Crash course: Ohm's Law for electricians what apprentices get wrong (part 5)

Ohm's Law is not optional knowledge

Every apprentice memorizes V = I x R for the test, then forgets it by the first rough-in. That is a mistake. Ohm's Law is the backbone of every load calculation, voltage drop check, and troubleshooting call you will ever run. If you cannot move between volts, amps, and ohms in your head on a ladder, you are guessing.

This is part 5 of the crash course. We are skipping the textbook derivations and going straight to what trips people up in the field. If you have been in the trade five years and still cannot explain why a motor pulls more current on a low-voltage feeder, read on.

The three forms, and when each one matters

V = I x R gets the headlines, but you will use the rearranged versions more often. I = V / R tells you how much current a load will draw. R = V / I tells you what resistance you are measuring when you meter a circuit. Keep all three on the tip of your tongue.

Apprentices get burned when they confuse resistance with impedance. On DC and purely resistive AC loads (heaters, incandescent, resistive elements), Ohm's Law is clean. On inductive loads (motors, transformers, ballasts) you are dealing with impedance Z, not just R, and power factor enters the picture. That is why a 10 ohm motor winding does not mean the motor draws 12 amps on a 120 V circuit.

• Resistive load: use R, straight Ohm's Law applies.
• Inductive load: use Z, and expect inrush current 6 to 10 times running current.
• Mixed circuit: trust nameplate FLA, not your resistance math.

Power: the fourth variable nobody memorizes properly

P = V x I is half the Power Wheel. The other half is P = I squared x R, and that is the one that matters when you are sizing conductors and worrying about heat. Double the current and you quadruple the heat dissipated in the wire. That is why NEC 310.15 ampacity tables are non-negotiable and why voltage drop calculations per NEC 210.19(A) Informational Note No. 4 recommend keeping branch circuits under 3 percent.

On a service call, P = I squared x R explains why a loose neutral or a corroded lug gets hot fast. Small increase in resistance at a high-current connection, and you are cooking the lug. That brown discoloration on a breaker terminal is Ohm's Law telling you something.

Field tip: if a lug is warm to the back of your hand through the deadfront, it is already failing. Kill the circuit, pull it, and retorque to the manufacturer spec per NEC 110.14(D).

Voltage drop: the calculation apprentices fudge

The mistake is treating voltage drop as a code rule. It is not. NEC 210.19 and 215.2 only have it as an Informational Note, not an enforceable requirement in most cases. But the physics is real, and your customer will feel it.

The working formula for single phase: VD = 2 x K x I x L / CM, where K is 12.9 for copper, L is one-way length in feet, and CM is the conductor circular mils from Chapter 9 Table 8. For three phase, swap the 2 for 1.732. Round up your wire size when the answer exceeds 3 percent of nominal voltage on branch circuits, or 5 percent combined feeder plus branch.

1. Calculate load current from nameplate or demand factors.
2. Measure one-way run length, not total conductor length.
3. Run the formula at the expected load, not breaker size.
4. If over 3 percent, upsize one trade size and recheck.

Troubleshooting with Ohm's Law in your head

The best troubleshooters do not reach for a calculator. They estimate. A 1500 W heater on 120 V should draw 12.5 amps. If your clamp reads 8 amps, the element is partially open or the supply voltage has sagged. If it reads 18 amps, something is wrong upstream, possibly a low-voltage condition dragging current up on a constant-power load.

Motors follow the same logic inverted. Reduced voltage means increased current to maintain torque. That is why undersized feeders cook motor windings. A 10 percent voltage drop on a 3 phase motor can push current up 15 to 20 percent, and the overloads per NEC 430.32 will trip eventually, but not before insulation degrades.

Field tip: when a motor trips intermittently and the overloads look fine, meter voltage at the motor terminals under load, not at the disconnect. That is where Ohm's Law tells the truth.

What to drill until it is automatic

Stop looking up common values. A journeyman should know, cold, that a 20 A circuit at 120 V can handle 2400 VA, that a 15 A circuit tops out at 1800 VA, and that the 80 percent continuous load rule in NEC 210.19(A)(1) drops those to 1920 and 1440 VA respectively.

Same for 240 V and 480 V three phase. If someone asks what a 100 A 208Y/120 panel can deliver, you should say 36 kVA in under two seconds. That is P = 1.732 x V x I, run in your head. Do this math on every job for a month and it stops being math.

• 120 V single phase: amps x 120 = VA.
• 240 V single phase: amps x 240 = VA.
• 208Y/120 three phase: amps x 360 = VA (approximate, 1.732 x 208).
• 480Y/277 three phase: amps x 831 = VA.

Ohm's Law is not a test answer. It is how you read a panel, size a feeder, and diagnose a nuisance trip without pulling schematics. Drill it until it is reflex.