panelboard: the service disconnect must be marked with its ampere rating and be capable of being locked open; a service disconnect must be provided at the point of entrance of the service conductors.

Ampacity of a receptacle is limited by the branch-circuit overcurrent device; a 15 A receptacle may be installed on a 20 A circuit only where the load is limited to 15 A.

Motors require overload protection sized per the motor nameplate full-load current and short-circuit/ground-fault protection per Article 430, Table 430.52.

NFPA 70E requires an arc-flash risk assessment before work on or near energized equipment; energized work is permitted only where de-energizing introduces additional hazards or is infeasible.

Approach boundaries: the limited approach boundary for a 480 V system is 3 ft 6 in; the restricted approach boundary is 1 ft; the arc-flash boundary depends on the calculated incident energy.

Lockout/tagout: a lockout device must be applied by the person performing the work, and the tag must identify the person and the reason for the lockout.

Personal protective equipment is selected by incident energy (cal/cm²); Category 1 = 4 cal/cm², Category 2 = 8 cal/cm², Category 3 = 25 cal/cm², Category 4 = 40 cal/cm².

Licensing: most states require electricians to be licensed, with separate journeyman and master classifications; a master electrician typically may pull permits and supervise journeymen.

Contract law: a written contract should identify scope, price, schedule, payment terms, and change-order procedures; a change order must be signed by both parties to be enforceable.

Bonding and insurance: a surety bond guarantees performance or payment; general liability insurance covers third-party bodily injury and property damage; workers' compensation covers employee injuries.