Journeyman Electrician cheat sheet

Ohm's law: E = I × R; rearranged, I = E ÷ R and R = E ÷ I.

Power: P = E × I = I²R = E² ÷ R; one horsepower equals 746 watts.

Series circuit: the current is common to all components, total resistance is the sum of the resistances, and voltage divides in proportion to resistance.

Parallel circuit: the voltage is common, 1/R_total = 1/R1 + 1/R2 + ..., and current divides inversely with resistance.

Two resistors in parallel: R = (R1 × R2) ÷ (R1 + R2).

AC sine wave: RMS (effective) value = 0.707 × peak; peak value = 1.414 × RMS.

Impedance Z = √(R² + X²); power factor = true power (W) ÷ apparent power (VA) = cosine of the phase angle.

Balanced three-phase power P = √3 × E_line × I_line × power factor; single-phase P = E × I × power factor.

Conductor resistance R = K × L ÷ CM, with K ≈ 12.9 for copper and 21.2 for aluminum (ohm-circular mil per foot at 75°C).

Type NM cable is limited to dry locations; it is not permitted in wet locations, embedded in masonry, in raceways outdoors, or where subject to physical damage.

Type MC cable is permitted in dry, damp, and wet locations and, when listed, in concrete and hazardous locations.

Type AC (armored) cable contains a bonding strip that must be connected to the box and to the device grounding terminal.

EMT, IMC, and rigid metal conduit are recognized equipment grounding conductors; PVC conduit is not, so a separate EGC must be installed.

Raceway fill for over two conductors is 40% of the interior cross-sectional area; one conductor 53%, two conductors 31%, and 60% for nipples 24 in or less.

Box fill (NEC 314.16): 14 AWG = 2.0 in³, 12 AWG = 2.25 in³, 10 AWG = 2.50 in³ per conductor; each yoke-mounted device counts as two conductors.

Support spacing: NM cable within 12 in of a box and every 4.5 ft; EMT within 3 ft of a box and every 10 ft; RMC and IMC every 10 ft.

Minimum cover (Table 300.5): 24 in for direct-buried cable, 18 in for PVC conduit, 6 in for rigid metal conduit; 12 in is permitted for GFCI-protected 120 V, 20 A residential branch circuits.

No more than 360 degrees of total bends is permitted between pull points in a conduit run.

The grounding electrode conductor is sized from Table 250.66 by the largest ungrounded service conductor and is never required to be larger than 3/0 copper or 250 kcmil aluminum.

A single grounding electrode must have a resistance of 25 ohms or less; if it does not, a second electrode of a permitted type must be added (250.53(A)(2)).

The equipment grounding conductor is sized from Table 250.122 using the rating of the overcurrent device protecting the circuit.

The main bonding jumper connects the grounded service conductor to the equipment grounding terminal at the service and is sized per 250.28 and Table 250.102(C)(1).

Metal water piping systems must be bonded within 5 ft of the point where they enter the building (250.104).

Class A GFCI protection trips at 4 to 6 mA and is required in dwelling bathrooms, garages, outdoors, unfinished basements, crawl spaces, kitchen countertops, and within 6 ft of sinks.

Ground-fault protection of equipment (GFPE) typically operates at 30 mA and is used for large services, marinas, and certain snow-melting or pipeline equipment.

Ground resistance is measured by the fall-of-potential method; a clamp-on ground tester is valid only on grounded-loop systems.

Standard ampere ratings include 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, and 400 A.

NEC 240.4(D) limits protection to 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG copper conductors.

Next-size-up rule (240.4(B)): the next higher standard rating is allowed only if the device is 800 A or less, the ampacity does not match a standard size, and the circuit is not a multioutlet receptacle branch circuit.

The interrupting rating must equal or exceed the available fault current at the device terminals (110.9); series-rated combinations must be tested and listed as a system.

Class RK1, RK5, J, and CC fuses are commonly rated 200 kA interrupting; Class T fuses are current limiting with a 200 kA rating at 600 V.

Overcurrent devices for continuous loads are sized at not less than 125% of the continuous load plus 100% of the noncontinuous load (210.20, 215.3).

Devices rated 1200 A or more require arc-energy reduction means such as instantaneous trip settings or Zone Selective Interlocking (240.87).

Overcurrent devices must be readily accessible and must not be installed in bathrooms, clothes closets, or over steps (240.24).

The overcurrent device rating may not exceed the ampacity of the conductors it protects, except as specifically permitted by 240.4(B), (C), and (E) through (G).

Dwelling general lighting load is 3 VA per square foot; small-appliance circuits are 1500 VA each with a minimum of two 20 A circuits, and the laundry circuit is 1500 VA.

Commercial receptacle outlets are calculated at 180 VA each, so ten outlets equal 1800 VA.

Dwelling lighting demand factors (Table 220.42): first 3000 VA at 100%, 3001 through 120,000 VA at 35%, and the remainder at 25%.

Electric ranges and dryers use Table 220.55 demand factors; a single 8 kW range may be calculated at 8 kW from Column C.

Optional dwelling method (220.82): first 10 kVA at 100%, the remainder at 40%, plus the larger of heating or cooling at 100%.

A continuous load operates 3 hours or more; branch-circuit conductors and overcurrent devices are sized at 125% of that load.

Recommended voltage drop is 3% maximum on a branch circuit, 3% on a feeder, and 5% total (210.19(A) Informational Note).

Dwelling receptacle spacing: no point along the wall line may be more than 6 ft from an outlet; kitchen countertops require outlets so no point is more than 24 in from one.

Bathroom receptacles require a dedicated 20 A branch circuit supplying only bathroom outlets.

Ampacities come from Table 310.16, but the final ampacity is limited by the lowest termination temperature rating, typically 60°C for equipment rated 100 A or less unless marked otherwise (110.14(C)).

Adjusted ampacity = table ampacity × ambient temperature correction factor (Table 310.15(B)(1)) × conductor bundling adjustment factor (Table 310.15(C)(1)).

Bundling adjustment: 4-6 conductors = 80%, 7-9 = 70%, 10-20 = 50%, 21-30 = 45%, 31-40 = 40%, and 41 or more = 35%.

The neutral of a multiwire branch circuit is not counted as a current-carrying conductor unless the load is nonlinear and produces triplen harmonics.

Parallel conductors must be 1/0 AWG or larger and identical in length, material, size, insulation, and termination.

Minimum branch-circuit conductor sizes: 14 AWG copper for 15 A, 12 AWG for 20 A, and 10 AWG for 30 A circuits.

Insulation ratings: THHN is 90°C dry and damp; THWN is 75°C wet; THWN-2 and XHHW-2 are 90°C in wet locations.

Conduit run on a rooftop requires the ambient temperature adder from Table 310.15(B)(3)(c) before correction factors are applied.

Conductor ampacity must be at least the load, and at least 125% of a continuous load unless the assembly is listed for 100% operation.

Working space (110.26): 3 ft deep for Condition 1 (150 V or less to ground), 3.5 ft for Condition 2, and 4 ft for Condition 3; 30 in wide and 6.5 ft high.

Dedicated equipment space extends from the floor to 6 ft above the equipment or to the structural ceiling, whichever is lower.

Box volumes from Table 314.16(A): 4 × 4 × 1½ in = 21 in³, 4 × 4 × 2⅛ in = 32 in³, and 4-11/16 × 2⅛ in = 42 in³.

Box sizing for conduit (314.28): straight pulls require 8 times the trade size of the largest raceway; angle or U pulls require 6 times the largest plus the sum of the other raceways on the same side.

Panelboards must not be installed in bathrooms or clothes closets and must not be located over steps.

Support: EMT every 10 ft and within 3 ft of a box; RMC and IMC every 10 ft; PVC per Table 352.30.

A lighting and appliance branch panelboard is limited to 42 overcurrent devices unless listed otherwise.

Raceway penetrations of fire-rated assemblies must be sealed with a listed firestop method.

At the service only, the neutral is bonded to the enclosure; in subpanels the grounded and grounding conductors terminate on separate bars.

Motor branch-circuit conductors must have an ampacity of at least 125% of the motor full-load current taken from Tables 430.247 through 430.250, not the nameplate.

Motor branch-circuit protection (Table 430.52): inverse-time breaker typically 250%, dual-element fuse 175%, and non-time-delay fuse 300% of full-load current.

Overload protection (430.32) is set at 125% of full-load current for a motor with a 1.15 service factor or 40°C rise, and 115% otherwise.

The motor disconnecting means must be within sight of the controller and within 50 ft.

Hermetic refrigerant motor-compressor conductors are sized at 125% of rated-load current, with branch protection per 440.22.

Transformer protection (450.3): primary overcurrent device not more than 125% of rated primary current for transformers over 9 A; secondary not more than 125%.

Transformers must be accessible, ventilated, and installed with the working space required by 110.26.

Class 2 control circuits, such as 24 V HVAC controls, are power-limited by Article 725 and may use smaller conductors.

A motor controller must be rated not less than the motor horsepower and capable of starting and stopping the motor.

Class I locations contain flammable gases or vapors, Class II locations contain combustible dust, and Class III locations contain ignitable fibers or flyings.

Division 1 covers conditions that exist under normal operation; Division 2 covers conditions that exist only under abnormal operation.

Zone classification for gases uses Zone 0 (continuous), Zone 1 (likely), and Zone 2 (unlikely); dust uses Zones 20, 21, and 22.

Intrinsically safe circuits must be identified, commonly with light-blue conductors, and kept separated from other wiring (Article 504).

Explosion-proof equipment must be listed for the class, division, and gas group, and conduit seals are required within 18 in of the enclosure.

Health care facilities require redundant grounding, hospital-grade receptacles, and isolated power systems in critical-care areas (Article 517).

Permanently installed pools require an equipotential bonding grid, and receptacles are restricted near the pool edge (680.22, 680.26).

Temporary wiring on construction sites requires GFCI protection for 125 V, 15 A and 20 A receptacles (590.6).

Marinas and boatyards require 30 mA ground-fault protection for shore-power receptacles (Article 555).

Circuits are treated as energized until verified de-energized with a tester rated for the voltage; the tester is checked on a known live source before and after.

Lockout/tagout: each worker applies a personal lock and tag, and the isolation is verified by attempting to start the equipment (OSHA 1910.147).

NFPA 70E establishes the limited approach boundary, restricted approach boundary, and arc-flash boundary; PPE is selected from the incident energy.

Unqualified persons must remain at least 10 ft from exposed energized parts operating at up to 50 kV.

Temporary 125 V, 15 A and 20 A receptacles on construction sites require listed GFCI protection (590.6).

No more than 360 degrees of bends is permitted between pull points in a conduit run.

Ladder setup follows the 4-to-1 rule, extends 3 ft above the landing, and is secured with three-point contact maintained.

Insulation resistance is measured with a megohmmeter, while voltage and continuity are measured with a multimeter of the correct CAT rating.

Rubber insulating gloves must be electrically retested every 6 months and air-tested before each use.