Crash course: Voltage, amperage, and resistance basics step by step (part 3)

Why this trio matters in the field

Voltage, amperage, and resistance are the three values you measure, calculate, or estimate on every call. Get one wrong and you nuisance trip a breaker, smoke a motor, or fail an inspection. Part 3 ties the basics into how circuits actually behave under load, and where the NEC sets your hard limits.

This is a working refresher, not a textbook. If you can read a meter and a label, you can use this on your next service call. We are sticking to single phase and basic three phase residential and light commercial scenarios.

Ohm's Law on the truck

V = I x R. Voltage equals current times resistance. Rearrange it to find any value when you know the other two. Power follows from P = V x I, and that one pays the bills because it sets your conductor and overcurrent sizing under NEC 210.19 and 210.20.

Memorize three field versions. Current draw of a known load: I = P / V. Voltage drop across a wire: V = I x R. Resistance of an unknown element: R = V / I. That covers most of what you will calculate in a panel or at a junction box.

• 120V, 1500W heater: I = 1500 / 120 = 12.5A. Sits fine on a 20A circuit.
• 240V, 4500W water heater element: I = 4500 / 240 = 18.75A. Needs a 30A circuit per the nameplate and 422.13.
• 277V ballast pulling 0.9A: P = 277 x 0.9 = 249W. Useful when checking lighting loads against 220.12.

Voltage: nominal vs what you actually read

Nominal voltages per ANSI C84.1 and referenced throughout the NEC are 120, 208, 240, 277, and 480. Your meter will rarely show those exact numbers. Service voltage utilization range is typically plus or minus 5 percent. A 120V circuit reading 114V to 126V is normal. Below 108V under load, start looking for a problem.

Always measure under load, not just open circuit. A loose neutral or a corroded lug can read fine de-energized and collapse the moment a motor kicks on. Voltage drop across a connection is your fastest tell.

Field tip: if you read 120V hot to neutral but only 110V hot to ground, you have a neutral problem, not a hot problem. Chase the neutral path back to the panel before you touch anything else.

Amperage: where the code lives

Current is what heats conductors, trips breakers, and welds contacts. The NEC sizes nearly everything around it. Conductor ampacity comes from Table 310.16, derated for ambient temperature (310.15(B)(1)) and conduit fill (310.15(C)(1)). Continuous loads get the 125 percent multiplier under 210.19(A) and 215.2(A).

Know the difference between inrush, running, and locked rotor amps. A 5HP motor pulling 28A FLA can hit 170A plus on startup. That is why 430.52 lets you size short circuit protection well above the running current, and why a too-tight breaker nuisance trips on every start.

1. Read the nameplate FLA, not the horsepower marketing number.
2. Apply 125 percent for the largest motor in a group (430.24).
3. Verify your overload, not just the breaker, is sized to the motor (430.32).
4. Check terminal temperature ratings, 60C or 75C, before pulling ampacity from the table (110.14(C)).

Resistance and voltage drop

Resistance shows up two ways: as a property of the conductor, and as a fault you are hunting. Copper at 75C is roughly 1.98 ohms per 1000 feet for #12, and 1.24 for #10. Chapter 9 Table 8 has the full list, and Table 9 gives you AC impedance for longer runs where reactance matters.

NEC 210.19 Informational Note 4 recommends keeping branch circuit voltage drop under 3 percent, and total drop under 5 percent including the feeder. It is not enforceable, but inspectors and load calcs lean on it. For a 20A circuit at 120V, 3 percent is 3.6V. At #12 copper, that caps your one way run around 75 feet before you upsize.

Field tip: if a homeowner complains lights dim when the AC kicks on, measure voltage at the panel and at the affected outlet during the inrush. More than a 5V dip points to undersized feeders or a loose lug, not a bad bulb.

Putting it together on a service call

Most diagnostic work is just Ohm's Law applied with a meter. A breaker trips, you measure current with a clamp and compare to the nameplate. A light flickers, you measure voltage drop under load. A receptacle is dead, you measure resistance to ground and to neutral with the circuit isolated.

Build the habit of writing down three numbers on every call: voltage at the source, current at the load, and voltage at the load under operation. Those three readings catch 90 percent of residential and light commercial faults before you ever pull a device.

• Source voltage out of range? Utility or service issue.
• Current above nameplate? Load problem or wrong sizing.
• Voltage drop excessive? Conductor, connection, or distance issue.

Part 4 will move into grounding and bonding, where these same three values decide whether a fault clears in a half cycle or burns until something gives.