Crash course: Voltage, amperage, and resistance basics exam prep (part 4)

Ohm's Law: The Only Equation That Matters on Day One

Voltage pushes, amperage flows, resistance fights back. Memorize V = I x R and you can derive everything else. P = V x I gets you watts. Stop trying to feel your way through service calls when 30 seconds with a calculator gives you the answer.

For exam prep, you will be asked to solve for one variable when given the other two. A 240V circuit pulling 20A across a heating element: resistance is 12 ohms, power draw is 4,800W. That is the entire exercise. The trick is reading the question and knowing which variable they handed you.

If the math feels wrong, check your units before checking your meter. Milliamps vs amps and kilowatts vs watts trip up more apprentices than any wiring diagram.

Voltage: Potential Difference, Not a Thing You Can See

Voltage is the pressure between two points. It exists relative to a reference, usually ground or neutral. A hot conductor reads 120V to neutral, 0V to another hot on the same phase, and 208V or 240V to a hot on a different phase depending on your service.

NEC 210.6 governs branch circuit voltage limitations for occupancy types. Dwelling units cap most lighting and receptacle circuits at 120V nominal between conductors. Commercial spaces allow higher voltages with conditions on luminaire mounting heights and disconnect locations.

• 120V: standard residential receptacles and lighting
• 208V: three-phase wye commercial, line to line
• 240V: single-phase residential, line to line (split phase)
• 277V: commercial lighting, line to neutral on 480Y/277V
• 480V: commercial and industrial three-phase, line to line

Know what you are working on before you touch it. Voltage testers lie when batteries are weak. Use a solenoid tester or a known-live verification before and after lockout per NFPA 70E.

Amperage: What Actually Heats the Wire

Current is the flow rate of electrons through a conductor. Amps determine conductor sizing, breaker rating, and whether your terminations stay tight or melt. NEC 310.16 ampacity tables drive every conductor selection you make in the field.

The 80% continuous load rule under NEC 210.19(A) and 215.2(A) is non-negotiable for any load running three hours or more. A 20A breaker handles 16A continuous. Size feeders and branch circuits accordingly or you will be back fixing nuisance trips on a Saturday.

When a breaker trips and feels warm, do not just reset it. Pull the load calc, check terminations, and verify the conductor matches the OCPD. Heat is the symptom, not the problem.

Resistance: Conductors, Connections, and Why Things Burn

Resistance opposes current flow and converts electrical energy into heat. Copper at 75 degrees C has roughly 0.0008 ohms per foot for #12 AWG. That number doubles for #14, halves again for #10. Voltage drop math under NEC 210.19(A) Informational Note No. 4 recommends 3% on branch circuits, 5% total including feeders.

Loose connections are resistance you did not design for. A backstabbed receptacle on a 15A circuit can develop a few ohms of contact resistance over time, dropping 10V or more under load and dumping that wattage as heat directly into the device. This is how outlets melt and structures burn.

1. Torque every termination to the manufacturer spec, NEC 110.14(D)
2. Strip to the gauge mark on the device or wire nut
3. Pigtail multi-wire branches at the device, not through the device
4. Re-torque after one heating cycle on critical loads

Putting It Together: Real Calculations You Will See

A 1,500W space heater on a 120V circuit pulls 12.5A. Continuous load math says you need a 20A circuit minimum (12.5 / 0.8 = 15.6A required breaker capacity). Run #12 copper THHN, single phase, no derating issues, and you are inside NEC 240.4(D) small conductor rules.

Now drop the same heater on a 240V circuit. Current halves to 6.25A. Voltage drop on a 100 foot run drops by a factor of four because both current and resistance contribution change. This is why commercial HVAC and shop equipment runs at higher voltages: less copper, less heat, smaller conduit.

• Single phase amps: I = W / V
• Three phase amps: I = W / (V x 1.732 x PF)
• Voltage drop: VD = 2 x K x I x D / CM (single phase)
• Power factor on motors: usually 0.8 to 0.9 unless corrected

Exam Traps and Field Reality

Test questions love to mix units and add irrelevant data. Read twice, identify what they are asking for, then plug numbers. If the question gives you horsepower instead of watts, multiply by 746 to convert. If it gives you BTU/hr, divide by 3.412 to get watts.

The bigger trap is in the field. Code minimums are minimums, not targets. NEC 90.1(B) is explicit that the Code is not a design specification. A circuit that passes inspection at 80% loaded with 3% voltage drop is legal but borderline. Build with margin and your callbacks drop to zero.