AR Electrician cheat sheet

Ohm's law: E = I × R, with I = E/R and R = E/I; power P = I × E = I²R = E²/R in a resistive circuit.

Series circuit: current is common to all elements, resistances add (R_T = R1 + R2 + ...), and voltage divides in proportion to each resistance.

Parallel circuit: voltage is common to all branches, conductances add (1/R_T = 1/R1 + 1/R2 + ...), and branch currents divide inversely with resistance.

For a sine wave the RMS value is 0.707 × peak and the peak is 1.414 × RMS; 120 V RMS has a peak of about 170 V.

Balanced three-phase power: P = √3 × line voltage × line current × power factor; power factor equals kW divided by kVA.

Conductor resistance: R = K × L / CM, where K is about 12.9 for copper and 21.2 for aluminum at 75 °C, L is in feet, and CM is circular mils.

Voltage drop: single-phase VD = 2 × K × I × L / CM; three-phase VD = 1.732 × K × I × L / CM.

Ampacity is adjusted for ambient temperature (Table 310.15(B)(1)) and for more than three current-carrying conductors in a raceway (Table 310.15(C)(1)).

North American AC systems operate at 60 Hz, so one cycle has a period of about 16.7 milliseconds.

Nonmetallic-sheathed cable (NM-B) is limited to dry locations and is not permitted in wet locations, inside raceways, or where subject to physical damage.

EMT must be supported within 3 ft of a box or fitting and at intervals not exceeding 10 ft thereafter.

Minimum earth cover: rigid metal conduit 6 in; PVC Schedule 40 or 80 is 18 in for conduit 2 in and smaller and 24 in for larger sizes.

MC cable with listed conductors may be installed in wet locations; armored cable (Type AC) is a dry-location wiring method.

Liquidtight flexible metal conduit and liquidtight flexible nonmetallic conduit are permitted in wet locations, but LFMC is not permitted where subject to physical damage.

Rigid PVC conduit requires expansion fittings where thermal expansion or contraction would be 1/4 in or more in a run.

Raceways must be installed complete between boxes and fittings before conductors are pulled in (300.18).

Junction, outlet, and pull boxes must remain accessible; boxes may not be buried in walls, ceilings, or floors.

A one-family dwelling service disconnecting means must be rated not less than 100 A, be a single means, and be located nearest the point of entry of the service conductors.

Overhead service-drop clearances: 10 ft above finished grade at the drip loop, 12 ft over residential driveways, 18 ft over public streets and alleys, and 8 ft over roof surfaces.

Dwelling service and feeder conductors may be sized at 83 percent of the service rating under 310.12 for a 120/240 V, single-phase, three-wire service.

Dwelling kitchens require at least two 20 A small-appliance branch circuits, and no other outlets may be supplied from those circuits.

Receptacle spacing rule: no point along a dwelling wall line may be more than 6 ft from a receptacle, and any wall space 2 ft or wider needs one.

Recommended voltage drop is a maximum of 3 percent on a branch circuit and 5 percent total for feeder plus branch circuit.

GFCI protection is required for dwelling bathroom, garage, outdoor, kitchen countertop, laundry, basement, and within 6 ft of sink receptacles, plus unfinished accessory buildings.

AFCI protection is required for branch circuits supplying outlets in dwelling living areas, including bedrooms, closets, hallways, and similar rooms.

Feeders must have ampacity at least equal to the calculated load (215.2) and a neutral sized for the maximum unbalanced load per 220.61.

A single ground rod must have a resistance to earth of 25 ohms or less, or it must be supplemented by one additional electrode at least 6 ft away.

The grounding electrode conductor is sized from Table 250.66 based on the largest service-entrance or feeder conductor; the equipment grounding conductor is sized from Table 250.122 based on the overcurrent device.

If circuit conductors are increased in size for voltage drop, the equipment grounding conductor must be increased proportionally (250.122(B)).

A metal underground water pipe is an electrode only where it is in direct contact with earth for 10 ft or more; it must be bonded within 5 ft of where it enters the building.

The main bonding jumper connects the grounded service conductor to the equipment grounding conductor and the grounding electrode conductor at the service disconnecting means.

A separate building or structure supplied by a feeder requires a disconnecting means, a grounding electrode system, and an equipment grounding conductor run with the feeder.

Metal raceways, cable sheaths, enclosures, and metal piping likely to become energized must be bonded to the equipment grounding conductor.

Gas piping must be bonded but may not be used as a grounding electrode (250.104(B)).

Bonding jumpers around water meters, insulating joints, and Flexible connections keep the grounding path electrically continuous.

Standard ampere ratings (240.6) include 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, and 200 A.

Next-size-up rule (240.4(B)): the next higher standard overcurrent device is permitted where conductor ampacity is 800 A or less, the load is not a multioutlet receptacle circuit, and the next standard size does not exceed 800 A.

An overcurrent device must have an interrupting rating not less than the available fault current at its terminals (110.9); series-rated combinations are permitted only where listed as a system.

Feeder tap rules (240.21): 10 ft tap not extending beyond a switchboard or panelboard, 25 ft feeder tap, 25 ft transformer secondary tap, and 25 ft outdoor tap.

Ground-fault protection of equipment is required for solidly grounded wye services of 1000 A or more, more than 150 V to ground, with a maximum ground-fault pickup of 1200 A.

Continuous loads require overcurrent devices and conductors sized at 125 percent of the continuous load plus 100 percent of the noncontinuous load.

Overload devices protect conductors and equipment from sustained excess current, while short-circuit protective devices clear high-current faults.

Selective coordination is required for emergency system overcurrent devices so that a fault is cleared by the device nearest the fault (700.32).

Box fill volumes per conductor (314.16): 14 AWG = 2.0 in³, 12 AWG = 2.25 in³, 10 AWG = 2.5 in³, and 8 AWG = 3.0 in³.

Each cable clamp counts as one conductor volume, and each device yoke counts as two conductor volumes for 14 and 12 AWG.

Conduit fill limits: 53 percent for one conductor, 31 percent for two conductors, and 40 percent for three or more conductors.

At least 6 in of free conductor must be left at each outlet and junction box, extending at least 3 in beyond the box opening (300.14).

NM cable must be secured within 12 in of each box and supported at intervals not exceeding 4.5 ft.

Conductor ampacity comes from Table 310.16; the 60 °C column applies to terminations rated 100 A or less unless the equipment is listed otherwise (110.14(C)).

Grounded conductors are identified white or gray, equipment grounding conductors green or green with yellow stripes, and ungrounded conductors by phase color.

For mixed conductor sizes, conduit fill is calculated by total conductor area from Chapter 9, Table 5 compared with the raceway area from Table 4.

Dwelling general lighting load is 3 VA per square foot; each small-appliance branch circuit is 1500 VA and the laundry circuit is 1500 VA.

Table 220.42 demand factors for dwelling lighting: first 3000 VA at 100 percent, the next 3000 VA at 35 percent, and the remainder at 25 percent.

Table 220.55 permits a demand of 8 kW for one household electric range rated not over 12 kW.

Receptacle outlets in other than dwelling units are calculated at 180 VA per outlet, with demand factors applied for large numbers.

The optional dwelling calculation method (220.82) uses 100 percent of the first 10 kVA plus 40 percent of the remaining load.

The feeder or service neutral carries the maximum unbalanced load and may be reduced to 70 percent of the range and dryer loads (220.61).

The largest motor load must be increased by 25 percent when calculating a feeder or service (430.24, 220.50).

Commercial lighting loads use the demand factors of Table 220.42 and continuous loads are taken at 125 percent.

Motor branch-circuit conductors must have an ampacity of at least 125 percent of the motor full-load current from Table 430.250 (430.22).

Motor overload protection is typically set at 115 to 125 percent of the motor nameplate full-load current (430.32).

Short-circuit and ground-fault protection values from Table 430.52 include 250 percent for an inverse-time breaker and 175 percent for a dual-element time-delay fuse.

A motor disconnecting means must be within sight of the controller and motor, and rated at least 115 percent of the motor full-load current.

Motor feeder conductors must have an ampacity of 125 percent of the largest motor plus the sum of the other motor full-load currents (430.24).

Table 430.250 full-load currents include 14 A for a 10 hp, 460 V motor and 28 A for a 5 hp, 230 V motor.

Hermetic refrigerant motor-compressors are covered by Article 440 and use rated-load current and branch-circuit selection current rather than Table 430.250 values.

HVAC equipment requires a disconnecting means within sight of and not more than 50 ft from the equipment (440.14).

Class I locations contain flammable gases or vapors, Class II locations contain combustible dust, and Class III locations contain ignitable fibers or flyings; each is divided into Division 1 and Division 2.

Zone classification uses Zone 0, 1, and 2 for gases and vapors and Zone 20, 21, and 22 for dusts.

Permanently installed pools, spas, and hot tubs require equipotential bonding of metal parts with a solid 8 AWG copper conductor (Article 680).

Patient care areas of health care facilities require a redundant equipment grounding path in addition to the raceway (Article 517).

Emergency systems are covered by Article 700, legally required standby systems by Article 701, and optional standby systems by Article 702.

Class 1, Class 2, and Class 3 remote-control and signal circuits are covered by Article 725, and fire alarm circuits by Article 760.

Agricultural buildings require an equipotential plane in areas where livestock are kept (Article 547).

Marinas, boatyards, and floating buildings are covered by Article 555, and commercial garages by Article 511.

Working space about electrical equipment must be at least 36 in deep (condition 1, 600 V or less), 30 in wide, and 6 ft 6 in high (110.26).

The dedicated equipment space above and in front of switchboards and panelboards must be kept free of piping, ducts, and leaky equipment (110.26(E)).

Overcurrent devices may not be located in bathrooms or over the steps of a stairway, and panelboards may not be installed in clothes closets.

Equipment rooms and working spaces must be provided with illumination and must not be used for storage.

Arc-rated clothing and personal protective equipment are selected per NFPA 70E based on the incident energy or arc flash PPE category of the task.

Lockout/tagout procedure: de-energize the equipment, verify the absence of voltage, apply a lock and tag, and release stored energy.

The authority having jurisdiction interprets the code, may grant approval of alternate methods and materials, and may require plans and specifications (90.4).

Electrical work requires a permit and is inspected at rough-in and at final completion before the system is energized.