Crash course: Voltage, amperage, and resistance basics step by step (part 2)

Part 2: Putting Ohm's Law to work

Part 1 covered the basics: voltage pushes, current flows, resistance opposes. Now apply it. On the job you rarely get all three values handed to you. You measure two and calculate the third, or you back into the answer from nameplate data and a known supply.

Ohm's Law is V = I x R. Power is P = V x I. Memorize both. Every load calc, voltage drop check, and breaker sizing decision starts here.

Voltage drop: the calc that saves callbacks

NEC 210.19(A) Informational Note 4 recommends branch circuits not exceed 3% voltage drop, with total feeder plus branch under 5%. It is not a hard code rule for most installs, but inspectors and engineers treat it like one on long runs.

For single phase, use VD = (2 x K x I x L) / CM. K is 12.9 for copper, 21.2 for aluminum. L is one-way length in feet. CM is circular mils from Chapter 9 Table 8. For three phase, swap the 2 for 1.732.

• 120V circuit, 3% drop = 3.6V max
• 240V circuit, 3% drop = 7.2V max
• 277V circuit, 3% drop = 8.31V max
• 480V circuit, 3% drop = 14.4V max

Run the numbers before you pull wire on anything past 100 feet. A 20A circuit at 150 feet on #12 copper drops about 4.6V at 120V. That is 3.8%, over the recommendation. Bump to #10.

Field tip: keep a voltage drop calculator on your phone, but learn the formula. When the calculator app crashes on a basement slab and the GC is staring at you, paper and a pencil still work.

Sizing conductors and overcurrent protection

Ampacity comes from NEC Table 310.16 for most installations. Pick the column that matches your insulation rating and the termination temperature rating per 110.14(C). For circuits 100A or less, you are stuck on the 60C column unless every termination point is rated 75C.

Overcurrent protection follows 240.4. Standard ampacities are in 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, and up. Small conductor rule in 240.4(D) caps #14 at 15A, #12 at 20A, #10 at 30A regardless of what the table says.

Continuous loads get the 125% bump per 210.20(A) and 215.3. A 16A continuous load needs a 20A breaker and conductors rated 20A minimum at the terminal temperature column.

Reading nameplates and converting to amps

Most equipment lists watts or VA, not amps. Convert before you size anything.

1. Single phase: I = P / V. A 1500W heater on 120V draws 12.5A.
2. Three phase: I = P / (V x 1.732 x PF). A 10kW resistive load on 480V three phase draws about 12A.
3. Motors: ignore the nameplate amps for branch circuit and feeder sizing. Use NEC Table 430.250 FLC values per 430.6(A)(1).

That last one trips up apprentices every time. The motor nameplate is for overload sizing, the table is for everything else. Two different numbers, two different jobs.

Resistance in the real world

Pure resistance is rare outside heaters and incandescent lamps. Real loads have inductance and capacitance, which is why power factor matters on motor and electronic loads. For code calcs you usually treat the load as resistive unless the manufacturer gives you a PF below 1.0.

Where resistance bites you is connections. A loose lug, corroded splice, or undersized terminal heats up under load. NEC 110.14 covers terminal connections, and 110.3(B) requires you to follow listing instructions, which means torque values. A connection at 50% of spec torque can run 30C hotter than one at full spec.

Field tip: torque every lug to spec with a calibrated wrench, not by feel. UL 486A-486B testing showed feel-based tightening was within spec less than 25% of the time. Document it if the spec sheet calls for it.

Putting it together on a service call

You get called to a panel that trips a 20A breaker every afternoon. Walk through it with the basics. Measure voltage at the panel, should be 120V nominal, accept 114 to 126 per ANSI C84.1. Measure current on the circuit under load with a clamp meter. If it reads 18A and the breaker holds for an hour then trips, you have a thermal trip from a continuous load on a non-continuous-rated circuit.

Check the ambient at the panel. NEC 310.15(B) ampacity correction factors apply when the panel sits in a hot mechanical room or attic. A 20A circuit in 50C ambient on THHN derates to 16.4A at the 75C column. The breaker is doing its job.

Solution path: split the load, upsize conductors and breaker if the equipment allows, or move the panel out of the heat. Three options, all rooted in voltage, current, and resistance. The math you learned in week one of trade school is still the math that closes tickets.