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

Why these three matter on the job

Voltage, amperage, and resistance are the load-bearing concepts behind every conductor sizing, breaker selection, and fault calculation you do. Get sloppy with the basics and you misread a panel schedule, undersize a feeder, or miss why a 20A circuit keeps tripping under a 14A load. The NEC assumes you already know this cold.

This is part one. We cover what each quantity is, how it behaves in the field, and the failure modes you actually see in residential and light commercial work. Part two will cover power, voltage drop math, and three-phase.

Voltage: the pressure

Voltage (E or V) is electrical potential difference, measured in volts. Think of it as pressure pushing electrons through a conductor. No difference in potential, no current. That is why a bird on a single line does not get cooked... there is nothing for current to flow toward.

Common nominal system voltages you will meet in the field, per NEC 220.5(A), which standardizes calculations at these values:

• 120V single phase: general lighting, receptacles, small appliances
• 240V single phase: ranges, dryers, water heaters, residential services
• 208Y/120V three phase: light commercial
• 480Y/277V three phase: industrial, commercial lighting

Voltage to ground is what shocks you. NEC 250.4(A)(1) requires the system to be grounded so that voltage to earth stays predictable and overcurrent devices clear faults fast. A floating ungrounded system can sit with hundreds of volts to ground and you would never know until you become the path.

Amperage: the flow

Amperage (I) is the rate of electron flow, measured in amps. One amp is roughly 6.24 quintillion electrons per second past a point, but the number you actually care about is the one stamped on the breaker. Amperage is what heats conductors, trips breakers, and welds contacts shut.

Conductor ampacity comes from NEC Table 310.16 with adjustments for ambient temp (310.15(B)(1)) and conduit fill (310.15(C)(1)). A 12 AWG THHN copper conductor is good for 30A at 90C in the table, but the termination temperature rating per 110.14(C) usually drags you back down to the 75C column, so 25A. Then derate further if you have more than three current carrying conductors in the raceway.

Tip from the field: if a 20A circuit is running a steady 16 to 18A under normal load, you are at or near the 80% continuous load limit in NEC 210.19(A)(1)(a). Either reduce the load or pull a 30A circuit with proper conductors. Nuisance trips are the breaker telling you the truth.

Resistance: the opposition

Resistance (R) is opposition to current flow, measured in ohms. Every conductor, splice, termination, and load has resistance. Copper has less than aluminum at the same size, which is why aluminum feeders are typically one or two sizes larger for the same ampacity (compare 310.16 columns).

Where resistance bites you in the field:

1. Loose terminations: a backed-out lug builds resistance, resistance builds heat, heat carbonizes insulation, and you have a melted breaker on a callback
2. Long runs: voltage drop is just current times conductor resistance, and NEC 210.19(A) Informational Note 4 recommends keeping branch circuit drop under 3%
3. Corroded splices: especially in wet locations or dissimilar metals, resistance climbs over time and the joint cooks itself

Insulation resistance is the other side of the coin. You want infinite resistance between conductors and ground. A megger reading below 1 megohm on a feeder is a problem, even if the circuit still energizes.

Ohm's Law: the one equation you cannot dodge

E = I times R. Voltage equals current times resistance. Rearrange for whatever you are solving:

• I = E / R (current draw given voltage and load resistance)
• R = E / I (apparent resistance from measured voltage and current)
• E = I times R (voltage drop across a known resistance)

Quick field example. A 240V water heater element measures 12 ohms cold. Expected current: 240 / 12 = 20A. If your clamp reads 8A, the element is failing open or one leg is dead. If it reads 30A, the element is shorting and the breaker should already be on its way out.

Where new journeymen get burned

The mistakes that show up on punch lists and incident reports are almost always basics, not exotic theory. Watch for these:

• Confusing voltage to ground with voltage between phases on a 208Y/120V system. Line to line is 208, line to neutral is 120
• Assuming a breaker protects the load. It protects the conductor (NEC 240.4). The load needs its own overcurrent or thermal protection
• Ignoring continuous load rules. NEC 210.20(A) requires the OCPD to be sized at 125% of continuous load plus 100% of non-continuous
• Treating neutral current as zero on a multiwire branch circuit feeding nonlinear loads. Harmonics on the neutral are real and 210.4(A) addresses MWBC handling

Tip from the field: before you troubleshoot anything, verify your meter on a known live source, take your reading, then verify the meter again. A bad meter has killed more electricians than bad math.

Lock these three quantities in and the rest of the code starts to make sense as engineering, not memorization. Part two builds on this with power, three-phase relationships, and voltage drop sizing you can do on a job trailer.