Crash course: Voltage, amperage, and resistance basics for industrial electricians (part 1)

What voltage, amperage, and resistance actually are

Voltage is electrical pressure. It's the potential difference between two points, measured in volts. No pressure, no push. A 480V feeder has more push than a 120V branch circuit, which is why it moves more power through smaller conductors.

Amperage is flow. It's the rate of electron movement through a conductor, measured in amps. This is what does the work, and it's what heats your conductors, trips your breakers, and kills people. When you're sizing wire or setting overcurrent protection, you're managing amperage.

Resistance opposes flow. Measured in ohms, it shows up in every conductor, connection, and load. Low resistance in a fault path means high fault current. High resistance in a connection means heat, voltage drop, and eventually failure.

Ohm's Law on the job

V = I x R. Voltage equals current times resistance. Rearrange it however you need: I = V/R, R = V/I. This is the equation you'll use more than any other in the field, whether you're troubleshooting a dead circuit or calculating voltage drop on a long run.

Power follows: P = V x I. A 240V load pulling 20 amps is moving 4,800 watts. That same 4,800 watts at 480V only pulls 10 amps, which is why industrial systems run higher voltages. Less current, smaller conductors, lower losses.

• Find current: I = P / V
• Find voltage drop: Vd = I x R (conductor resistance from Chapter 9, Table 8)
• Find resistance of a load: R = V / I
• Find power: P = V x I, or P = I squared x R

If a 120V circuit reads 118V at the panel and 104V at the load, you've got 14V of drop. That's 11.7%, well past the NEC 210.19(A) Informational Note recommendation of 3% on branch circuits. Check your terminations before you blame the wire.

Voltage drop and why it matters

NEC 210.19(A) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2 recommend a maximum 3% drop on branch circuits and feeders, with a combined total not exceeding 5%. These are recommendations, not mandates, but ignoring them gets you nuisance trips, dim lights, motors that won't start, and VFDs that fault on undervoltage.

For long runs, upsize the conductor. A 100A feeder going 200 feet on #3 copper might calc out fine on ampacity but drop 4% under load. Bumping to #1 cuts it to roughly 2.5%. Use Chapter 9, Table 8 for DC resistance, or Table 9 for AC impedance on three-phase calcs.

Three-phase voltage drop uses a different multiplier than single-phase. Single-phase: Vd = 2 x K x I x L / CM. Three-phase: Vd = 1.732 x K x I x L / CM. K is approximately 12.9 for copper, 21.2 for aluminum.

Series vs parallel circuits

In series, current is the same through every component, voltages add up, and resistances add up. Break one point and the whole circuit dies. This is why your control circuits, E-stops, and safety interlocks are wired in series. One open contact and the load drops out.

In parallel, voltage is the same across every branch, currents add up, and total resistance drops below the smallest individual resistor. This is how your branch circuits feed multiple receptacles, and why adding loads to a circuit raises total amperage. It's also why a parallel conductor set per NEC 310.10(G) shares current proportional to impedance, which is why the conductors must be the same length, size, material, and termination type.

If parallel conductors aren't identical, current splits unevenly. The conductor with lower impedance carries more, runs hotter, and may exceed its ampacity even though the total load looks fine. This is a code violation and a fire waiting to happen.

AC vs DC behavior

DC is steady-state. Voltage and current don't reverse direction. Resistance is the only opposition. Battery banks, control circuits, solar arrays before the inverter, and most VFD DC bus sections are DC.

AC alternates, typically 60 Hz in North America. On AC systems, you deal with impedance, not just resistance. Impedance includes resistance plus reactance from inductance (motors, transformers, ballasts) and capacitance (power factor correction, long cable runs). This is why a motor's locked-rotor amps are far higher than its full-load amps. Inrush is mostly inductive.

1. DC: opposition is resistance only
2. AC resistive load (heaters, incandescent): impedance roughly equals resistance
3. AC inductive load (motors, transformers): impedance includes inductive reactance
4. AC capacitive load (PF correction, VFD inputs): impedance includes capacitive reactance

Measuring it correctly in the field

Voltage is measured in parallel, across the component or between two points. Meter leads on L1 and L2, or hot to neutral. Always verify your meter on a known live source before and after testing per NFPA 70E 120.5(7), the live-dead-live test.

Current is measured in series, or with a clamp meter around a single conductor. Never break a live circuit to insert a meter in series above low-voltage control work. Use a clamp. For accurate readings on small currents, loop the conductor through the clamp jaw multiple times and divide by the number of turns.

Resistance is measured with the circuit de-energized and isolated. A megger reads insulation resistance at 500V, 1000V, or higher per the equipment rating. A standard DMM reads continuity and low-ohm values for windings, contacts, and grounding paths. NEC 250.4 and 250.118 govern grounding paths, and a high-resistance ground is as bad as no ground at all.