Crash course: Voltage, amperage, and resistance basics quick reference (part 1)

Why these three matter on every job

Voltage, amperage, and resistance are the load-bearing concepts behind every calculation on a service call. Misread one and you mis-size a conductor, trip a breaker, or burn a terminal. The NEC assumes you already know them cold before you open Chapter 3.

This is part 1 of a refresher. Keep it tight, keep it usable, and tie each concept to something you actually do in the field.

Voltage: the push

Voltage is electrical pressure, measured in volts. It is the potential difference between two points. No difference, no current. A 240V circuit has twice the push of a 120V circuit, which is why dryers and ranges run on it. NEC 210.6 governs voltage limits for branch circuits in dwellings and other occupancies.

Nominal voltages you see daily: 120/240V single phase, 120/208V wye, 277/480V wye in commercial. The utility delivers nominal; the actual reading at the panel can swing several percent. NEC 210.19(A) Informational Note No. 4 recommends sizing conductors so voltage drop on a branch circuit does not exceed 3 percent, with combined feeder and branch under 5 percent.

• Measure phase to phase and phase to ground before energizing anything.
• 277V to ground will kill you faster than 120V. Treat 480V wye like the hazard it is.
• Low voltage at the load usually means a long run, undersized conductor, or a loose lug upstream.

Amperage: the flow

Amperage, or current, is the rate of electron flow in coulombs per second. One amp equals one coulomb per second. Current is what does the work, and current is what cooks insulation when it exceeds conductor ampacity. NEC Table 310.16 is the reference you will open more than any other for ampacity in raceways or cables.

Overcurrent protection sizing lives in NEC 240.4. Conductors must be protected at their ampacity after applying correction and adjustment factors from 310.15(B) and (C). The 60/75/90 degree column you use depends on the termination rating, per NEC 110.14(C), not the conductor insulation rating alone.

Field tip: when a 12 AWG THHN shows up on a 90 degree column at 30 amps, you still cannot put it on a 30 amp breaker if the breaker terminals are rated 75 degrees. Terminations rule. Read 110.14(C) twice.

Resistance: the opposition

Resistance opposes current flow, measured in ohms. Conductors have low resistance, insulation has very high resistance, and connections sit somewhere in between depending on torque, oxidation, and contact area. A loose neutral is just a high-resistance connection generating heat where it should not be.

Temperature changes resistance. Copper rises about 0.4 percent per degree C. That is why long runs in hot attics measure differently in August than in February, and why NEC 310.15(B) gives ambient temperature correction factors. Apply them; they are not optional.

• Bonding and grounding resistance: NEC 250.4 lays out the performance requirements.
• Ground rod resistance to earth: NEC 250.53(A)(2) requires a supplemental electrode unless a single rod tests at 25 ohms or less.
• Insulation resistance testing on motors and feeders catches problems before they become callbacks.

Ohm's Law, in your head

V = I times R. Rearrange as needed: I = V over R, R = V over I. Power follows: P = V times I, which gets you to P = I squared times R for heat dissipation. That last form is why a loose connection at 20 amps gets hot fast. Double the current, quadruple the heat.

Most of what you do in the field is one of three problems: find the current a load will draw, find the voltage drop on a run, or find the resistance of a fault path. Ohm's Law and the Code tables cover all three.

1. Load draws 1500 watts at 120V. I = 1500 / 120 = 12.5 amps. Size the circuit and OCPD accordingly.
2. Voltage drop on 100 feet of 12 AWG copper at 15 amps, 120V: roughly 4.8V, or 4 percent. Upsize to 10 AWG.
3. Fault current to ground through 0.25 ohms at 120V: 480 amps. Your OCPD better clear it fast.

Field tip: keep a voltage drop wheel or a calculator app on your phone. Doing the math wrong on a 200 foot run costs you a return trip and a homeowner's patience.

Putting it together at the panel

Every breaker in a panel is a statement about all three quantities. The voltage rating tells you the system it serves. The amp rating tells you the conductor it protects, governed by 240.4. The interrupting rating, AIC, tells you the available fault current it can safely clear, per NEC 110.9. Mis-spec any one and you have a code violation or a hazard.

Before you leave the truck, know the service voltage, the calculated load, and the available fault current at the point of installation. Those three numbers drive almost every decision you make once the cover comes off. Part 2 will cover single phase versus three phase, power factor, and how these basics scale up on commercial gear.