Crash course: Ohm's Law for electricians no jargon edition (part 2)

Quick recap, then onward

Part 1 covered the basics: V = I x R. Volts push, amps flow, ohms resist. Now we put it to work on the job, where you are actually sizing conductors, chasing voltage drop, and figuring out why a motor keeps tripping.

This part adds the power piece (watts) and shows how to use the whole thing without a calculator when you are standing on a ladder. Keep the formulas in your head, not in a book you left in the truck.

The power piece: watts

Power is what the load actually does. Heat, light, torque. The formula is P = V x I. Twelve amps at 120 volts is 1,440 watts. That is your hair dryer, your space heater, your shop vac. If you know any two values, you can solve for the third.

Combined with Ohm's Law you get the full wheel: P = I squared x R, and P = V squared / R. You rarely need the squared versions in the field, but they matter when you are thinking about heat in a conductor or a bad connection.

• P = V x I (basic power)
• V = I x R (Ohm's Law)
• I = P / V (find the load current fast)
• R = V / I (find resistance from a reading)

Field shortcut: watts divided by volts gives amps. A 1,500 watt heater on a 120V circuit pulls 12.5 amps. On a 20A circuit with anything else running, you are already close to the edge per NEC 210.23.

Voltage drop, the real-world version

NEC 210.19(A) Informational Note 4 and 215.2(A)(4) recommend keeping voltage drop at 3 percent on branch circuits and 5 percent total. Not a hard rule, but ignore it and you get nuisance trips, dim lights, and motors that run hot.

The quick single-phase formula: VD = (2 x K x I x L) / CM. K is 12.9 for copper, 21.2 for aluminum. L is one-way length in feet. CM is circular mils from Chapter 9 Table 8. Three-phase drops the 2 and multiplies by 1.732 instead.

If you hate the formula, remember this: long runs and small wire equal voltage drop. A 20A load on #12 copper at 120V will hit 3 percent drop around 60 feet one way. Past that, bump up a size.

Troubleshooting with Ohm's Law

A meter plus Ohm's Law finds faults faster than guessing. If a 120V circuit reads 118V at the panel and 104V at the receptacle under load, that 14V drop across the conductor tells you exactly where the problem lives. Loose neutral, corroded splice, undersized wire. Pick one and start looking.

Same trick for a dead branch circuit. If you read 120V hot to ground but nothing hot to neutral, your neutral is open. If you read 60V on both legs of a multiwire circuit, you have a shared neutral issue. Phantom voltage will read high impedance, so use a loaded meter or a wiggy to confirm.

1. Measure voltage at the source with the load off.
2. Measure at the load with it running.
3. Subtract. That drop is your conductor plus every connection in between.
4. Divide drop by current for the resistance of the fault path.

Sizing and overcurrent sanity checks

Ohm's Law will not pick your breaker, but it tells you if the load matches the circuit. NEC 240.4 requires conductors be protected at their ampacity per 310.16. A 30A breaker on #12 copper is a code violation and a fire waiting to happen. Run the math before you land the wire.

Continuous loads per NEC 210.20(A) need the OCPD rated at 125 percent of the continuous load. A 16A continuous load needs a 20A breaker minimum. Use P / V to get the current, then apply the factor.

On the job tip: motors are not resistive loads. Locked rotor current can be 6 to 8 times FLA. Size the conductors per NEC 430.22 and the overload per 430.32, not off a simple Ohm's Law guess.

Common mistakes that cost you time

Guys burn hours on problems Ohm's Law would have caught in two minutes. The big ones are usually simple. Assuming voltage at the panel matches voltage at the load. Forgetting that parallel resistances drop, not add. Treating inrush like steady state.

Another one: measuring resistance on an energized circuit. You will blow the meter and maybe your hand. Kill power, verify dead, then ring out the circuit. Low resistance hot to ground with the breaker off is a short or a bonded neutral, not a ghost.

• Always measure under load, not just open circuit.
• Check both ends of long runs for drop.
• Remember temperature raises resistance. A hot conductor reads higher ohms than a cold one.
• Phantom voltage is not real voltage. Load the meter or trust a solenoid tester.

Put it in your pocket

You do not need to memorize every formula. Keep V = I x R and P = V x I in your head, know where Chapter 9 Table 8 lives for voltage drop math, and remember that every weird reading has a reason. Ohm's Law is the reason, nine times out of ten.

Next time a circuit acts up, do not grab a new breaker first. Grab your meter, take two readings, and do the math. The answer is almost always sitting right there in the numbers.