Crash course: Voltage, amperage, and resistance basics field cheat sheet (part 2)

Ohm's Law: The One Equation You Actually Use

Voltage equals current times resistance. E = I × R. Rearrange it three ways and you have most of what you need on a service call. Volts push, amps flow, ohms resist. If you know any two, you solve for the third.

Power follows from there. P = I × E, or P = I² × R. A 1500W heater on a 120V circuit pulls 12.5A. Same heater on 240V pulls 6.25A. Double the voltage, halve the current for the same load. That is why motor branch circuits and large appliances run at higher voltage: less current, smaller conductors, less voltage drop.

Memorize the wheel or write it inside your meter case. You will use it more than any code table.

Voltage: What You Read at the Terminals

Nominal is not actual. NEC 220.5(A) sets nominal voltages at 120, 208, 240, 277, 480. In the field you will see anything from 114 to 126 on a 120V leg depending on POCO regulation, time of day, and the load on the transformer. ANSI C84.1 allows ±5% at the service. Anything outside that, call it in.

Always check phase to phase, phase to neutral, and phase to ground. A 208/120 wye and a 240/120 high leg delta will both read 120V on two legs, but the third leg on the high leg delta reads 208V to neutral. Land a 120V load on that B phase and you cook it. NEC 110.15 requires that high leg to be marked orange.

Before you pull a meter or open a panel, verify your tester on a known live source, take your reading, then verify the tester again. Live, dead, live. Three checks. Every time.

Amperage: What Actually Burns Things Down

Current is what melts insulation, trips breakers, and shocks people. NEC 310.16 gives you allowable ampacity for conductors based on insulation type, ambient temp, and conductor count in the raceway. A 12 AWG THHN copper conductor is rated 30A in the 90°C column, but you size the breaker off the 60° or 75° column per NEC 110.14(C). For most terminations under 100A, that means 12 AWG = 20A, 10 AWG = 30A, 8 AWG = 40A.

Apply derating before you sign off. More than three current-carrying conductors in a raceway? NEC 310.15(C)(1) cuts your ampacity. Ambient over 30°C? NEC 310.15(B) hits you again. Both can stack. A bundle of nine THHN 12s in a hot attic is not a 20A circuit anymore.

• Continuous load (3+ hours): size conductors and OCPD at 125% per NEC 210.19(A)(1) and 210.20(A).
• Motor circuits: branch conductors at 125% of FLC per NEC 430.22.
• Neutral on a 3-phase wye with nonlinear loads: count it as a current-carrying conductor per NEC 310.15(E)(3).

Resistance: Where Your Connections Live or Die

Copper has resistance you can ignore on short runs and resistance you cannot ignore on long ones. NEC Chapter 9 Table 8 gives ohms per 1000 feet. 12 AWG uncoated copper is about 1.93 Ω/kFT. A 100 foot run, there and back, is 200 feet of conductor, roughly 0.39 Ω. Push 16A through that and you drop about 6.2V. On a 120V circuit, that is over 5%. NEC 210.19(A) Informational Note 4 recommends 3% on branch circuits, 5% total.

The resistance that actually kills equipment is at the connections. A loose lug, a back-stabbed receptacle, an aluminum splice without antioxidant. Bad terminations build heat, oxidize, build more heat. That is the loop that ends in a melted panel.

If a breaker or lug feels warm under normal load, it is already failing. Kill the circuit, retorque to the manufacturer's spec, and inspect for discoloration. NEC 110.14(D) requires torque per listing.

Reading the Panel: Putting It Together

You roll up to a tripped 20A breaker on a kitchen counter circuit. Customer says the microwave and the toaster were on. Microwave nameplate, 1500W. Toaster, 1200W. At 120V, that is 12.5A plus 10A. 22.5A on a 20A circuit. Breaker did its job.

Now check the breaker itself. Voltage leg to leg, leg to neutral, leg to ground. Resistance to ground on the dead conductor. If the conductor reads continuous to ground with the breaker off and the load disconnected, you have a fault, not just an overload. NEC 210.8(A) requires GFCI protection on those countertop receptacles, so verify the GFCI is functional before you close it back up.

1. Verify de-energized. Lockout, tagout, test.
2. Inspect terminations for heat, discoloration, looseness.
3. Measure insulation resistance if you suspect a fault.
4. Retorque to spec. Restore power. Verify with a load.

Field Cheat Sheet

Keep these numbers in your head. They cover most residential and light commercial calls without opening the book.

• 120V × 20A = 2400W max. Continuous, 1920W.
• 240V × 30A dryer circuit = 7200W max.
• 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A, 8 AWG = 40A, 6 AWG = 55A (75°C column).
• Voltage drop rule of thumb: 12 AWG good to about 50 ft at 16A on 120V before you exceed 3%.
• GFCI required: bathrooms, kitchens, garages, outdoors, basements, laundry, within 6 ft of sinks. NEC 210.8.
• AFCI required: most dwelling unit branch circuits per NEC 210.12.

Voltage, amperage, resistance. Three quantities, one equation, every call. Get the readings, do the math, trust the code.