Capítulo 5 de 513% del examen

Electrical Theory, Working Space and Job-Site Safety

Theory questions on a journeyman exam are straightforward if you keep four formulas and two system relationships at hand, and they often appear disguised inside a load or transformer calculation. The safety half of this chapter covers the Article 110 clearances that inspectors check first and the field practices that keep the work survivable.

Ohm's Law and Power

Nearly every DC and single-phase resistive question reduces to one of four relationships built from voltage, current, resistance and power. Write down the two you are given and solve for the third rather than trying to recall a single combined formula. Watch the wording carefully, since power questions frequently supply current and resistance rather than current and voltage.

Ohm's law
E equals I times R, so current equals voltage divided by resistance.
Power from voltage and current
P equals E times I for a resistive or unity power factor load.
Power from current and resistance
P equals I squared times R, which is the form used for conductor heating and voltage drop losses.
Power from voltage and resistance
P equals E squared divided by R, useful when only the applied voltage and element resistance are known.

Series, Parallel and AC Circuits

Series circuits share one current and divide the voltage, while parallel circuits share one voltage and divide the current, and every series-parallel problem is solved by reducing branches until only one resistance remains. In AC circuits, inductance and capacitance add reactance that combines with resistance vectorially rather than arithmetically. Power factor is the ratio of true power in watts to apparent power in volt-amperes.

Series resistance
Total resistance is the sum of the individual resistances, and current is the same everywhere in the loop.
Parallel resistance
For two branches, total resistance equals the product divided by the sum; total resistance is always less than the smallest branch.
Impedance
Z equals the square root of R squared plus X squared, where X is the net reactance.
Reactance
Inductive reactance equals 2 pi f L and capacitive reactance equals 1 divided by 2 pi f C, so frequency changes affect them in opposite directions.
Power factor
Power factor equals true power in watts divided by apparent power in volt-amperes.

Three-Phase Systems and Transformers

The square root of three, about 1.732, is the constant that connects line and phase quantities in three-phase systems, and knowing which quantity it applies to in wye versus delta is the whole game. Transformers change voltage and current inversely while keeping volt-amperes nearly constant, so a step-down transformer always raises available secondary current. Load calculations on three-phase systems begin by converting kVA into line current.

Wye relationships
Line voltage equals 1.732 times phase voltage, and line current equals phase current.
Delta relationships
Line voltage equals phase voltage, and line current equals 1.732 times phase current.
Three-phase power
True power equals 1.732 times line voltage times line current times power factor.
Three-phase current from kVA
Line current equals volt-amperes divided by 1.732 times line voltage.
Transformer ratio
Primary voltage divided by secondary voltage equals secondary current divided by primary current; volt-amperes are essentially equal on both sides.

Working Space and Equipment Clearances

Article 110 defines the space a worker needs to stand in front of energized equipment and examine, adjust or service it. Depth depends on the voltage and on what is directly across from the exposed live parts, described as three conditions of increasing hazard. Width, height and dedicated space above the equipment are fixed requirements that do not vary with voltage.

Depth by condition
For 151 to 600 volts to ground: 3 feet for Condition 1, 3 and 1/2 feet for Condition 2 with a grounded surface opposite, and 4 feet for Condition 3 with exposed live parts on both sides.
2023 NEC §110.26(A)(1)
Width
Working space must be at least 30 inches wide or as wide as the equipment, whichever is greater, and doors must open at least 90 degrees.
2023 NEC §110.26(A)(2)
Height
Headroom must be at least 6 and 1/2 feet or the height of the equipment, whichever is greater.
2023 NEC §110.26(A)(3)
Two entrances
Equipment rated 1,200 amperes or more and over 6 feet wide generally requires an entrance at each end of the working space.
2023 NEC §110.26(C)(2)
Dedicated space
The footprint of a panelboard extends from the floor to 6 feet above the equipment or to the structural ceiling, whichever is lower, and must stay clear of foreign piping and ducts.
2023 NEC §110.26(E)(1)
Termination temperature
Conductor ampacity at a termination is limited by the temperature rating of the terminal, not by the conductor insulation alone.
2023 NEC §110.14(C)

Electrical Safety Practice

Code compliance protects the installation, but work practices protect the electrician, and exams increasingly test both. The controlling ideas are that equipment is treated as energized until proven otherwise, that only the person who applied a lock removes it, and that the correct personal protective equipment is chosen from a documented assessment rather than from habit. Arc flash is a distinct hazard from shock and requires its own protective strategy.

Lockout and tagout
Each worker applies an individual lock and tag to the disconnecting means, and only that worker removes it.
Live-dead-live testing
Prove the test instrument on a known source, test for absence of voltage, then prove the instrument again on the known source.
Arc flash labeling
Equipment likely to require examination or servicing while energized must be field or factory marked to warn of potential arc-flash hazards.
2023 NEC §110.16
Personal protective equipment
Arc-rated clothing, insulating gloves with leather protectors, face and eye protection and hearing protection are selected based on the assessed shock and arc-flash hazards.
Approach boundaries
Shock protection boundaries limit how close unqualified and qualified persons may come to exposed energized parts without insulation or protective equipment.
Workmanlike installation
Electrical equipment must be installed in a neat and workmanlike manner, with unused openings closed and equipment protected from damage.
2023 NEC §110.12
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Last updated: July 2026

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