Crash course: Voltage, amperage, and resistance basics for apprentices (part 3)

Why Part 3 matters

Parts 1 and 2 covered what voltage, amperage, and resistance are. This one is about how they bite you on the job. Ohm's Law is not classroom theory. It is the math behind every nuisance trip, every hot lug, and every burned receptacle you will ever troubleshoot.

If you can run V = IR in your head while standing on a ladder, you stop guessing. You start diagnosing. That is the difference between an apprentice who pulls wire and a journeyman who solves problems.

Ohm's Law on a real circuit

A 120V branch circuit feeds a load drawing 12 amps. The conductor, terminations, and load together present 10 ohms. Plug it in: 120 = 12 x 10. Clean.

Now somebody backstabs a receptacle and the connection corrodes. That joint adds 2 ohms of resistance. Voltage at the load drops, current shifts, and the bad termination dissipates power as heat. P = I squared R. At 12 amps through 2 ohms, that joint is throwing 288 watts into a plastic device box. That is how fires start.

Field tip: if a receptacle feels warm to the back of your hand, kill the circuit and pull it. Warm devices are failing devices, every time.

Voltage drop, the silent killer

NEC 210.19(A) Informational Note 4 recommends branch circuit voltage drop not exceed 3 percent, with combined feeder and branch not exceeding 5 percent. It is a recommendation, not a rule, but ignore it and equipment suffers. Motors run hot. Electronics reset. LED drivers fail early.

The quick field formula for single phase, copper, is VD = (2 x K x I x L) / CM, where K is 12.9 for copper, I is current in amps, L is one-way length in feet, and CM is circular mils of the conductor. For a 100 foot run of #12 copper (6530 CM) at 16 amps, that is (2 x 12.9 x 16 x 100) / 6530, or about 6.3 volts. On a 120V circuit, that is 5.25 percent. Over the recommendation. Upsize to #10.

• Under 100 feet, #12 is usually fine for a 20A circuit.
• 100 to 150 feet, plan on #10.
• Over 150 feet, run the math, do not eyeball it.

Amperage and conductor sizing

NEC 310.16 gives ampacity tables for conductors in raceway or cable. A #12 THHN copper at 75C column is good for 25 amps, but NEC 240.4(D)(5) caps the overcurrent device at 20 amps for #12 copper. The table value is not the breaker size. Apprentices mix this up constantly.

Then come the derates. Ambient temperature correction per NEC 310.15(B)(1), and adjustment for more than three current-carrying conductors in a raceway per NEC 310.15(C)(1). Four to six CCCs, you derate to 80 percent. Stuff a panel feeder with neutrals and travelers and you can lose a wire size fast.

1. Start with the load calculation.
2. Pick the conductor from NEC 310.16 at the correct temperature column.
3. Apply ambient and CCC derates.
4. Verify against the OCPD rules in NEC 240.4.

Resistance, grounding, and bonding

Resistance is not just about the load. The equipment grounding conductor exists to give fault current a low impedance path back to the source so the breaker trips fast. NEC 250.4(A)(5) calls this out directly. High resistance in that path means slow trips, or no trip at all, and energized metal parts you can touch.

This is why loose bonding bushings, painted-over ground bars, and missing locknuts matter. Every bad mechanical connection adds impedance. A ground rod measured at 25 ohms per NEC 250.53(A)(2) is not a fault clearing path. It is a reference to earth. The EGC clears faults, not the rod. Get that straight and a lot of grounding confusion disappears.

Field tip: torque every lug to the manufacturer's spec, every time. NEC 110.14(D) requires it. Hand tight is not a spec.

Putting it together on a service call

A homeowner says their microwave keeps tripping the breaker. You meter the circuit at 118V no load. Microwave nameplate says 12.5 amps. You start it, voltage at the receptacle sags to 102V. That is a 16V drop under load on a 20 foot run. Something in that circuit has resistance it should not have.

You pull the receptacle. Backstabbed, discolored, and the neutral pigtail is barely making contact. Re-terminate on the screws, torque to spec, voltage holds at 117V under load. Breaker stops tripping. You did not need a wiring diagram. You needed Ohm's Law and a meter.

• Always meter under load, not just no load.
• A voltage drop greater than 3 percent on a short run means a connection problem.
• Heat at a termination is the tell, every time.

What to carry in your head

You do not need to memorize every table. You do need V = IR, P = IE, and the 3 percent and 5 percent voltage drop targets locked in. Everything else you can look up in the book or in BONBON while you are standing at the panel.

The trades reward people who can think on their feet. The math is simple. The discipline of actually using it is what separates the apprentices who advance from the ones who stay apprentices.