Crash course: Voltage, amperage, and resistance basics with examples (part 5)

Ohm's Law: The One Equation That Runs the Trade

Voltage pushes, current flows, resistance fights back. Every troubleshooting call, every conductor sizing decision, every nuisance trip traces back to the same triangle: V = I x R. Memorize it cold. You'll use it more than your tape measure.

Voltage (V) is electrical pressure, measured in volts. Current (I) is the rate of electron flow, measured in amperes. Resistance (R) is opposition to flow, measured in ohms. Power (P) ties them together: P = V x I. If you know any two values, you can solve for the rest in your head before you climb the ladder.

Quick field math you should never need a calculator for:

• 120V circuit, 12 ohm load: I = 120 / 12 = 10A
• 240V baseboard heater, 1500W: I = 1500 / 240 = 6.25A
• 20A breaker at 120V: max continuous load is 16A per NEC 210.19(A)(1) (80% rule)
• Voltage drop across 0.5 ohm of wire at 15A: V = 15 x 0.5 = 7.5V (way too much)

Voltage: Pressure, Not Flow

Voltage is potential difference. No difference, no current, no matter how much copper you run. That's why a bird sits on a 13.8kV line and goes home for dinner. Both feet, same potential.

Standard residential service in the US is 120/240V single phase. Commercial buildings typically run 208Y/120V or 480Y/277V three phase. NEC 220.5(A) sets the nominal voltages used for load calculations: 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, and 600. Use these for math, not what your meter actually reads.

Voltage drop is where rookies get burned. NEC recommends (does not require) keeping branch circuit drop under 3%, with total drop under 5% in 210.19(A) Informational Note 4 and 215.2(A)(1) Informational Note 2. On a 120V circuit, that's 3.6V max on the branch. Long runs to detached garages and well pumps are the usual culprits.

Field tip: If a homeowner complains lights dim when the AC kicks on, you're chasing voltage drop or a loose neutral, not a bad bulb. Pull the panel cover and torque every lug to spec before you start swapping breakers.

Amperage: What Actually Cooks Things

Current is what melts insulation, trips breakers, and kills people. Voltage gets the headlines, but amps do the damage. 100mA across the chest stops a heart. That's why GFCIs trip at 5mA and AFCIs watch for arc signatures.

Conductor ampacity comes from NEC Table 310.16 for the common stuff. Memorize the 60/75/90 degree columns for 14, 12, 10, and 8 AWG copper:

1. 14 AWG: 15A (60C), 20A (75C), 25A (90C), but 240.4(D) caps it at 15A overcurrent protection
2. 12 AWG: 20A / 25A / 30A, capped at 20A by 240.4(D)
3. 10 AWG: 30A / 35A / 40A, capped at 30A by 240.4(D)
4. 8 AWG: 40A / 50A / 55A, no small-conductor cap above this size

Apply derating for ambient temp (310.15(B)(1)) and conduit fill (310.15(C)(1)) before you call the run done. More than three current-carrying conductors in a raceway and you're knocking ampacity down by 80%, 70%, 50%, or worse.

Resistance: The Silent Killer of Connections

Resistance shows up two ways on the job: the load itself, and unwanted resistance in your splices, terminations, and conductors. The first is design. The second is how your work fails.

A loose wire nut might read 0.5 ohms instead of near zero. At 15A, that's 7.5V dropped and 112W dissipated as heat right at the connection. That's a melted backstab waiting to happen. NEC 110.14(D) now requires torque values per listing or manufacturer instructions. Carry a torque screwdriver. It's not optional anymore.

Common resistance values worth knowing:

• Good ground rod: 25 ohms or less per NEC 250.53(A)(2), or drive a second rod
• Bonded equipment to ground: under 1 ohm, ideally under 0.1
• Healthy motor winding: matches manufacturer spec, phases within 5% of each other
• Open circuit on a multimeter: OL or infinity

Putting It Together on a Service Call

Customer says the dishwasher quit. You pull the kick plate, set your meter to AC volts, and read 121V at the receptacle. Voltage is fine. Switch to amps with a clamp on the hot conductor while the unit tries to start: 0.2A and dropping. The motor isn't drawing. Now you check resistance across the motor leads with the unit unplugged: open. Burned winding. Replace the motor or the appliance.

That whole diagnostic took three measurements and two minutes because V, I, and R told you the story in order. Voltage present, current absent, resistance infinite. The math points to exactly one answer.

Field tip: Always measure voltage first, current second, resistance last (with power off). Skipping the order is how you blow up a meter or miss an obvious upstream problem.

The Numbers You Should Have Memorized

If you're reaching for a calculator on basic stuff, you're slow. These should be reflex by your second year:

• Watts to amps at 120V: divide by 120 (1500W = 12.5A)
• Watts to amps at 240V: divide by 240 (4500W water heater = 18.75A, needs 30A circuit per 422.13)
• Three phase amps: I = P / (V x 1.732 x PF)
• 80% continuous load rule: 210.19(A)(1) and 210.20(A)
• Voltage drop formula single phase: VD = (2 x K x I x L) / CM, where K = 12.9 for copper

Ohm's law isn't a textbook exercise. It's the framework that turns a confused service call into a 10 minute diagnosis. Get fluent, stay fluent, and the trade gets a lot easier.