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

Why these three matter

Voltage, amperage, and resistance are the three numbers you live by. Get them wrong and you trip breakers, smoke conductors, or kill someone. Get them right and the rest of the trade falls into place: conductor sizing, OCPD selection, voltage drop, fault current, the lot.

This is part one. We are starting with the fundamentals you should already know cold but probably explain to apprentices three times a week. Part two will cover three-phase, power factor, and how these scale on commercial gear.

Voltage: the pressure

Voltage is electrical pressure measured in volts (V). It is the potential difference between two points. No difference, no current flow, even if the wire is fat and the load is hungry. That is why a bird on a single conductor does not fry... no second reference point.

On commercial jobs you will see standard nominal systems per NEC Article 220 and Table 220.5: 120/208V wye, 277/480V wye, and the occasional 120/240V high-leg delta in older buildings. Always verify with a meter before you commit to a conductor or device rating. Nominal is not actual.

• 120V: receptacles, lighting branch circuits, small appliances
• 208V: light commercial HVAC, kitchen equipment, single-phase loads off a 3-phase wye
• 277V: commercial fluorescent and LED lighting
• 480V: motors, large HVAC, distribution feeders

Amperage: the flow

Amps measure current, the actual movement of electrons through the conductor. Voltage pushes, amps flow. This is what heats your conductors, trips your breakers, and welds your lugs when something goes sideways.

Conductor ampacity comes from NEC 310.16 (formerly 310.15(B)(16) before the 2020 cycle reorganization). Always derate for ambient temperature per Table 310.15(B)(1) and bundling per 310.15(C)(1) when you have more than three current-carrying conductors in a raceway. A 12 AWG THHN rated 30A on the chart is not 30A in a 110 degree F attic with seven other circuits in the same EMT.

Field tip: when a breaker nuisance trips, clamp the line under load before you swap the breaker. Nine times out of ten the breaker is doing its job and the load grew without anyone telling you.

Resistance: what fights back

Resistance, measured in ohms, is opposition to current flow. Every conductor has it. Every connection has it. Every load is essentially a controlled resistance (or impedance, once you bring AC and reactance into it, but save that for part two).

Two resistances will eat your lunch on commercial work: conductor resistance over distance, and connection resistance at terminations. The first shows up as voltage drop. The second shows up as a hot lug, a discolored breaker, or a fire. NEC 110.14(D) requires torque values from the listing, and the 2017 cycle made calibrated torque tools mandatory for terminations rated above what you can feel with a screwdriver.

Ohm's law in the field

V = I × R. Memorize the triangle, but more importantly memorize what each variable tells you on a service call.

1. Voltage reads low at the load but normal at the panel: resistance went up somewhere in between. Check terminations and splices.
2. Amperage climbs over time on a fixed load: resistance is dropping (insulation failure) or the load is growing.
3. Resistance reads infinite where it should not: open conductor, blown fuse element, or a tripped internal protector.
4. Resistance reads near zero where it should not: short to ground or short between conductors. Lock it out before you energize.

The power formula rides shotgun: P = V × I for DC and resistive AC. On a 277V circuit pulling 8A, you have roughly 2,216W of load. That number drives breaker sizing, conductor sizing per NEC 215.2 for feeders, and your voltage drop calc per 210.19(A) Informational Note 4 (3% on branch, 5% total).

Voltage drop, the silent killer

Voltage drop is where Ohm's law gets expensive. The NEC recommendation is informational, not mandatory, but the manufacturer specs on motors, electronics, and LED drivers are not. Drop too much voltage and equipment runs hot, controllers reset, and warranties evaporate.

Quick field formula for single-phase: VD = (2 × K × I × D) / CM, where K is 12.9 for copper, I is amps, D is one-way distance in feet, and CM is circular mils from NEC Chapter 9 Table 8. For three-phase, swap the 2 for 1.732. Run it before you pull the wire, not after the inspector flags the lighting on the back of the warehouse.

Field tip: any run over 100 feet on a 20A branch circuit at full load deserves a voltage drop check. On 277V lighting feeders running the length of a big-box store, assume you are upsizing from 12 AWG to 10 AWG and budget for it on the bid.

What to take to the truck

Volts push, amps flow, ohms resist. Every problem you troubleshoot is one of those three drifting from where it should be. Your meter tells you which one. The NEC tells you what the limits are. Your hands and your torque wrench keep the resistance at the terminations from becoming the next chapter of an incident report.

Part two picks up with three-phase math, power factor, and why the 480V switchgear room has different rules than the 120V receptacle on the breakroom counter.