Crash course: Voltage, amperage, and resistance basics quick reference (part 3)

Ohm's Law: The Only Equation You Need Memorized

Voltage pushes, amperage flows, resistance fights back. E = I × R. Rearrange it however the problem demands: I = E/R when you know voltage and resistance, R = E/I when you're sizing a load. Power follows from there: P = E × I, or watts equals volts times amps.

On a 120V branch circuit feeding a 1,500W heater, current draw is 12.5A. That single calculation tells you whether a 15A breaker holds or trips, whether your 14 AWG conductor is legal under NEC 240.4(D)(3), and whether the load qualifies as continuous under NEC 210.19(A)(1) (over three hours means size at 125%).

Keep the wheel diagram in your head, not on your phone. When you're upside down in an attic at 4pm in August, you won't have time to scroll.

Voltage: What's Actually at the Terminals

Nominal voltages are nominal. NEC 220.5(A) lets you calculate loads using 120, 208, 240, 277, or 480. Real-world readings drift. A 120V receptacle measuring 114V under load is fine. Below 108V (90% of nominal), you're outside the ANSI C84.1 utilization range and equipment will misbehave.

Voltage drop is not an NEC violation in most cases, it's a recommendation. NEC 210.19(A) Informational Note 4 suggests 3% on branch circuits, 5% combined with the feeder. On a 120V, 20A circuit running 100 feet of 12 AWG copper, you're already at roughly 3.6V drop at full load. Push it to 150 feet and you're outside the recommendation.

If a customer complains about dim lights when the AC kicks on, measure voltage at the panel and at the load while cycling the compressor. More than 3% delta means undersized conductors or a loose connection upstream.

Amperage: Sizing, Continuous Loads, and the 80% Rule

Breakers and conductors don't get sized to the load. They get sized to the load times 1.25 if the load runs three hours or more. That's NEC 210.20(A) for branch circuit OCPD and 215.2(A)(1) for feeders. Miss it on a commercial lighting job and the breaker thermal-trips at 2am.

Ampacity tables (NEC 310.16) assume 30°C ambient and three current-carrying conductors in a raceway. The moment you exceed either, derate. Common adjustments to remember:

• 4 to 6 CCCs in a raceway: 80% per Table 310.15(C)(1)
• 7 to 9 CCCs: 70%
• Ambient 41 to 45°C on 75°C insulation: 0.87 multiplier per Table 310.15(B)(1)(1)
• Rooftop conduit within 7/8 inch of the roof: add 33°C to ambient per 310.15(B)(2)

Stack derates multiplicatively. A 12 AWG THHN rated 30A at 90°C, in a raceway with 6 CCCs, on a 110°F rooftop, drops fast. Run the math before you pull wire.

Resistance: The Hidden Variable That Burns Houses Down

Conductor resistance is published in NEC Chapter 9, Table 8. 12 AWG copper is 1.98 ohms per 1,000 feet at 75°C. That's the easy part. The resistance that kills is at terminations: loose lugs, backstabbed receptacles, oxidized aluminum, corroded ground rods.

A connection with 0.5 ohms of resistance carrying 15A dissipates 112 watts at the joint. That's a soldering iron behind your wall plate. Torque every termination to manufacturer spec. NEC 110.14(D) requires it, and as of the 2017 cycle, inspectors are checking.

Thermal imaging a panel under load takes 90 seconds and finds problems no megger or multimeter will. If you're not carrying a thermal camera in 2026, you're working blind.

AC vs DC: Where the Math Bends

Everything above is true for DC and resistive AC loads. The moment you add inductance (motors, transformers, ballasts) or capacitance, current and voltage stop being in phase. Power factor enters the picture.

True power (watts) = E × I × PF. Apparent power (VA) = E × I. A 10 hp motor at 240V single-phase pulls roughly 50A FLA per NEC Table 430.248, but the nameplate watts won't match volts times amps. Size conductors and OCPD off the table values, not Ohm's Law calculations, per NEC 430.6(A)(1).

For service calculations, use VA. For heat dissipation and energy bills, use watts. Confusing the two is how feeders end up undersized on motor-heavy installs.

Field Quick-Reference Numbers

Memorize these. They cover 80% of residential and light commercial calls:

1. 120V × 15A = 1,800 VA circuit capacity, 1,440 VA continuous
2. 120V × 20A = 2,400 VA circuit capacity, 1,920 VA continuous
3. 240V × 30A = 7,200 VA (typical dryer or small AC)
4. 240V × 50A = 12,000 VA (range, EV charger, welder)
5. 14 AWG Cu = 15A max, 12 AWG = 20A, 10 AWG = 30A, 8 AWG = 40A, 6 AWG = 55A at 75°C per 310.16

When voltage, current, and resistance disagree with what the meter says, the meter wins. Trust readings, verify connections, and never assume the last guy did it right.