Crash course: Voltage, amperage, and resistance basics exam prep (part 1)

Voltage: the push

Voltage is electrical pressure. It is the potential difference between two points that drives current through a conductor. No pressure, no flow. Measured in volts (V), and on the job you will see it expressed nominally: 120V, 208V, 240V, 277V, 480V.

Nominal is not actual. A 120V circuit can read anywhere from about 114V to 126V and still be within ANSI C84.1 service voltage range A. Always meter before you assume. NEC 220.5(A) requires you to use nominal voltages (120, 120/240, 208Y/120, 240, 347, 480Y/277, 600) for load calculations unless other voltages are specified.

Single phase versus three phase changes everything downstream. A 120/240V single-phase residential service gives you two hots 180 degrees apart. A 208Y/120V three-phase wye gives you three hots 120 degrees apart with a shared neutral. Know which one you are standing in front of before you pull a cover.

Amperage: the flow

Amperage is the rate of electron flow past a point per second. One amp equals one coulomb per second. Measured with a clamp meter on the conductor, never across it. Current is what does the work, and current is what kills people and starts fires.

Conductor ampacity is governed by NEC Table 310.16 for the most common installations (not more than three current-carrying conductors in a raceway, ambient 30 degrees C). A 12 AWG THHN copper conductor is rated 30A at the 90 degree column, but the 60/75 degree termination rules in NEC 110.14(C) usually knock you down to 25A or 20A depending on the equipment listing.

• 14 AWG copper: 15A breaker max, 60 degree column
• 12 AWG copper: 20A breaker max
• 10 AWG copper: 30A breaker max
• 8 AWG copper: 40A or 50A depending on termination temp
• 6 AWG copper: 55A at 60 degrees, 65A at 75 degrees

Resistance: the brake

Resistance opposes current. Measured in ohms. Every conductor, connection, and load has it. Copper has very little, which is why we use it. A loose wire nut or a corroded lug has a lot, which is why panels burn down.

Resistance increases with conductor length and decreases with conductor cross-section. That is the entire reason voltage drop exists. NEC 210.19(A) Informational Note 4 recommends keeping branch circuit voltage drop to 3 percent or less, and total drop (feeder plus branch) to 5 percent or less. Not a code requirement, but a quality benchmark inspectors and engineers will hold you to.

If a 20A circuit is tripping intermittently and the load math checks out, ohm out the connections. A high-resistance splice in a junction box will heat up under load, expand, and trip the breaker on thermal sensing long before you find a short.

Ohm's Law on the job

V = I x R. Voltage equals current times resistance. Rearrange it as needed: I = V/R, R = V/I. This is the single most useful equation in the trade. Memorize it cold.

Power follows: P = V x I. Watts equal volts times amps. For three phase: P = V x I x 1.732 x power factor. Resistive loads (heat strips, incandescent) have a power factor of 1. Motors and ballasts do not, which is why nameplate FLA matters more than calculated current on motor work.

1. Sizing a 240V, 4500W water heater: I = 4500/240 = 18.75A. Code requires 125 percent for continuous loads per NEC 422.13, so 23.4A minimum. Use a 30A breaker on 10 AWG.
2. Voltage drop on a 100 foot run of 12 AWG at 16A, 120V: VD = 2 x 1.93 ohms/1000ft x 100ft x 16A / 1000 = 6.2V, or 5.2 percent. Upsize to 10 AWG.
3. A motor pulling 12A at 240V single phase: P = 240 x 12 = 2880W apparent. Real power depends on PF, usually around 0.8 for induction motors.

Putting it together at the panel

Voltage is supplied by the utility and verified with a meter at the line side of the main. Amperage is what the loads draw, summed and demand-factored per NEC Article 220. Resistance is everywhere, and managing it through proper terminations, correct conductor sizing, and clean splices is most of what separates a clean install from a callback.

For exam prep, expect questions that combine all three. You will get a load in watts, a voltage, and be asked for the breaker size. Or a conductor size, a length, and a load, asked for the voltage drop. Practice the algebra until you do not need a calculator for the common values.

On the test, watch the units. Watts vs VA, line-to-line vs line-to-neutral, single phase vs three phase. Half the wrong answers come from missing a 1.732 or dividing by 240 when you should have divided by 120.

Quick reference for the truck

Tape these to the inside of your meter case or save them in Ask BONBON. They cover 80 percent of field math.

• Ohm's Law: V = IR, P = VI
• Three phase power: P = V x I x 1.732 x PF
• Continuous load factor: 125 percent (NEC 210.19, 215.2, 230.42)
• Voltage drop target: 3 percent branch, 5 percent total
• Common nominal voltages: 120, 208, 240, 277, 480
• Copper 75 degree ampacity quick: 14/20, 12/25, 10/35, 8/50, 6/65, 4/85, 2/115, 1/0/150

Part 2 covers series vs parallel circuits, Kirchhoff's laws, and how power factor shows up on the test and on the meter.