Crash course: Voltage, amperage, and resistance basics with examples (part 2)

Why Part 2 matters

Part 1 covered the definitions. This one puts numbers on the bench and shows how voltage, amperage, and resistance behave when you actually load a circuit. If you can predict the math before you pull the trigger on the meter, you troubleshoot faster and you stop chasing ghosts.

Every example below assumes a healthy 120V or 240V supply, copper conductors, and standard ambient. Adjust for temperature and conductor fill per NEC 310.15 when you leave the bench.

Ohm's Law on real loads

V = I x R. Rearrange it any direction you want. The trick on the job is knowing which two values you can measure cleanly and which one you have to calculate.

Take a 1500W heater on a 120V branch. Current draw is 1500 / 120, so 12.5A. Element resistance is 120 / 12.5, so 9.6 ohms cold. Measure cold resistance with the heater unplugged and you should land near that number. Read 0.2 ohms and the element is shorted. Read open and the element is fried.

• Volts known, amps known: R = V / I
• Volts known, resistance known: I = V / R
• Amps known, resistance known: V = I x R
• Watts and volts known: I = P / V

Voltage drop in the field

NEC 210.19(A) Informational Note 4 recommends keeping branch circuit drop at 3% or less, with total drop on feeder plus branch at 5% or less. It is not enforceable code, but inspectors and POCOs lean on it and your customer feels it when motors hum and lights dim.

Quick math for single phase, two wire copper: VD = (2 x K x I x D) / CM. K is 12.9 for copper, D is one way distance in feet, CM is circular mils from Chapter 9 Table 8. For a 20A load on #12 (6530 CM) running 100 feet, that is (2 x 12.9 x 20 x 100) / 6530, which is 7.9V. On a 120V circuit that is 6.6%. Bump it to #10 and you cut the drop roughly in half.

Long run, marginal load, customer complains lights flicker when the fridge kicks on. Before you tear into the panel, measure voltage at the receptacle with the offending load running. If you see more than 6V drop from no load to full load, your conductor is undersized or your terminations are loose.

Resistance, continuity, and what your meter is telling you

Resistance readings only mean something on a de-energized, isolated circuit. Energized resistance readings are garbage and dangerous. Lock out, verify dead, then ohm.

Healthy 14 AWG copper runs about 2.525 ohms per 1000 feet at 75C per Chapter 9 Table 8. So 50 feet of #14 should read roughly 0.13 ohms loop. Your leads alone read 0.2 to 0.4 ohms, so always null them or subtract. A reading of 3 ohms on that same run says you have a corroded splice, a backstabbed receptacle, or a wirenut that never bit.

1. Open the circuit at the breaker and verify with your tester.
2. Disconnect the load so you are reading conductor only.
3. Short the far end and measure loop resistance from the panel.
4. Compare to expected value from Table 8 plus terminations.

Amperage, breakers, and the 80% rule

NEC 210.19(A)(1) and 210.20(A) require branch circuit conductors and OCPD to carry 125% of continuous load. Translation: a continuous load (3 hours or more) can only use 80% of the breaker rating. A 20A circuit feeding a continuous load tops out at 16A.

Common spot where this bites: a 1500W bathroom heater, a 1440W space heater on an office circuit, or a long string of LED high bays. They look harmless on the nameplate, then you stack two of them and trip the breaker every afternoon when the building warms up and the elements draw a hair more.

• Continuous load: 3 hours or more at full current
• Branch conductor ampacity: load x 1.25, then size per 310.16
• OCPD: load x 1.25, round up to standard size per 240.6(A)
• Receptacles on 20A circuits: 5-20R only when supplying a single receptacle, 5-15R or 5-20R on multi outlet, see 210.21(B)

Putting it together: a service call walkthrough

Customer says a 240V well pump keeps tripping its 30A two pole breaker. Nameplate is 5HP, 240V, 28A FLA. FLA times 1.25 is 35A, but motor circuits use 430.52, so the breaker can be sized up to 250% of FLA, which is 70A for an inverse time breaker. The 30A is too small from day one.

Before you upsize, measure. Clamp each leg under load. If you read 32A and the nameplate says 28A FLA, the motor is loaded past spec, likely a worn impeller or a stuck check valve. Megger the windings phase to ground per the manufacturer spec. If insulation reads above 1 megohm and current is high, the problem is mechanical, not electrical.

Tripping breakers are almost never the breaker. They are doing exactly what they were sized to do. Find the load, find the fault, find the wire, then decide if the OCPD is correct per 240.4 and 430.52.

Field shortcuts worth memorizing

You will not always have a calculator handy on a ladder. These are the back of the hand numbers that solve 80% of what comes up.

1. Watts to amps at 120V: divide watts by 120, or roughly by 100 for a fast estimate.
2. Watts to amps at 240V single phase: divide by 240, or by 200 for an estimate.
3. Watts to amps at 208V three phase: divide by 360.
4. #12 copper carries 20A, #10 carries 30A, #8 carries 40A, #6 carries 55A at 75C per 310.16.
5. Voltage drop doubles when you double the run, halves when you go up two wire sizes.

Keep these in your head and you can size, troubleshoot, and sanity check without pulling out the codebook on every job. The codebook still wins ties, every time.