Crash course: Voltage, amperage, and resistance basics field cheat sheet (part 5)

Ohm's law is the whole game

Voltage pushes, current flows, resistance pushes back. V = I × R. That equation governs every troubleshooting call you will ever take. Memorize it cold, then learn to rearrange it without thinking: I = V/R, R = V/I.

Power follows right behind: P = V × I. Combine the two and you can solve anything on a residential or light commercial job with the numbers stamped on the nameplate. A 1500W heater on a 120V circuit pulls 12.5A. A 240V dryer rated 5500W pulls about 23A, which is why it lands on a 30A breaker per NEC 210.19.

Keep these four forms on the back of your meter or in your head:

• V = I × R (find voltage)
• I = V / R (find current, the troubleshooting form)
• R = V / I (find resistance, the dead-circuit form)
• P = V × I (find wattage or verify load)

Voltage: what your meter actually reads

Voltage is potential difference. Your meter reads between two points, never at one point. A hot conductor reads 120V to neutral, 120V to ground, and 0V to another hot on the same leg. Cross legs in a single-phase 240V panel and you get 240V hot to hot.

Three-phase changes the math. 208Y/120V wye gives you 120V phase to neutral and 208V phase to phase. 480Y/277V gives 277V to neutral, 480V phase to phase. Know which system you are on before you energize anything. NEC 110.4 requires equipment voltage ratings to match the supply.

Phantom voltage will lie to you. A high-impedance meter on a long unloaded run can read 40 to 80V from induced coupling. Put a wiggy or a low-Z setting on it and the ghost drops to zero. Trust loaded readings, not floating ones.

Amperage: the number that trips breakers and starts fires

Current is what does the work and what kills you. Conductors are sized by ampacity in NEC Table 310.16, and overcurrent protection per NEC 240.4 has to match the conductor, not the load. 14 AWG copper gets a 15A breaker, 12 AWG gets 20A, 10 AWG gets 30A. No exceptions on general-purpose branch circuits.

Continuous loads, anything running 3 hours or more, get sized at 125% per NEC 210.20(A). A 16A continuous load needs a 20A breaker and 12 AWG conductors rated for at least 20A. Miss this and the breaker nuisance trips, or worse, holds while the conductor cooks inside the wall.

Clamp meter rules of thumb when you are reading a circuit:

1. Clamp one conductor at a time, never the whole cable, or the fields cancel and you read zero.
2. Inrush on motors can be 6 to 8 times running current for a fraction of a second.
3. If neutral current equals hot current on a 120V circuit, the load is balanced and clean.
4. Neutral current on a multiwire branch circuit should equal the difference between the two hots, not the sum.

Resistance: the dead-circuit diagnostic

Resistance is measured with the circuit de-energized and isolated. Power on, you read voltage and current. Power off, you read resistance and continuity. Mix those up and you blow a meter or worse.

Useful resistance benchmarks in the field: a good ground rod connection per NEC 250.53(A)(2) should be 25 ohms or less, or you drive a second rod. A heating element reads its rated resistance plus or minus 10%, calculated from R = V²/P. A 4500W 240V water heater element should read about 12.8 ohms. Open reads OL, shorted reads near zero, and a partial short reads somewhere weird in between.

Insulation resistance is a different animal. A megger at 500V or 1000V on a motor winding should read megohms, not ohms. NETA and IEEE 43 call for 100 megohms minimum on most motors after temperature correction. Anything under 1 megohm and the winding is wet, contaminated, or failing.

Voltage drop: the silent code violation

NEC 210.19 informational note recommends 3% drop on branch circuits, 5% total including the feeder. It is a recommendation, not a rule, but inspectors and engineers treat it like one on long runs. Undersized conductors on a long pull cause motors to overheat, LED drivers to flicker, and electronics to fail early.

Quick formula for single-phase: VD = (2 × K × I × L) / CM, where K is 12.9 for copper, L is one-way length in feet, CM is circular mils from Chapter 9 Table 8. Three-phase swaps the 2 for 1.732. Or just remember: a 20A load on 12 AWG copper drops about 3% at 100 feet. Past that, upsize.

If a homeowner complains that the lights dim when the well pump kicks on, you are not chasing a bad neutral first. You are measuring voltage drop on the pump circuit under load. Nine times out of ten the conductor is undersized for the run.

Putting it together on a service call

Walk in with this sequence and you will solve most residential and light commercial calls in under an hour. Verify voltage at the source. Verify voltage at the load. Compare. A drop greater than a few volts under load points to a loose connection, undersized conductor, or failing breaker.

If voltage is good and the load will not run, kill the power, lock it out per NEC 110.25 and OSHA 1910.333, and ohm out the load. Open element, open winding, or shorted contactor coil will all show in the resistance reading. The numbers do not lie if your meter is on the right setting and your leads are good.