Crash course: Voltage, amperage, and resistance basics with examples (part 4)

Ohm's Law: The Only Math You Actually Need

Voltage, amperage, and resistance are not abstract concepts. They are what kills you, melts your conductors, and trips your breakers. Ohm's Law ties them together: V = I × R. Voltage equals current times resistance. Rearrange it depending on what you know.

Voltage (V) is electrical pressure, measured in volts. Current (I) is the flow of electrons, measured in amps. Resistance (R) is opposition to flow, measured in ohms. Add power (P = V × I) and you can size almost anything in the field without pulling out a textbook.

• V = I × R (find voltage)
• I = V ÷ R (find current)
• R = V ÷ I (find resistance)
• P = V × I (find watts)

Voltage: Pressure in the Pipe

Standard residential service in the US is 120/240V single-phase. Commercial and industrial often run 208Y/120V or 480Y/277V three-phase. Voltage drop matters once you start pulling long branch circuits. NEC 210.19(A) Informational Note 4 recommends keeping branch circuit voltage drop at 3% or less, with combined feeder and branch not exceeding 5%.

Quick field check: a 120V circuit reading 114V at the receptacle is sitting at 5% drop. Anything below that and you start cooking motor windings, dimming LED drivers, and getting nuisance trips. Measure under load, not at idle. A no-load reading lies.

If a homeowner complains the microwave keeps tripping the GFCI on a long garage run, check voltage drop before you swap the device. Nine times out of ten the run is undersized 14 AWG on a 20A circuit pushing 80 feet.

Amperage: What Actually Sizes Your Wire

Current is what generates heat in a conductor. That heat is what NEC Table 310.16 is built around. Ampacity ratings assume specific insulation types, ambient temperature, and conductor bundling. Derate when conditions change per 310.15(B) and (C).

For a quick gut check: 14 AWG copper at 60°C is good for 15A, 12 AWG for 20A, 10 AWG for 30A. But that is the ampacity, not the breaker size. Continuous loads (three hours or more) require the conductor and OCPD sized to 125% of the load per NEC 210.19(A)(1) and 210.20(A).

1. Calculate the load in amps (P ÷ V).
2. Multiply continuous portions by 1.25.
3. Pick a conductor with sufficient ampacity from 310.16.
4. Apply derating factors for ambient temp and conductor count.
5. Pick the OCPD per 240.4.

Resistance: Where the Heat Comes From

Every connection, splice, and termination adds resistance. A loose lug on a 200A feeder with even 0.01 ohms of contact resistance dissipates 400 watts as heat (P = I² × R). That is why torque specs matter and why thermal imaging finds problems before they become fires.

Wire itself has resistance per NEC Chapter 9 Table 8. A 100-foot run of 12 AWG copper has roughly 0.193 ohms. Push 16 amps through it and you drop about 3.1V on the run, or 6.2V round trip. That is your voltage drop in one calculation.

Anytime you open a panel for service, retorque the lugs to manufacturer spec. NEC 110.14(D) made this explicit in 2017. A calibrated torque screwdriver pays for itself the first time you avoid a callback.

Worked Example: Sizing a 240V Water Heater Circuit

A 4500W 240V residential water heater. Find the current: I = P ÷ V = 4500 ÷ 240 = 18.75A. Water heaters are continuous loads per NEC 422.13, so multiply by 1.25: 18.75 × 1.25 = 23.4A.

That puts you on a 30A breaker minimum, with 10 AWG copper conductors at 60°C from Table 310.16. Run is 50 feet from the panel. Check voltage drop: 10 AWG copper is roughly 1.21 ohms per 1000 ft. Round trip 100 ft × 0.00121 × 18.75A = 2.27V drop, under 1%. You are well within spec.

• Load: 4500W at 240V
• Current: 18.75A (23.4A continuous adjusted)
• OCPD: 30A double-pole per 240.6(A)
• Conductor: 10 AWG Cu
• Disconnect: required within sight per 422.31(B)

Field Habits That Keep You Out of Trouble

Always measure voltage before you trust the breaker is off. Test your meter on a known live source, test the circuit, then test the meter again. NFPA 70E calls this the live-dead-live test, and it is non-negotiable.

Keep an Ohm's Law wheel taped to the inside of your panel cover or saved offline on your phone. When a load is acting up, check current draw against nameplate, then check voltage at the load under operating conditions. The numbers will tell you whether you are looking at a wiring problem, a load problem, or a source problem.

• Voltage off at the load: open conductor, tripped OCPD, or loose connection upstream.
• Voltage present but low under load: voltage drop or high resistance connection.
• Current higher than nameplate: failing motor, locked rotor, or short to ground.
• Current at zero with voltage present: open winding or controller fault.