Crash course: Voltage, amperage, and resistance basics for apprentices (part 2)

Picking up where Part 1 left off

Part 1 covered the basics: voltage pushes, amperage flows, resistance opposes. Ohm's Law ties them together. Now we put those numbers to work on real circuits, real conductors, and real failures you will actually see on the job.

If you skipped Part 1, the short version is V = I x R. Voltage in volts, current in amps, resistance in ohms. Memorize the triangle. You will use it every day.

Voltage drop is not optional math

NEC 210.19(A) Informational Note No. 4 recommends branch circuit voltage drop stay at or below 3%, with combined feeder and branch circuit drop at or below 5%. Informational notes are not enforceable, but the AHJ, the engineer of record, and your foreman all care. Motors care more. A 240V motor fed at 220V pulls more current to make the same horsepower, and that extra current cooks windings.

The single phase formula most apprentices learn first:

• VD = (2 x K x I x D) / CM
• K = 12.9 for copper, 21.2 for aluminum at 75 deg C
• I = load in amps, D = one way distance in feet, CM = circular mils of the conductor

For three phase, swap the 2 for 1.732 (square root of 3). Run the numbers before you pull wire on any home run over about 100 feet. Upsizing from #12 to #10 on a long 20A circuit is cheaper than a callback.

On a 120V, 20A receptacle circuit running 150 feet, #12 copper drops roughly 4.6V, which is 3.8%. Bump it to #10 and you are back under 3%. Do this math at the truck, not after the drywall is up.

Amperage, ampacity, and why they are not the same word

Amperage is what the load draws. Ampacity is what the conductor can carry continuously without exceeding its temperature rating. NEC 310.16 is the table you live in for conductors in raceway or cable, rated up to 2000V.

Three things knock ampacity down: ambient temperature (310.15(B)(1)), more than three current carrying conductors in a raceway (310.15(C)(1)), and continuous loads (210.19(A)(1) and 215.2(A)(1) require 125% sizing). Stack all three on a hot attic run and your shiny #12 THHN is no longer a 20A conductor.

• 4 to 6 CCCs in a raceway: 80% adjustment
• 7 to 9 CCCs: 70%
• Ambient 96 to 104 deg F on 75 deg C wire: 0.88 multiplier
• Continuous load: size at 125% of the load

Resistance in the real world

Conductor resistance is in NEC Chapter 9, Table 8. It is also where apprentices first see why aluminum and copper behave differently on long runs. Resistance climbs with temperature, climbs with length, and drops as cross sectional area grows. That is why a #6 carries more than a #10, and why a hot conductor under load gets even hotter.

Connections add resistance the table does not show. A loose lug, a backstabbed receptacle, a corroded ground rod clamp. Any of those create a series resistance that drops voltage and dumps heat exactly where you do not want it. NEC 110.14(D) now requires a torque tool for terminations rated for one. Use it. Feel is not a torque spec.

If a breaker is warm but not tripping, check the lugs and the busbar stab before you blame the breaker. Resistance you can feel is resistance you can measure, and it is almost always a connection.

Putting Ohm's Law on a service call

You arrive at a 240V baseboard heater that is not putting out heat. Element reads 19 ohms cold. Apply Ohm's Law: I = V / R, so 240 / 19 = 12.6A. Power is V x I, so 240 x 12.6 = roughly 3000W. That matches the nameplate. Element is good.

Now you measure 215V at the heater under load. Voltage drop tells you the problem is upstream: a long run, a loose terminal, a shared neutral, or an undersized conductor. The heater is innocent. The math pointed you at the right wall.

1. Read the nameplate. Volts, watts, amps.
2. Measure the actual voltage at the load, under load.
3. Compare measured current to calculated current.
4. If they disagree, the problem is the conductor, the connection, or the source.

Test gear and habits that keep you alive

A CAT III 600V or CAT IV 600V meter is the minimum for residential and light commercial. Verify it on a known live source before and after every measurement (NFPA 70E live dead live). A non contact tester is a screening tool, not a verification tool. NEC 110.16 flash warning labels exist because arc flash does not care that you were "just checking."

Build the habit of writing down voltage, amperage, and resistance readings on every troubleshooting call. Patterns show up across calls: the same 8V drop on every long run in a track home, the same 0.4 ohm increase across every aluminum splice in a 1970s panel. That notebook makes you faster than the guy with twenty years and no notes.