Crash course: Ohm's Law for electricians with examples (part 5)

Why Ohm's Law still matters on the job

Ohm's Law is the first tool out of the bag when something doesn't add up in the field. Voltage drop on a long run, a breaker that won't stop tripping, a motor pulling more than the nameplate says... most of it traces back to V = I × R. Know it cold and you troubleshoot faster than the apprentice with the clamp meter.

This part of the crash course skips the physics lecture and runs straight at the math you actually use between the panel and the device. Three variables, one triangle, and a handful of worked examples you can reuse on Monday.

The three forms you need memorized

Voltage (V, in volts), current (I, in amps), and resistance (R, in ohms) are the legs of the triangle. Cover the one you want, and the other two tell you how to get there.

• V = I × R ... find voltage when you know current and resistance.
• I = V / R ... find current when you know voltage and resistance.
• R = V / I ... find resistance when you know voltage and current.

Power rides along with the Watt's Law extension: P = V × I. Combine them and you get P = I² × R, which is the one you reach for when you're chasing heat in a conductor or connection.

Example 1: Voltage drop on a 120V branch circuit

Run a 14 AWG copper circuit 80 feet to a 12A load on 120V. Copper resistance for 14 AWG is roughly 2.525 ohms per 1000 feet. Round trip is 160 feet, so total conductor resistance is 160 × (2.525 / 1000) = 0.404 ohms.

Voltage drop: V = I × R = 12 × 0.404 = 4.85V. That's about 4% on a 120V circuit, right at the edge of the NEC 210.19(A) Informational Note recommendation of 3% on branch circuits (5% combined feeder and branch). Bump it to 12 AWG and you cut the drop to roughly 3V.

Field tip: if a homeowner complains about dim lights at the end of a long garage run, measure voltage at the receptacle under load, not at the panel. The drop hides until the circuit is working.

Example 2: Why a connection is cooking

A loose lug on a 40A feeder heats up because resistance at the bad joint climbs. Say it measures 0.05 ohms at the splice. Power dissipated at that single point: P = I² × R = 40² × 0.05 = 80 watts. That's a soldering iron burning inside your junction box.

This is the physics behind NEC 110.14(D) torque requirements. Every loose terminal becomes a resistor, and every resistor in series with the load turns current into heat. Thermal imaging crews earn their keep finding these before the insulation does.

• Torque to the listed value, every time. No "good and tight" by feel.
• Re-torque aluminum after the first heat cycle if the listing calls for it.
• If a lug is discolored, cut it off and redo the termination. You cannot polish a bad connection.

Example 3: Sizing a resistive load

A 240V baseboard heater is rated 1500W. What's the current draw, and what's the effective resistance?

Current: I = P / V = 1500 / 240 = 6.25A. Resistance: R = V / I = 240 / 6.25 = 38.4 ohms. Now you know what your meter should read across the element cold (close to 38 ohms, slightly lower since resistance rises with temperature). An open element reads OL. A shorted one reads near zero and trips the breaker the moment you energize it.

Per NEC 424.3(B), fixed electric space heating is a continuous load, so size the branch circuit conductors and overcurrent device at 125% of 6.25A, which is 7.8A. A 15A circuit handles it, but now you know why stacking two heaters on one circuit is a bad call.

Example 4: Ground fault math sanity check

Ohm's Law is also how you sanity-check a ground fault path. On a 120V circuit with a bolted fault, if the total fault loop impedance is 0.3 ohms, the fault current is I = V / R = 120 / 0.3 = 400A. Plenty to open a 20A breaker in its instantaneous region, which is what NEC 250.4(A)(5) is asking for when it requires an effective ground-fault current path.

If that impedance climbs to 3 ohms because of a poor bonding jumper or a long EGC run, fault current drops to 40A. The breaker still opens, but it takes longer, and the equipment sits energized during the delay. That's why sizing the EGC per NEC 250.122 and keeping terminations tight is not optional.

Field tip: when you megger a circuit and see a few hundred kilo-ohms line to ground, do the math. Leakage current at 120V through 200k ohms is 0.6 mA. One nuisance trip on a GFCI and you've found your problem before the customer does.

Quick reference you can keep in your head

1. V = I × R, I = V / R, R = V / I. That's the triangle.
2. P = V × I, and P = I² × R when you're thinking about heat.
3. Voltage drop on a run: double the one-way length, multiply by ohms per foot, multiply by current.
4. Continuous loads (3 hours or more) get the 125% treatment per NEC 210.19(A) and 215.2(A).
5. Loose connections are resistors. Resistors at 40A get hot fast.

Part 6 picks up with AC specifics: power factor, impedance, and why your clamp meter reads different numbers than your calculator predicts on motor loads.