Crash course: Voltage, amperage, and resistance basics for apprentices (part 5)

Three quantities, one relationship

Voltage pushes. Amperage flows. Resistance opposes. Every circuit you touch on a job site obeys Ohm's Law: V = I × R. Memorize it cold. If you know any two, you can solve for the third with basic algebra.

Voltage (V) is electrical pressure, measured in volts. Amperage (I) is the rate of electron flow, measured in amperes. Resistance (R) is the opposition to that flow, measured in ohms. A 120V circuit with a 12-ohm load draws 10 amps. Same circuit, drop the resistance to 6 ohms, you pull 20 amps. That is why a short circuit pulls hundreds of amps before the breaker trips.

Power (watts) is the fourth quantity you need on the truck: P = V × I. A 1500W space heater on 120V pulls 12.5 amps. That puts it at 80% of a 15A circuit, which is the continuous load ceiling per NEC 210.19(A)(1).

Voltage in the real world

Residential service in North America is split-phase 120/240V. You get 120V from any hot to neutral, and 240V across both hots. Commercial buildings typically run 208Y/120V or 480Y/277V three-phase wye systems. Know which one you are standing in front of before you pull a cover.

Voltage drop matters once runs get long. NEC 210.19(A) Informational Note 4 recommends keeping branch circuit voltage drop under 3%, and total drop (feeder plus branch) under 5%. A 120V circuit should not drop below roughly 114V at the load. Undersized conductors on long runs cause motors to overheat, LED drivers to flicker, and electronics to reset.

If a homeowner complains their garage freezer keeps tripping on hot days, measure voltage at the receptacle while it is running. Anything below 110V on a 120V nominal circuit and you are chasing a voltage drop or a loose connection, not a bad freezer.

Amperage and conductor sizing

Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. NEC Table 310.16 is the one you live in for conductors rated 0 to 2000V. A 12 AWG copper THHN at the 75°C column carries 25 amps, but the small conductor rule in NEC 240.4(D)(5) caps its overcurrent protection at 20 amps.

Continuous loads (those operating three hours or more) require the branch circuit and overcurrent device to be sized at 125% of the load per NEC 210.19(A)(1) and 210.20(A). A 16-amp continuous load needs a 20-amp breaker minimum.

• 14 AWG copper: 15A maximum overcurrent
• 12 AWG copper: 20A maximum
• 10 AWG copper: 30A maximum
• 8 AWG copper: 40A or 50A depending on termination temp rating
• 6 AWG copper: 55A or 65A depending on termination temp rating

Derate for ambient temperature (Table 310.15(B)(1)) and for more than three current-carrying conductors in a raceway (Table 310.15(C)(1)). Stack both factors. A bundle of seven 12 AWG THHN in a hot attic can drop ampacity below the 20A breaker protecting it, which is a code violation and a future fire.

Resistance, impedance, and what the meter shows you

Pure resistance only tells the full story on DC and on purely resistive AC loads like heaters and incandescent lamps. On AC circuits with motors, transformers, or electronic ballasts, you are dealing with impedance (Z), which combines resistance with inductive and capacitive reactance. Your clamp meter reads the resulting current, not the resistance.

For troubleshooting, ohms get you to the answer fast. Power off, locked out, leads on the load: open circuit reads OL, dead short reads near zero, a healthy heating element reads a specific value you can compute from its nameplate. A 4500W water heater element on 240V should measure roughly 12.8 ohms (R = V² / P).

Never ohm out a circuit that is energized. You will either smoke the meter, blow the fuse in the meter, or take an arc to the face. Verify dead with a known-good tester before the leads touch anything.

Putting it together on the job

Most service calls come down to one of three failures: too much resistance somewhere it should not be (loose lug, corroded splice, failing contact), not enough resistance where there should be (insulation breakdown, ground fault, shorted winding), or a voltage source that has shifted (open neutral, lost phase, undersized service).

Train yourself to think in all three quantities at once. If a 240V dryer circuit reads 208V hot to hot, you have lost a leg. If a 20A circuit is tripping at 14 amps of measured load, the breaker is weak or the connection at the lug is heating up and adding resistance. If a motor is humming and not starting, check voltage at the motor leads under load, not at the panel.

1. Verify the source: measure voltage at the panel, hot to neutral and hot to hot.
2. Verify the path: check continuity and resistance with the circuit dead and locked out.
3. Verify the load: measure current draw with a clamp meter under normal operation.
4. Compare to nameplate and to NEC ampacity tables. Anything outside spec is your suspect.

Ohm's Law is not academic. It is the diagnostic framework every good troubleshooter runs in their head before they pull the first wire nut.