Crash course: Voltage, amperage, and resistance basics field cheat sheet (part 3)

Ohm's Law: the only equation you actually need

Voltage pushes, amperage flows, resistance fights back. V = I × R. Memorize it once, use it forever. Every troubleshooting call you take, every conductor you size, every breaker that nuisance trips, traces back to this triangle.

Power follows right behind: P = V × I. A 1500W space heater on a 120V circuit pulls 12.5 amps. That's why your customer's bedroom 15A breaker keeps tripping when they plug it in next to the hair dryer. You don't need a calculator for this. You need it in your head before you climb the ladder.

Three combos worth burning into memory:

• V = I × R (volts equal amps times ohms)
• I = V / R (amps equal volts divided by ohms)
• P = V × I (watts equal volts times amps)

Voltage: what you measure, what you expect

Nominal is not actual. NEC 220.5(A) lets you calculate loads at 120, 208, 240, 277, 480. In the field your meter reads 118, 122, 244, 478. ANSI C84.1 allows ±5% at the service, so anything from 114 to 126 on a 120V circuit is technically in spec. Below that, motors run hot and electronics misbehave.

Voltage drop is where rookies get burned. NEC 210.19(A) Informational Note 4 recommends keeping branch circuit drop under 3%, and total drop (feeder plus branch) under 5%. On a 120V circuit, that's 3.6 volts you can lose before equipment starts complaining. Long runs to detached garages, well pumps, and outbuildings eat that budget fast.

If a homeowner says "the lights dim when the fridge kicks on," check voltage at the panel under load before you start pulling devices. Nine times out of ten it's a loose neutral or an undersized feeder, not a bad receptacle.

Amperage: sizing, derating, and the 80% rule

Conductor ampacity lives in NEC Table 310.16. A 12 AWG copper THHN at 75°C is good for 25 amps... but the asterisk in 240.4(D) caps it at 20 amps for overcurrent protection. Read the table, then read the footnotes. The footnotes are where the money is.

Continuous loads (3 hours or more) get the 80% treatment per NEC 210.19(A)(1) and 215.2(A)(1). A 16 amp continuous load needs a 20 amp circuit. A 24 amp continuous load needs a 30 amp circuit. EV chargers, electric heat, sign lighting, anything that runs for shifts at a time. Size the conductor and the OCPD at 125% of the continuous load plus 100% of the non-continuous.

Derating stacks. Ambient temperature correction (Table 310.15(B)(1)), more than three current-carrying conductors in a raceway (Table 310.15(C)(1)), and the terminal temperature rating per 110.14(C). Apply them in order. The smallest result wins.

Resistance: what your meter is actually telling you

Resistance is the diagnostic gold mine. A good 14 AWG copper conductor reads roughly 2.5 ohms per 1000 feet at 75°F. A motor winding that should read 3 ohms phase-to-phase but reads 0.2 ohms is shorted. A heating element that reads OL is open. Insulation resistance to ground should be in the megohms... if it's not, you've found your fault.

Continuity testing on a de-energized circuit catches what voltage testing misses. Loose backstabs, corroded splices, broken neutrals inside a romex staple. Set your meter to the lowest ohm range, zero your leads, and walk the circuit.

• Wire-to-wire short: near zero ohms where there should be infinite
• Open conductor: OL where there should be a few ohms
• Ground fault: low resistance from hot or neutral to EGC
• Bad splice: fluctuating reading when you wiggle the joint

Putting it together on a service call

Customer says a circuit is dead. You measure 120V hot to ground, 0V hot to neutral. The neutral is open somewhere. That's voltage telling you a story. Now you walk the circuit, kill power, and ohm out the neutral conductor back to the panel. Resistance confirms the break. You found it without guessing.

Customer says a 20A breaker trips after 20 minutes. Clamp the circuit. If you read 18 amps and it's a continuous load, the breaker is doing its job, the circuit is undersized per 210.19(A)(1). If you read 8 amps and it still trips, you've got a thermal issue at the breaker, a loose lug, or a parallel path heating up the panel.

Always measure before you replace. A breaker that trips is not automatically a bad breaker. Nine times out of ten it's the load, the connection, or the conductor, and swapping the breaker just resets the clock on the real problem.

Field cheat sheet: numbers to keep in your head

You won't always have time to flip to NEC Chapter 9 Table 8. These are the ones working electricians keep on the tip of their tongue. Verify against the current code cycle adopted in your jurisdiction, but these will get you through the day.

1. 120V circuit, 3% drop budget = 3.6V. 240V circuit = 7.2V.
2. 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A (60°C/75°C residential rule of thumb per 240.4(D)).
3. Continuous load = 125% sizing. Always.
4. Standard receptacle: 1.5A typical draw, 15 or 20A circuit per 210.21(B).
5. GFCI required per 210.8(A) in kitchens, baths, garages, outdoors, basements, laundry, within 6 ft of sinks.
6. AFCI required per 210.12 in nearly all dwelling unit living areas.
7. Megger reading under 1 megohm to ground? Suspect. Under 100k ohms? Faulted.

Voltage, amperage, resistance. Push, flow, fight. Know what each one should be, measure what each one is, and the gap between those two numbers is where the fault lives.