Crash course: Ohm's Law for electricians with photos (part 2)

Quick recap from part 1

Ohm's Law: V = I x R. Power: P = I x V. Rearrange as needed. Part 1 covered the basics on a bench. Part 2 is the field version: real panels, real loads, real troubleshooting.

The math does not change between a textbook and a 200A service. What changes is that voltage sags, terminations heat up, and neutrals carry current you did not expect. Ohm's Law tells you why.

Voltage drop on long branch circuits

NEC 210.19(A) Informational Note No. 4 recommends branch-circuit voltage drop stay under 3%, with a combined feeder plus branch under 5%. That is guidance, not a hard rule, but it keeps motors and electronics happy. To calculate it, you need conductor resistance per foot (Chapter 9, Table 8 or Table 9) and the load current.

Single-phase two-wire VD = 2 x K x I x L / CM, where K is roughly 12.9 for copper at 75C, I is current in amps, L is one-way length in feet, and CM is the circular mils of the conductor. For three-phase, swap the 2 for 1.732.

• #12 Cu, 20A, 100 ft one-way: VD is about 6.4V on 120V, or 5.3%. Upsize to #10.
• #10 Cu, 30A, 75 ft one-way: VD is about 5.9V on 240V, or 2.4%. Good.
• Add a 25% starting inrush on a motor and recheck, especially at the compressor.

Reading a panel with a clamp meter

A clamp meter plus Ohm's Law solves most field mysteries. Measure voltage line to neutral and line to ground. Measure current on each ungrounded conductor. If a 20A circuit reads 17A and the breaker trips intermittently, you are inside the thermal curve of the breaker, not tripping on a short.

When a receptacle reads 118V unloaded but sags to 104V under a 12A load, the upstream impedance is roughly (118 - 104) / 12, or about 1.17 ohms. That is way too high for a branch circuit. Suspect a backstabbed receptacle, a loose wire nut, or a corroded neutral splice in a box upstream.

If voltage drops more under load than the math predicts for the wire size and length, stop calculating and start opening boxes. A bad termination is almost always the answer.

Sizing conductors for real loads

NEC 310.16 gives ampacity by insulation temperature rating. NEC 110.14(C) caps the terminal rating, usually 75C for equipment over 100A and 60C for equipment 100A or less unless marked otherwise. You size the conductor for the smaller of the two, then apply Ohm's Law to check voltage drop at the far end.

Continuous loads under NEC 210.19(A)(1) and 215.2(A)(1) require the branch or feeder to be rated at 125% of the continuous current. For a 16A continuous load, you need a 20A circuit and a conductor rated for 20A after any derating per 310.15(B) and (C).

1. Calculate load current from P = I x V.
2. Multiply continuous portions by 1.25.
3. Pick a conductor at the terminal temperature rating.
4. Derate for ambient and conductor count.
5. Verify voltage drop at full load.

Troubleshooting with resistance checks

Power off, locked out, verified dead: now your ohmmeter earns its keep. A good heating element reads close to its nameplate resistance. A 4500W element at 240V pulls 18.75A and reads about 12.8 ohms. Open reads OL. Shorted to the sheath reads low to ground.

Motor windings should read balanced phase to phase within a few percent. A megger at 500V or 1000V per NEC 110.7 spirit, with winding to ground above 1 megohm on a warm motor, is the rough field threshold. Below that, the motor is on borrowed time.

• Heater element open: infinite ohms, no current, no heat.
• Heater shorted to ground: low ohms to case, trips GFCI or breaker.
• Contactor coil open: no pull-in, coil reads OL.
• Contactor coil shorted turns: low ohms, buzzes, overheats.

Power, heat, and why terminations fail

Power dissipated at a bad connection is P = I squared x R. A loose lug with 0.05 ohms carrying 40A dissipates 80W in a space the size of a thumbnail. That is a soldering iron inside your panel. This is why NEC 110.14(D) now requires torque values from the manufacturer, and why thermal imaging finds problems a voltmeter misses.

The same physics explains why aluminum on a backstab or a steel screw without antioxidant fails over years. Microscopic gaps, oxidation, thermal cycling. Resistance creeps up, I squared R climbs, the terminal browns, the insulation chars, and then you get a call.

Torque every lug to spec on new work and retorque after the first heat cycle on large feeders. A 10 second check today prevents a 2 AM callback next winter.

Field math you should know cold

Keep these in your head. They cover 90% of what comes up on a service call, and they all fall out of V = I x R and P = I x V.

1. Watts to amps at 120V: divide watts by 120. At 240V single phase: divide by 240. At 208V three-phase: divide by 360.
2. #14 Cu is 20A ampacity at 75C but limited to 15A per NEC 240.4(D)(3).
3. Every doubling of current quadruples heat at a connection.
4. A 1% voltage drop on 120V is 1.2V. If you see more than that unloaded, start looking upstream.

Ohm's Law is not a classroom exercise. It is the lens you use to read a panel, pick a wire, and find the bad splice before it finds you.