Crash course: Voltage, amperage, and resistance basics quick reference (part 5)

Ohm's Law: The Only Equation You Need on the Truck

V = I × R. Voltage equals current times resistance. Rearrange it three ways and you can solve almost any field problem without pulling out a calculator app. Memorize the wheel: V = IR, I = V/R, R = V/I.

Power rounds it out: P = VI, or P = I²R, or P = V²/R. A 1500W heater on 120V pulls 12.5 amps. That same heater on 240V pulls 6.25 amps. Half the current, same heat output, which is why resistive loads are often spec'd at higher voltage to cut conductor size.

Keep the units honest. Volts, amps, ohms, watts. If you start mixing milliamps and kilowatts in the same scratch calc, you will land a decimal point in the wrong column and undersize a feeder.

Voltage: Potential, Not Flow

Voltage is electrical pressure between two points. It exists whether or not current is flowing. A dead-ended branch circuit still reads 120V to ground at the breaker, even with no load. That is why you test before you touch, every time, on every conductor.

Standard nominal system voltages per NEC 220.5(A) are 120, 120/240, 208Y/120, 240, 480Y/277, and 480. Use these for load calculations regardless of what your meter actually reads in the field. Utility tolerance under ANSI C84.1 is typically +/- 5%, so 120V can legitimately sit anywhere from 114V to 126V at the service.

Voltage drop is the field killer. NEC 210.19(A) Informational Note 4 recommends branch circuits not exceed 3% drop, and the combined feeder plus branch not exceed 5%. Long runs at 120V on #12 will bite you fast. Bump the wire size or step up the voltage.

Tip: When a motor runs hot and slow on a long run, check voltage at the motor terminals under load, not at the panel. A 10% drop under inrush will cook windings over time.

Amperage: What Actually Does the Work

Current is the flow of electrons, measured in amps. It is what trips your breaker, heats your conductor, and shocks the apprentice. Voltage threatens; current kills. As little as 100 mA across the chest can stop a heart, which is the whole reason GFCI exists at 4-6 mA trip thresholds per UL 943.

Conductor ampacity comes from NEC Table 310.16 for the 0-2000V range. Read the column matching your insulation rating (typically 75°C for terminations under 110.14(C)) and apply correction factors for ambient temp and conduit fill. A #12 THHN is rated 30A in the 90°C column but you size to 75°C, so 25A, and the breaker max is still 20A under 240.4(D).

Continuous load rule: any load expected to run 3 hours or more is sized at 125% per 210.19(A)(1) and 215.3. A 16A continuous load needs a 20A breaker and conductors rated for 20A. Get this wrong on a lighting branch and you will be back replacing breakers that nuisance trip on Mondays.

Resistance: Why Connections Matter More Than Wire

Resistance is opposition to current flow, measured in ohms. Copper conductors have very low resistance per foot. The problems live at terminations, splices, and corroded lugs. A loose neutral can read 0.5 ohms cold and 50 ohms hot under load, dropping enough voltage to dim half the building.

Insulation resistance is the inverse problem. Megger between conductors and to ground. Anything reading under 1 megohm on a 600V circuit is suspect per IEEE 43 minimum acceptable values. New installations should read in the hundreds of megohms or higher.

• Loose lug: heats up, oxidizes, gets worse over time
• Wet splice: low resistance to ground, GFCI nuisance trip or fault
• Aluminum to copper without listed connector: galvanic corrosion, eventual failure
• Over-torqued lug: cold flow on stranded aluminum, high resistance joint
• Under-torqued lug: poor contact, arcing, glowing connection

Torque every termination to the manufacturer spec. NEC 110.14(D) made this a code requirement, not a suggestion. Carry a calibrated torque screwdriver and use it.

Series vs Parallel: Reading a Circuit at a Glance

Series circuits share one current path. Resistances add: R_total = R1 + R2 + R3. Voltage divides across the loads. Christmas-light failure: one bulb opens, the whole string drops.

Parallel circuits share voltage across each branch. Currents add at the node, resistances combine as 1/R_total = 1/R1 + 1/R2 + 1/R3. This is how every receptacle circuit in a house works. Each device sees the full 120V regardless of what else is plugged in.

Real branch circuits are mixed. The hot conductor from breaker to first device is series with everything downstream. The loads themselves run parallel. A bad backstabbed neutral on the third device in a string acts like a series fault that takes out everything beyond it while leaving the first two working fine. That is your signature symptom for a loose neutral mid-circuit.

Field Math You Will Actually Use

Here is the short list worth memorizing before you climb the ladder:

1. Single-phase amps: I = P / V. A 1200W load on 120V draws 10A.
2. Single-phase amps with PF: I = P / (V × PF). Add power factor for motors.
3. Three-phase amps: I = P / (V × 1.732 × PF). The 1.732 is the square root of 3.
4. Voltage drop, single-phase: VD = (2 × K × I × L) / CM, where K = 12.9 for copper, L = one-way length in feet, CM = circular mils from Chapter 9 Table 8.
5. Voltage drop, three-phase: same formula, swap the 2 for 1.732.
6. Watts to BTU: 1 watt = 3.412 BTU/hr. Useful when sizing transformer rooms.

Tip: If you only remember one number, remember 12.9. That is your copper K-value for voltage drop and it will get you 90% of the way on any field calc.

Stay ahead of the math, stay ahead of the callback. Code minimums get you legal. Understanding the why behind the numbers gets you reliable installs that hold up after the truck pulls away.