Crash course: Voltage, amperage, and resistance basics for DIY homeowners (part 3)

Part 3: Putting Ohm's Law to Work in Real Houses

Parts 1 and 2 covered the theory. Voltage pushes, amperage flows, resistance fights back. V = I × R. Now we use it on actual residential jobs where the math decides whether a circuit holds or trips, whether a wire stays cool or cooks insulation.

This part is for the homeowner who can already strip a 12 AWG conductor without nicking it and wants to stop guessing. Every number below ties back to NEC 2023, and every shortcut assumes copper at 75°C terminations unless noted.

Sizing a Branch Circuit From Load, Not Habit

Most DIY mistakes start with picking a breaker first. Wrong order. Calculate the continuous load, apply the 125% rule per NEC 210.19(A) and 210.20(A), then size the conductor and overcurrent device together.

A 1500W bathroom heater pulling 12.5A continuous on 120V needs a circuit rated for 12.5 × 1.25 = 15.6A minimum. That kicks you to a 20A circuit on 12 AWG copper, not 14 AWG on 15A. Same heater on a 240V circuit pulls 6.25A and lives comfortably on a 15A breaker with 14 AWG.

• Continuous load: any load expected to run 3 hours or more (NEC Article 100).
• Conductor ampacity: NEC Table 310.16, 75°C column for residential.
• Breaker upsize for continuous loads: NEC 210.20(A), 125%.
• Small appliance branch circuits in kitchens: minimum two 20A circuits, NEC 210.11(C)(1).

Voltage Drop: The Number Inspectors Skip and Loads Notice

NEC 210.19(A) Informational Note 4 recommends 3% drop on branch circuits, 5% total feeder plus branch. It is not enforceable, but motors, LED drivers, and resistive heaters care. A 20A circuit running 80 feet on 12 AWG at full load drops about 3.8V on 120V, which is 3.2%. Bump to 10 AWG and you cut it under 2%.

Quick field formula for single-phase copper: VD = (2 × K × I × L) / cmil, where K = 12.9 for copper, I is amps, L is one-way length in feet, and cmil is the conductor's circular mils from Chapter 9 Table 8. Memorize the cmil values for 14, 12, 10, and 8 AWG and you can size in your head on the truck.

Field tip: any homerun over 75 feet on a 20A circuit, default to 10 AWG. The wire costs a few dollars more. The callback to fix a flickering kitchen costs an afternoon.

Resistance Math for Troubleshooting

A meter across a de-energized load tells you what should draw current. A 1500W toaster on 120V should read about 9.6 ohms cold (R = V² / P = 14400 / 1500). Read 80 ohms, the element is failing open. Read 0.5 ohms, you have a short to chassis or a pinched conductor.

For motors and transformers, cold resistance is lower than running impedance because reactance hasn't kicked in yet. Do not condemn a motor on DC resistance alone. Compare phase to phase on a three-phase motor, and any leg more than 10% off the others is suspect.

1. Kill power, lock out, verify dead with a known-good meter.
2. Disconnect the load from the circuit so you read the load only.
3. Measure resistance across the load terminals.
4. Compare to V² / P or to the manufacturer's spec.
5. Measure load to ground. Anything under 1 megohm on a 120V circuit is a red flag.

GFCI, AFCI, and Why Tiny Currents Trip Them

A Class A GFCI trips at 4 to 6 mA of imbalance between hot and neutral. That is 0.004A, roughly the current through 30,000 ohms at 120V. Wet skin can drop body resistance to 1000 ohms, which puts 120 mA across a person before the breaker on the panel even notices. The GFCI sees the imbalance and clears in under 25 ms.

NEC 210.8(A) requires GFCI protection for 125V through 250V receptacles in bathrooms, kitchens, garages, basements, laundry, outdoors, and within 6 feet of sinks and tubs. NEC 210.12 requires AFCI protection on most 120V 15A and 20A branch circuits in dwelling units. AFCIs look at high-frequency signatures of arcing, not raw current, so a loose backstab can trip one without ever tripping the thermal breaker.

Field tip: nuisance AFCI trips on a refrigerator circuit usually mean a shared neutral or a damaged conductor at the box, not a defective breaker. Pull the device, inspect for nicks, and re-land on screws not stabs.

Putting It Together on a Real Panel

Take a 200A service feeding a 1980s ranch. Owner wants to add an EV charger, a heat pump water heater, and finish the basement. Service load calc per NEC 220 Part III tells you whether the existing panel takes it. Voltage drop tells you what conductor runs to the detached garage charger. NEC 210.8 and 210.12 tell you what protection the new basement circuits need.

Ohm's law is the spine of every one of those decisions. Voltage sets the system. Amperage sets the conductor and the breaker. Resistance sets the load and the safety margin. Get those three right and the rest of the code reads like a checklist instead of a puzzle.