Crash course: Voltage, amperage, and resistance basics field cheat sheet (part 4)

Ohm's Law: The One Equation You Run Everything Through

Voltage pushes, amperage flows, resistance fights back. V = I × R. Rearrange it: I = V/R, R = V/I. If you can run those three formulas without thinking, you can size a conductor, predict a voltage drop, and spot a bad connection before it burns.

Power follows: P = V × I. A 1500W heater on a 120V circuit pulls 12.5A. That number drives breaker selection, conductor sizing per NEC 210.19, and whether you're tripping the 15A circuit the homeowner swears "just stopped working."

Memorize the wheel or keep it on your phone. Either way, every troubleshooting call eventually comes back to these four variables.

Voltage: What You're Actually Reading

Voltage is potential difference, measured between two points. Hot to neutral on a standard residential branch circuit reads nominal 120V, hot to hot on a split-phase service reads 240V. Three-phase commercial reads 208V or 480V line to line depending on the system.

NEC 220.5(A) sets the nominal voltages used for load calculations: 120, 120/240, 208Y/120, 240, 480Y/277, 480, and 600. Field readings drift. ANSI C84.1 allows utilization equipment a range of roughly 114V to 126V on a 120V nominal system. Anything outside that, start looking at the source.

Reading 208V where you expected 240V? You're on a high-leg or three-phase system, not split-phase. Check the panel schedule before you connect a 240V single-phase load.

Amperage: Load, Ampacity, and Why They're Different

Amperage is current flow. Load is what the equipment draws. Ampacity is what the conductor can carry continuously without exceeding its temperature rating. Confuse the two and you'll undersize a feeder.

NEC 310.16 gives you the ampacity tables. A 12 AWG copper THHN conductor at the 75°C column carries 25A, but NEC 240.4(D)(5) caps the overcurrent device at 20A for that wire size in most applications. The conductor can carry more, the code says you don't get to use it that way.

Continuous loads change the math. NEC 210.19(A)(1) and 215.2(A)(1) require feeders and branch circuits to be sized at 125% of the continuous load plus 100% of the non-continuous load. Miss that on a lighting calc and your inspector will catch it.

• Branch circuit conductor: 125% continuous + 100% non-continuous
• OCPD rating: also 125% continuous + 100% non-continuous, NEC 210.20(A)
• Equipment terminations: check the listed temperature rating, NEC 110.14(C)

Resistance: The Invisible Variable That Burns Houses Down

Resistance is opposition to current flow, measured in ohms. Conductors have it, connections have it, loads are built around it. The problem isn't the resistance you designed for. It's the resistance you didn't know was there.

A loose lug, a backstabbed receptacle, a corroded splice... each adds resistance, each drops voltage across the bad joint, each dissipates that voltage as heat. P = I²R. Two amps through a half-ohm bad connection puts 2 watts into that single point. Not much. Twenty amps through the same bad joint puts 200 watts into a wire nut. That's how fires start.

Pull a thermal camera across a panel under load before you leave a service call. Anything more than 10°C hotter than its neighbors gets retorqued or replaced.

Voltage Drop: The Calculation You Skip at Your Peril

NEC 210.19 Informational Note No. 4 recommends branch circuits not exceed 3% voltage drop, with the combined branch and feeder not exceeding 5%. It's a recommendation, not a mandate, but PV systems under NEC 690 and certain motor circuits do enforce it.

The single-phase formula: VD = (2 × K × I × D) / CM. K is 12.9 for copper, 21.2 for aluminum at 75°C. D is one-way distance in feet, CM is the conductor's circular mil area from Chapter 9 Table 8. For a 200 foot run of 12 AWG copper at 16A: VD = (2 × 12.9 × 16 × 200) / 6530 = 12.6V. That's 10.5% on a 120V circuit. Bump to 10 AWG, recalculate, get under 3%.

1. Get the actual one-way length, not the panel-to-panel rough estimate
2. Use the load you'll actually run, not the breaker size
3. Round up wire size, never down
4. Recheck on long motor branch circuits, NEC 430.6 governs the load value

Field Test Sequence: Voltage, Then Continuity, Then Resistance

Order matters. Always verify your meter on a known live source before testing a dead circuit, then verify again after. Live-dead-live. NFPA 70E 120.5 calls it out for a reason.

Test voltage first to confirm the circuit state. Lock out, then test continuity to confirm conductor integrity. Last, measure resistance to ground and between conductors with an insulation tester if the situation calls for it. NEC 110.7 requires installations to be free from short circuits, ground faults, and any connections to ground other than as required.

• L-N and L-G should read nominal voltage hot, near zero dead
• N-G should read under 2V on a healthy circuit, anything higher means a shared neutral or a bonding issue
• Insulation resistance to ground on a 600V class conductor: minimum 1 megohm per the old rule of thumb, manufacturer spec governs

Three variables, one law, a handful of code references. Run the numbers before you run the wire.