Crash course: Voltage, amperage, and resistance basics quick reference (part 4)

Ohm's Law: the only equation you need memorized

V = I × R. Voltage equals current times resistance. Rearrange it however the job demands: I = V/R when you know the supply and the load, R = V/I when you're checking a suspect connection. Power follows: P = V × I, and by substitution P = I²R or P = V²/R.

On a 120V circuit pulling 12A, the load resistance is 10 ohms and power draw is 1440W. That's roughly 80% of a 15A branch, the practical ceiling per NEC 210.19(A) and 210.20(A) for continuous loads. Burn this triangle into muscle memory because every troubleshooting decision in the field traces back to it.

If the math doesn't match what your meter reads, the meter is right and your assumptions are wrong. Recheck the conductor size, the connections, and the actual load before second-guessing the instrument.

Voltage: potential, not flow

Voltage is electrical pressure between two points. No reference, no reading. That's why a "hot" conductor measures 120V to ground but 0V to another conductor on the same phase, and 240V across two opposing phases in a standard residential split-phase service.

Common nominal system voltages you'll see on a service call:

• 120/240V single-phase, 3-wire: residential and small commercial
• 120/208V wye, 3-phase, 4-wire: light commercial, multi-tenant
• 277/480V wye, 3-phase, 4-wire: industrial, large commercial lighting
• 240V delta with high-leg (208V to neutral on B phase): older commercial, NEC 110.15 requires orange ID on the high leg

Voltage drop is governed by NEC 210.19(A) Informational Note No. 4 (branch circuits) and 215.2(A)(2) Informational Note No. 2 (feeders): aim for 3% on the branch, 5% combined. Drop kills equipment quietly. Motors run hot, electronics brown out, and the homeowner blames the appliance.

Amperage: what actually does the work

Current is the flow of electrons, measured in amps. It's what heats conductors, trips breakers, and welds contacts shut. Size every conductor and overcurrent device to the load, not the other way around.

Reference points worth keeping handy:

1. 14 AWG copper: 15A max, NEC 240.4(D)(3)
2. 12 AWG copper: 20A max, NEC 240.4(D)(5)
3. 10 AWG copper: 30A max, NEC 240.4(D)(7)
4. Continuous loads (3 hours or more): conductor and OCPD sized at 125% of the load per NEC 210.19(A)(1) and 210.20(A)

Ampacity tables in NEC 310.16 assume specific conditions. Derate for ambient temp above 30°C per Table 310.15(B)(1) and for more than three current-carrying conductors in a raceway per 310.15(C)(1). Skip the derate and the insulation cooks long before the breaker notices.

Resistance: the variable that breaks things

Resistance opposes current. Conductors have it, connections have it, and loads are built around specific values of it. The danger is that resistance climbs slowly: a backstabbed receptacle, a loose lug, a corroded splice. Each adds heat, and heat raises resistance further until something fails.

Use a low-resistance ohmmeter (a DLRO or a quality DMM on the lowest range) when you suspect a bad termination. A clean copper-to-copper connection should read effectively zero. Anything above a fraction of an ohm in a power circuit is suspect.

Thermal imaging on an energized panel under load finds high-resistance joints faster than any meter. A 20°F delta between identical lugs means torque it, clean it, or replace it.

Putting it together on a service call

The diagnostic loop is short. Measure voltage at the source, measure voltage at the load under operation, and the difference is your drop. Drop divided by current gives you the resistance hiding in the run. If that resistance lives in 4 inches of terminations instead of 80 feet of conductor, you found your problem.

Quick field sequence for a "low voltage at the receptacle" complaint:

• Verify nominal voltage at the panel, hot to neutral and hot to ground
• Energize the load and read voltage at the device, note the drop
• Drop greater than 5% under load with proper conductor sizing points to a connection, not a calculation
• Pull the device, check for backstab terminations and torque per NEC 110.14(D) and the manufacturer's marking

Three numbers to commit to memory

Most residential and light commercial troubleshooting comes down to three quick references. 1.732 is the square root of 3, the multiplier for line-to-line current and power in a balanced 3-phase system. 12.9 ohms per 1000 feet is the approximate DC resistance of 12 AWG copper at 75°C, useful for fast voltage drop estimates. 1875W is the practical continuous load limit on a 20A, 120V circuit (20A × 120V × 0.80, roughly).

Code is the floor, not the ceiling. The numbers above keep the work safe and legal. Experience tells you when to tighten the margins... a 30A circuit feeding a hard-starting compressor doesn't care that the conductor passes 240.4(D). Size for the inrush and the duty cycle, not just the nameplate.

Voltage tells you the system has potential. Amperage tells you it's working. Resistance tells you whether it's working safely. Read all three on every call and the failures stop being surprises.