Crash course: Voltage, amperage, and resistance basics with calculations (part 2)

Ohm's Law: The Only Equation You Need on the Truck

V = I × R. Voltage equals current times resistance. Rearrange it as needed: I = V/R, R = V/I. That's the spine of every load calc, every voltage drop check, every troubleshooting call you'll run today.

Power rounds out the picture: P = V × I. Combine the two and you get P = I²R, the formula that explains why a loose connection cooks itself. Double the current through a bad splice and you quadruple the heat.

Memorize the wheel, but more importantly, memorize what each variable feels like in the field. Voltage is pressure. Current is flow. Resistance is restriction. Power is the work being done, and the heat being dumped.

Part 1 Recap, Then Forward

Part 1 covered the definitions: volts push, amps move, ohms resist. Watts are the result. If any of that is fuzzy, go back. Everything below assumes you can identify each quantity on a nameplate without thinking.

This installment is about applying the math to real conductors, real loads, and real code. We're working in the territory where Article 210 (branch circuits), Article 215 (feeders), and Article 220 (load calcs) live.

Voltage Drop: When Code Becomes Math

NEC 210.19(A) Informational Note 4 recommends branch circuit voltage drop not exceed 3%, with total (feeder plus branch) at 5%. It's not enforceable, but it is the number engineers and inspectors expect you to hit.

Single-phase voltage drop formula: VD = (2 × K × I × D) / CM. K is 12.9 for copper, 21.2 for aluminum. I is load current. D is one-way distance in feet. CM is the conductor's circular mils from Chapter 9, Table 8.

Run a 20A load 150 feet on #12 copper at 120V:

• VD = (2 × 12.9 × 20 × 150) / 6530 = 11.85V
• 11.85 / 120 = 9.9% drop. Not acceptable.
• Bump to #8 (16,510 CM): VD = 4.69V, or 3.9%. Still over. Go to #6.

This is why long runs eat conductor budget fast. Always size by ampacity first (Table 310.16), then verify drop.

If a homeowner complains their well pump hums but won't start at the end of a 300 foot run, voltage drop is your first suspect, not the pump. Measure under load, not at rest.

Resistance, Temperature, and the Connections That Kill You

Conductor resistance rises with temperature. Chapter 9, Table 8 gives DC resistance at 75°C. A conductor running hot under load has measurably more resistance than the same wire on a cold morning, which is part of why ampacity tables are tied to insulation temperature ratings (60°C, 75°C, 90°C columns in 310.16).

Connection resistance is where most field failures live. A properly torqued lug at 110.14(D) values has near-zero resistance. A loose lug might read 0.5 ohms. Push 30A through that and you're dissipating 450 watts at the connection. That's a space heater inside your panel.

1. Torque every termination to the manufacturer's spec. NEC 110.14(D) makes this a code requirement, not a suggestion.
2. Re-torque aluminum after the first heat cycle when the spec calls for it.
3. Use an IR thermometer or thermal camera on panels under load. Hot lugs glow before they fail.

Working Single-Phase and Three-Phase Loads

Single-phase: I = P / V. A 1500W heater on 120V draws 12.5A. Same heater on 240V draws 6.25A. Doubling the voltage halves the current, which is why commercial gear runs higher voltage. Smaller conductors, less drop, lower I²R losses.

Three-phase: I = P / (V × 1.732 × PF). The 1.732 is the square root of 3. A 10 kW three-phase load at 480V with unity power factor draws 12.03A per leg. Same 10 kW single-phase at 240V draws 41.7A. The math alone justifies why you pull three-phase to any motor over a few horsepower.

Power factor matters once motors and ballasts enter the picture. A 0.8 PF motor pulls 25% more current than the same kW resistive load. Article 460 covers capacitor correction if you're getting hammered on demand charges.

Field Calc Sequence

When you're sizing a circuit cold, work the steps in order. Skipping ahead is how undersized conductors and tripped breakers happen.

1. Identify the load in watts or amps from the nameplate, or calculate per Article 220.
2. Apply continuous load factor of 125% per 210.19(A)(1) and 215.2(A)(1) where it applies.
3. Size conductor by ampacity from 310.16, applying ambient and bundling adjustments per 310.15.
4. Calculate voltage drop. If over 3% branch or 5% total, upsize.
5. Size overcurrent protection per 240.4, respecting the small conductor rule in 240.4(D).
6. Verify equipment terminal temperature rating per 110.14(C). A 90°C conductor on a 75°C lug uses the 75°C column.

Always run the voltage drop calc before you pull wire, not after. Pulling 200 feet of #10 only to learn you needed #8 is a bad afternoon.

Part 3 will cover power factor, harmonics, and how to read a clamp meter without lying to yourself. Until then, trust the math, torque the lugs, and measure under load.