Crash course: Ohm's Law for electricians no jargon edition (part 1)

What Ohm's Law actually is

Ohm's Law ties together three things you measure every day: voltage (V), current (I), and resistance (R). The formula is V = I x R. Rearrange it and you get I = V / R, or R = V / I. That's the whole show.

Voltage is the push. Current is the flow. Resistance is what fights the flow. Change any one of them and the other two react. If you can hold this in your head, you can troubleshoot most residential and light commercial issues without pulling out a calculator.

Power (P) lives next door, measured in watts: P = V x I. The NEC leans on watts and volt-amperes constantly when sizing loads, so you'll bounce between the two formulas in the same job.

The pie chart in your head

Memorize three working forms and you're set for the field:

• V = I x R (find voltage when you know current and resistance)
• I = V / R (find current draw on a known circuit)
• P = V x I (find wattage, or back into amps from a nameplate)

That last one is the workhorse. A 1500W space heater on a 120V circuit pulls 1500 / 120 = 12.5 amps. Drop that on a 15A circuit with a TV and a lamp already running and you've found your nuisance trip before the homeowner calls you back.

For motors and inductive loads, treat the nameplate FLA as gospel and skip the math. NEC 430.6(A)(1) tells you to use the tables in 430.247 through 430.250 for sizing conductors and overcurrent, not the nameplate. Two different worlds, don't mix them.

Voltage drop, the one that bites

This is where Ohm's Law stops being trivia and starts saving callbacks. Conductors have resistance. Push current through them and you lose voltage along the way. NEC 210.19(A) Informational Note 4 recommends keeping branch circuit voltage drop under 3%, and the total feeder plus branch under 5%.

Quick field math for a single-phase run: VD = 2 x K x I x L / CM. K is roughly 12.9 for copper, L is one-way length in feet, CM is the circular mils of the conductor (look it up once, write it on the inside of your tool box lid). For three-phase, swap the 2 for 1.732.

Long run to a detached garage on #12? Run the numbers before you pull. A 120V, 15A circuit at 100 feet on #12 copper drops about 4.8V, right at 4%. Bump to #10 and you're back inside spec.

Sizing loads without guessing

Most service calls boil down to "why does this trip." Ohm's Law plus a clamp meter answers it in two minutes. Read the amps on the conductor, compare to the breaker rating, then check the continuous load rule in NEC 210.20(A): breaker and conductor sized at 125% of the continuous load.

Worked example. A bakery display case nameplate reads 1800W at 120V. Current is 1800 / 120 = 15A. Continuous load (runs 3 hours or more), so you need 15 x 1.25 = 18.75A minimum. That's a 20A breaker on #12 THHN, not a 15A. Miss this and the breaker holds for a week, then nuisance trips every afternoon when the case cycles harder.

1. Get the wattage or VA from the nameplate.
2. Divide by the circuit voltage to get amps.
3. Multiply by 1.25 if it's continuous.
4. Pick the breaker and conductor from NEC 240.6(A) and Table 310.16.

Resistance readings that mean something

Your meter on the ohms setting is Ohm's Law in reverse. You're applying a small known voltage and measuring current to back into resistance. That's why you de-energize before testing continuity, live voltage wrecks the reading and sometimes the meter.

Ground fault paths, bonding jumpers, and equipment grounding conductors should read near zero ohms. NEC 250.4(A)(5) requires the grounding path to be "permanent and electrically continuous" with low enough impedance to facilitate operation of the overcurrent device. If you're reading 5 ohms on a bond, something is loose, corroded, or missing.

On insulation resistance tests with a megger, anything under 1 megohm per kV of operating voltage is suspect. New install on 480V gear should read in the hundreds of megohms or higher.

What to carry in your head on every call

You don't need to recite formulas. You need three reflexes: watts divided by volts gives amps, amps times resistance gives voltage drop, and continuous loads need the 125% bump. Those three cover maybe 80% of residential and light commercial troubleshooting.

Part 2 will get into single-phase versus three-phase math, why 208V is not 240V, and how to calculate available fault current at a panel without a software tool. For now, run the numbers on your next two service calls before you reach for parts. The math is faster than the second trip to the truck.