Crash course: Voltage, amperage, and resistance basics exam prep (part 3)

Voltage, amperage, resistance: the working trio

Every circuit problem on the exam and on the job comes back to three quantities. Voltage (V) is electrical pressure measured in volts. Current (I) is flow measured in amperes. Resistance (R) is opposition to flow measured in ohms. Get these three locked in and most calculation questions collapse into arithmetic.

Ohm's Law ties them together: V = I times R. Rearranged, I = V / R and R = V / I. Power adds the fourth leg: P = V times I, measured in watts. The PIE wheel (P, I, E, R) is worth memorizing cold before exam day.

NEC itself doesn't redefine these terms. Article 100 gives you the definitions of voltage (nominal, to ground, between conductors), and the rest of the book assumes you can do the math. If you can't run Ohm's Law in your head for common 120V and 240V scenarios, slow down and drill it.

Voltage: nominal vs. actual, and why it matters

Nominal voltage is the label value: 120V, 208V, 240V, 277V, 480V. Actual voltage at the receptacle will swing a few percent above or below depending on load and distance. NEC 210.19(A) Informational Note No. 4 recommends sizing branch-circuit conductors so voltage drop does not exceed 3%, with a combined feeder-plus-branch limit of 5%.

Voltage to ground matters for clearance, working space, and shock hazard rules. NEC Table 110.26(A)(1) gives required working space depths based on voltage to ground and what's across the aisle. On a 480Y/277V system, voltage to ground is 277V, not 480V, which changes the working space category.

If a homeowner says their lights dim every time the AC kicks on, you're looking at voltage drop or a loose neutral before you're looking at anything exotic. Check terminations at the panel and meter base first.

Amperage: what actually trips the breaker

Current is what heats conductors and trips overcurrent devices. NEC 240.4 requires conductors be protected at their ampacity per Table 310.16, with the small-conductor rule in 240.4(D) capping 14 AWG copper at 15A, 12 AWG at 20A, and 10 AWG at 30A regardless of what the ampacity table says.

Continuous loads (three hours or more) get the 125% multiplier under 210.19(A)(1) and 215.2(A)(1). A 16A continuous load needs a 20A breaker and conductors rated for 20A minimum. Skip this and you'll fail an inspection or, worse, cook a feeder.

• Branch-circuit rating: not less than the noncontinuous load plus 125% of the continuous load.
• Feeder and service conductors: same 125% rule, per 215.2 and 230.42.
• Motor circuits: different animal, see Article 430. FLC from tables, not nameplate, for sizing conductors.

Resistance, impedance, and the field meter reality

Resistance is what a DMM reads on the ohms setting with the circuit dead. Impedance (Z) is the AC equivalent and includes inductive and capacitive reactance. For most resistive loads (heaters, incandescent, resistive ranges) R and Z are close enough that Ohm's Law works directly.

For motors, transformers, and electronic loads, you need to think in terms of impedance and power factor. That's why a 5 hp single-phase motor pulling 28A at 240V isn't 240 / 28 = 8.57 ohms in any useful sense. The nameplate FLA and Article 430 tables are your reference, not Ohm's Law.

Ground-fault and bonding paths live in the resistance world too. NEC 250.4(A)(5) requires the equipment grounding path to be permanent, continuous, and of low impedance. A high-resistance bond won't clear a fault fast enough to trip the OCPD, which is the entire point of bonding.

Voltage drop math you'll actually use

Single-phase voltage drop: VD = (2 times K times I times D) / CM, where K is 12.9 for copper and 21.2 for aluminum, I is current in amps, D is one-way distance in feet, and CM is circular mils of the conductor from Chapter 9, Table 8. For three-phase, replace the 2 with 1.732.

Practical rule of thumb on a 120V branch circuit: 12 AWG copper at 20A starts pushing 3% drop around 60 to 70 feet one way. Bump to 10 AWG when the run gets long, especially for well pumps, garage subpanels, and outbuildings.

Don't trust voltage drop apps blindly on the truck. Plug in your numbers, then sanity check against the formula once. If the app says 8 AWG for a 30 foot 20A run, something's wrong.

Putting it together: a 30 second exam approach

Read the question, identify which of V, I, R, or P you have and which you need. Write the formula before you touch the calculator. Confirm units (volts, amps, ohms, watts) and check whether the load is continuous, because that 125% multiplier shows up constantly.

For NEC code questions tied to these basics, the article numbers cluster: 210 for branch circuits, 215 for feeders, 220 for load calcs, 240 for overcurrent, 250 for grounding and bonding, 310 for conductor ampacity, 430 for motors. Know which neighborhood your question lives in and the lookup is fast.

1. Identify the quantity asked for.
2. Pull the right formula or NEC table.
3. Apply continuous load and derating factors before sizing.
4. Round up to the next standard breaker or conductor size per 240.6(A).
5. Sanity check against field experience.