Crash course: Voltage, amperage, and resistance basics for residential electricians (part 1)

Why these three matter on every job

Voltage, amperage, and resistance are the only variables you need to predict what a circuit will do before you energize it. Get them right and your conductor sizing, breaker selection, and voltage drop math all fall into place. Get them wrong and you trip breakers, cook insulation, or fail inspection.

Residential work hides a lot of this behind standard practice. A 15A bedroom circuit on 14 AWG copper feels automatic until you add a 100 foot run, a space heater, and a homeowner who wants to "just add one more outlet." Then the numbers matter again.

This part 1 covers the fundamentals and how they show up in dwelling unit work under NEC Chapters 1 through 4. Part 2 will cover load calcs, voltage drop, and AFCI/GFCI behavior.

Voltage: the pressure

Voltage is electrical pressure, measured in volts. In a typical single-family dwelling fed by a split-phase service, you have 240V between the two ungrounded (hot) legs and 120V from either hot to the grounded (neutral) conductor. NEC 210.6 sets the voltage limits for branch circuits in dwellings.

What you actually see at the receptacle varies. Utility nominal is 120/240V, but acceptable service voltage under ANSI C84.1 ranges roughly 114V to 126V at the point of delivery. If you are reading 108V at a kitchen counter receptacle with no load, the problem is upstream, not the device.

• 120V: general lighting, receptacles, small appliances
• 240V: ranges, dryers, water heaters, central AC, EV chargers
• 120/240V: appliances needing both legs and a neutral, like electric ranges

Amperage: the flow

Amperage is the rate of current flow, measured in amps. It is what actually heats conductors, trips breakers, and kills people. Sizing decisions hinge on it more than anything else.

NEC 210.19(A) requires branch circuit conductors to have an ampacity not less than the maximum load served, and not less than 125% of continuous loads. NEC 210.20(A) applies the same 125% rule to the overcurrent device. A 16A continuous load needs a 20A breaker and 12 AWG copper, not 14.

Field tip: if a circuit runs more than 3 hours at a stretch, treat it as continuous. Baseboard heat, EV chargers, and exterior lighting on photocells all qualify, even when the homeowner swears "it only runs sometimes."

Resistance: what fights the flow

Resistance, measured in ohms, is the opposition to current. Every conductor, connection, and load has some. In residential work, three sources of resistance bite you: long conductor runs, loose terminations, and corroded splices.

Conductor resistance is published in NEC Chapter 9, Table 8. For 12 AWG uncoated copper, that is 1.93 ohms per 1000 feet at 75 degrees C. Double the run length, double the resistance, double the voltage drop at a given current.

Loose terminations are the resistance you cannot calculate. A backstabbed receptacle with a marginal spring contact can read fine cold and glow red under a vacuum cleaner. NEC 110.14 covers termination requirements, and the torque values in 110.14(D) are not optional in the 2017 and later cycles.

Ohm's Law in the panel and at the device

V = I x R. That is the whole law. Voltage equals current times resistance. Rearranged: I = V/R, and R = V/I. Power follows from P = V x I, which is how you convert nameplate watts to the amps that actually drive your sizing.

Examples you will run into this week:

1. 1500W space heater at 120V: I = 1500/120 = 12.5A. Fine on a dedicated 20A circuit, marginal on a shared 15A.
2. 4500W water heater at 240V: I = 4500/240 = 18.75A. Continuous load, so 18.75 x 1.25 = 23.4A minimum. 30A breaker, 10 AWG copper per NEC 422.13.
3. 240V 50A EV charger: nameplate 50A continuous, so 50 x 1.25 = 62.5A. 70A breaker, 4 AWG copper at 75 degrees C per Table 310.16.

How the three interact in a real circuit

Picture a 100 foot run of 14 AWG copper feeding a 12A load at 120V. Resistance of the round trip is about 0.62 ohms. Voltage drop is I x R, so 12 x 0.62 = 7.4V. The load sees 112.6V, not 120V. That is 6.2% drop, past the 3% NEC 210.19(A) Informational Note recommendation for branch circuits.

The motor or resistive load now draws slightly more current to do the same work, conductors run hotter, and the cycle compounds on long runs. This is why NEC voltage drop guidance exists, even though it is informational rather than mandatory.

Field tip: on any branch circuit run over 75 feet at 15A or 20A, upsize one wire gauge by default. The labor cost is negligible at rough-in and you will never get a callback for dim lights or nuisance trips.

What to carry in your head

You do not need to memorize Chapter 9. You do need the reflexes: 120V or 240V, continuous or not, length of run, and the 125% rule. Everything else is lookup.

• 120V x 15A = 1800W circuit capacity, 1440W usable continuous
• 120V x 20A = 2400W circuit capacity, 1920W usable continuous
• 240V x 30A = 7200W circuit capacity, 5760W usable continuous
• Voltage drop target: 3% on branch, 5% total including feeder

Part 2 picks up with dwelling load calculations under NEC Article 220, voltage drop math on real runs, and how AFCI and GFCI devices read the same three variables you just learned.