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

What these three actually are

Voltage, amperage, and resistance are the three values that define every circuit in your house. Get them straight and the rest of the code starts to make sense. Get them wrong and you blow breakers, melt insulation, or kill someone.

Voltage (V) is electrical pressure, measured in volts. Amperage (I) is the rate of flow, measured in amps. Resistance (R) is opposition to flow, measured in ohms. They are tied together by Ohm's Law: V = I x R.

Residential service in the US is nominally 120/240V single-phase per NEC 220.5(A). Your receptacles deliver 120V. Your range, dryer, and HVAC pull 240V. Everything downstream of the panel is governed by those two numbers and the ampacity of the conductor feeding it.

Voltage: the pressure behind the wire

Voltage exists whether current is flowing or not. A dead-end hot conductor sitting in a junction box is still at 120V to ground. That is why you test before you touch, every time, even on a circuit you just turned off.

Voltage drop matters on long runs. NEC 210.19(A) Informational Note 4 recommends keeping branch circuit voltage drop under 3%, and total drop (feeder plus branch) under 5%. On a 120V circuit that is 3.6V max on the branch side. Undersized wire on a long run will cook motors and dim lights even when the breaker never trips.

• 120V: standard receptacles, lighting, most small appliances
• 240V: ranges, dryers, water heaters, EV chargers, central AC
• 208V: rare in homes, common in multi-family with three-phase service

Amperage: what actually does the work (and the damage)

Current is what heats conductors and what kills people. A 15A breaker is not protecting you, it is protecting the wire. The breaker trips before the conductor insulation breaks down. That is the entire job of overcurrent protection per NEC 240.4.

Conductor ampacity comes from NEC Table 310.16. For typical residential NM cable at 60 deg C terminations: 14 AWG copper is good for 15A, 12 AWG for 20A, 10 AWG for 30A. Do not exceed those numbers regardless of what the wire jacket is rated for. The terminations set the limit.

Field tip: if a homeowner has 14 AWG on a 20A breaker, swap the breaker to 15A before you do anything else. It is the fastest, cheapest fix on the list and it is almost always a previous owner's mistake.

Resistance: where the heat comes from

Every connection, splice, and termination has resistance. Good ones have nearly zero. Bad ones build resistance, drop voltage across themselves, and turn that voltage drop into heat. Power dissipated at a connection is P = I squared x R. Double the resistance and you double the heat at the same current.

This is why backstabbed receptacles fail, why aluminum-to-copper splices without proper connectors burn houses down, and why a loose neutral on a multiwire branch circuit will fry electronics. Resistance you cannot see is the most dangerous kind.

1. Torque every termination to the manufacturer spec, NEC 110.14(D) requires it
2. Use listed connectors for dissimilar metals, AL/CU rated only
3. Pigtail receptacles on shared neutrals so the device is not carrying through-current
4. Re-tighten lugs on a service or panel after the first heat cycle if the manufacturer calls for it

Ohm's Law on the job

You do not need a calculator for most of this. Three rearrangements cover 90% of field math: V = I x R to find voltage, I = V / R to find current, R = V / I to find resistance. Power is P = V x I, watts equals volts times amps.

Quick example: a 1500W space heater on a 120V circuit pulls 12.5A. Put two on the same 15A circuit and you are tripping breakers all afternoon. Put one on a 20A small-appliance circuit in a kitchen and you are violating NEC 210.52(B) because that circuit is reserved for countertop receptacles, not portable heat.

Field tip: memorize the wattage of the four loads you see most, microwave, hair dryer, space heater, window AC. You can size a circuit in your head while the homeowner is still describing the problem.

Why this matters before you pull a permit

Code is built on top of these three values. Breaker sizing is amperage. Wire sizing is amperage corrected for ambient temperature and bundling per NEC 310.15. GFCI and AFCI requirements per NEC 210.8 and 210.12 exist because of how current behaves under fault conditions. Grounding and bonding per NEC Article 250 exist to give fault current a low-resistance path back to the source.

If you are a homeowner planning a project, understanding voltage, amperage, and resistance is what lets you read a load calculation and know whether your panel can take another 240V circuit. If you cannot do that math, hire it out. Part 2 covers load calcs, branch circuit design, and how to read a panel schedule without guessing.