Craftsman Electricity cheat sheet

OSHA 29 CFR 1910 Subpart S and NFPA 70E govern workplace electrical safety; the NEC (NFPA 70) is an installation code, not a work-practice standard.

NFPA 70E arc flash PPE categories: Category 1 requires 4 cal/cm² arc-rated clothing; Category 2 requires 8 cal/cm²; Category 3 requires 25 cal/cm²; Category 4 requires 40 cal/cm².

An electrically safe work condition requires: disconnect, verify absence of voltage with a tested meter, lockout/tagout, and apply temporary protective grounds where induced or stored energy exists.

Approach boundaries at 50 V to 300 V: limited approach 3 ft 6 in for fixed circuit parts, restricted approach 1 ft 0 in, no unqualified entry inside the limited boundary.

Qualified person under NFPA 70E must be trained on voltage-rated tools, PPE selection, arc flash hazard analysis, and emergency response including CPR.

NEC 210.8 requires GFCI protection for 125 V receptacles in bathrooms, garages, outdoors, kitchens, laundry, basements, and within 6 ft of sinks; 2020 and later expands to 250 V receptacles in many of these areas.

NEC 210.12 requires AFCI protection for 120 V branch circuits in dwelling unit bedrooms, kitchens, laundry, family rooms, and similar areas; combination-type AFCI devices satisfy this.

Working space per NEC 110.26: 3 ft minimum depth at 0-150 V to ground, 3.5 ft at 151-300 V, 4 ft at 301-600 V; 30 in wide, 6.5 ft high, and clear of storage.

Ohm's law: V = I × R; power in a resistive circuit: P = V × I = I²R = V²/R.

Series circuit: R_total = R1 + R2 + ...; current is the same through each element; voltage divides in proportion to resistance.

Parallel circuit: 1/R_total = 1/R1 + 1/R2 + ...; voltage is the same across each branch; current divides inversely with resistance.

Three-phase power: P = √3 × V_LL × I × PF for balanced loads; single-phase: P = V × I × PF.

Impedance Z = √(R² + X²) where X = X_L − X_C; X_L = 2πfL; X_C = 1/(2πfC).

AC frequency and reactance: inductive reactance rises with frequency; capacitive reactance falls with frequency; series resonance occurs where X_L = X_C.

Conductor resistance: R = ρL/A; doubling the length doubles resistance; doubling the cross-sectional area halves resistance.

Transformer turns ratio: V_primary/V_secondary = N_primary/N_secondary; current ratio is inverse; power is conserved minus losses.

NEC Chapter 3 articles govern wiring methods: 300 general requirements, 310 conductors, 320-340 cable assemblies, 350-362 raceways, 370-376 boxes and cabinets.

NM-B (Romex) is permitted in dry, concealed locations in dwellings and many commercial occupancies; prohibited in exposed locations subject to physical damage and in hazardous locations.

EMT must be supported within 3 ft of each box and every 10 ft; IMC and RMC supported within 3 ft and every 10 ft; PVC Schedule 40 supported every 3 ft and Schedule 80 every 3 ft.

Box fill per NEC 314.16: each 14 AWG conductor counts 2.00 in³, 12 AWG 2.25 in³, 10 AWG 2.50 in³; each device yoke counts two conductor equivalents; each clamp one.

Conductor ampacity uses NEC Table 310.16; 14 AWG copper is 15 A, 12 AWG 20 A, 10 AWG 30 A, 8 AWG 50 A, 6 AWG 65 A at 60 °C in raceway.

Conduit fill per NEC Chapter 9 Table 1: one conductor 53%, two conductors 31%, over two conductors 40% of interior cross-sectional area.

Bend radius for cable per NEC 334.24: not less than five times the cable diameter for NM and SE cable.

Raceway supports: EMT within 3 ft of termination and every 10 ft; flexible metal conduit every 4.5 ft and within 12 in of termination.

Standard branch-circuit ratings per NEC 210.3: 15, 20, 30, 40, 50, and 60 A for general-purpose circuits; 15 and 20 A for lighting and receptacles.

Dwelling unit general lighting load per NEC 220.12: 3 VA per square foot; demand factors per Table 220.42 reduce load above 3,000 VA.

Small-appliance branch circuits per NEC 210.11(C)(1): two 20 A circuits required in dwelling kitchens; laundry circuit per 210.11(C)(2) at 20 A.

Receptacle spacing per NEC 210.52: no point along a wall is more than 6 ft from a receptacle; kitchen countertop receptacles within 24 in of any point.

Service calculation per NEC 220.82 optional method: 100% of first 10 kVA plus 40% of the remainder of general load, plus 100% of HVAC or 100% of the larger of heating or cooling.

Feeder and service conductor sizing uses Table 310.16 corrected for ambient temperature and adjusted for more than three current-carrying conductors per NEC 310.15.

Voltage drop on a branch or feeder: VD = 2 × L × I × R / 1000 for single-phase; NEC 210.19(A) informational note recommends 3% branch and 5% total.

Demand factors for ranges per NEC Table 220.55: one range up to 12 kW may be calculated at 8 kW; larger ranges add 5% per kW above 12 kW.

Grounded conductor (neutral) is white or gray; equipment grounding conductor is green, green with yellow stripe, or bare; ungrounded conductors are black, red, blue, or other colors.

NEC 250.24 requires the service neutral to be bonded to the grounding electrode conductor at the service disconnect only; downstream panels use a separate ground bar.

Grounding electrode system per NEC 250.50 includes metal water pipe (10 ft in contact), ground rod, concrete-encased electrode, and ground ring; all must be bonded together.

Equipment grounding conductor sizing per NEC Table 250.122: 15 A circuit uses 14 AWG copper, 20 A uses 12 AWG, 60 A uses 10 AWG, 200 A uses 6 AWG.

Standard overcurrent device ratings per NEC 240.6: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400 A.

Fuse types: dual-element time-delay fuses tolerate motor inrush; fast-acting fuses protect sensitive electronics; Class RK5 and RK1 are common branch-circuit fuses.

Circuit breaker types: thermal-magnetic, magnetic-only (motor circuit protectors), and electronic trip units; instantaneous trip typically 10-13× rating for thermal-magnetic.

NEC 250.32 requires a grounding electrode at each separate building or structure served by a feeder; the equipment grounding conductor must be run with the feeder.

Motor full-load current per NEC Tables 430.248 (single-phase) and 430.250 (three-phase); use table values, not nameplate, for sizing conductors and overloads.

Motor branch-circuit short-circuit and ground-fault protection per NEC 430.52: inverse-time breaker up to 250% of FLC, dual-element fuse up to 175%, non-time-delay fuse up to 300%.

Motor overload protection per NEC 430.32: 125% of FLC for motors with service factor 1.15 or greater; 115% for others.

Motor disconnect per NEC 430.102 must be within sight and within 50 ft of the motor; a lockable disconnect is required for remote motors.

Motor conductor sizing per NEC 430.22: 125% of FLC for continuous-duty motors; the equipment grounding conductor is sized per Table 250.122 based on the branch-circuit protective device.

HVAC equipment nameplate gives minimum circuit ampacity (MCA) and maximum overcurrent protection (MOCP); size conductors to MCA and breaker to MOCP.

Contactors and starters: NEMA sizes correlate with horsepower; a NEMA size 1 starter handles up to 10 hp at 460 V three-phase.

Thermostat wiring: R is 24 V hot, C is common, W is heat, Y is cooling, G is fan; a heat pump adds O/B reversing valve and aux heat (W2/E).

Transformer kVA calculation: kVA = (V × I) / 1000 for single-phase; kVA = (√3 × V_LL × I) / 1000 for three-phase.

Transformer primary and secondary overcurrent protection per NEC 450.3: primary only up to 125% of rated current for transformers over 1000 V; 250% for 600 V and below.

Delta-wye transformers provide a neutral and are common for 480 V to 208/120 V distribution; delta-delta is used for isolation and three-wire loads.

Transformer impedance typically 1.5-6% for dry-type; short-circuit current at secondary = full-load amps / percent impedance.

Single-phase transformer bank connected delta-delta can supply three-phase; open-delta (two transformers) supplies about 58% of the three-transformer capacity.

Buck-boost transformers correct voltage by small amounts (typically 12/24 V or 16/32 V) and are autotransformer-connected.

Transformer temperature rise ratings: 80 °C, 115 °C, and 150 °C rise for dry-type; insulation class 220 °C is common for 150 °C rise units.

Neutral grounding of separately derived systems per NEC 250.30: bond the neutral to the grounding electrode conductor at the transformer secondary or first disconnect.

Lockout/tagout per OSHA 1910.147: notify affected employees, shut down, isolate, apply locks and tags, release stored energy, verify zero energy.

Infrared thermography identifies loose connections and overloaded conductors; a temperature rise above 15 °C over similar components indicates a problem.

Motor insulation resistance testing uses a megohmmeter at 500 V or 1000 V DC; minimum acceptable insulation resistance is generally one megohm per kV of rated voltage plus one.

Continuity and voltage checks: verify meter on a known live source before and after testing to confirm the meter is functioning.

Ground-fault detection: a clamp-on ammeter on the grounding conductor can reveal leakage current; more than a few milliamps indicates a fault.

NEMA motor frame sizes and enclosures: ODP, TEFC, TENV; TEFC is common for general industrial use, TENV for clean indoor environments.

Vibration analysis and current signature analysis detect bearing wear, misalignment, and rotor bar defects in motors.

Troubleshooting sequence: verify symptoms, check power supply and fuses, check control circuit voltage, isolate the faulty component, test, and document findings.