Crash course: Voltage, amperage, and resistance basics with examples (part 1)

The three quantities that run every circuit

Voltage, amperage, and resistance are the only variables you need to size conductors, pick breakers, and troubleshoot a dead receptacle. Get these straight and the NEC tables stop feeling like a wall of numbers.

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

• Voltage pushes.
• Amperage flows.
• Resistance restricts.
• Change one, the other two react.

Voltage: what the source delivers

Standard residential service in the US is 120/240V single phase. Light commercial often runs 208Y/120V three phase. Industrial gear commonly sees 480Y/277V. NEC 220.5(A) tells you to use nominal system voltages (120, 208, 240, 277, 480) for load calculations, not whatever your meter reads on a Tuesday afternoon.

Voltage drop matters once a run gets long. NEC 210.19(A) Informational Note 4 recommends keeping branch circuit drop at or below 3%, with 5% total for feeder plus branch. Past that, motors run hot, electronics misbehave, and your customer calls back.

Field tip: if a 120V circuit reads 108V at the receptacle under load, you are sitting at 10% drop. Upsize the conductor or shorten the run before you chase a "bad outlet."

Amperage: what the load pulls

Amps are what trip breakers and melt insulation. Every device draws a predictable current based on its wattage and the supply voltage: I = P / V. A 1500W space heater on 120V pulls 12.5A. The same heater on 240V would pull 6.25A.

NEC 210.20(A) requires the overcurrent device to be sized at 125% of continuous load (anything running 3 hours or more) plus 100% of non-continuous load. That is why a 16A continuous load lands on a 20A breaker, not a 15A.

1. Identify the load in watts or VA.
2. Divide by nominal voltage to get amps.
3. Multiply continuous portions by 1.25.
4. Round up to the next standard breaker per NEC 240.6(A).
5. Pick the conductor from NEC Table 310.16 at the correct temperature column.

Resistance: the conductor and everything in it

Copper has low resistance. Aluminum has more. Connections, splices, and corroded terminals add their own. Resistance shows up as heat, voltage drop, and nuisance trips. NEC Chapter 9 Table 8 gives DC resistance per 1000 ft for every common conductor size, which is your starting point for any drop calculation.

A loose neutral on a multiwire branch circuit is resistance in the wrong place. It pushes voltage onto the high-impedance leg and can swing 120V loads up toward 240V. NEC 300.13(B) bars dependence on device terminations for neutral continuity in multiwire circuits for exactly this reason.

• Loose lug: high resistance, hot termination.
• Undersized neutral on shared circuits: voltage imbalance.
• Long aluminum runs: voltage drop you must calculate, not guess.

Ohm's Law on the truck

You will use three forms of the same equation more than any other math in this trade. Memorize them and you will troubleshoot faster than the guy still digging through his phone.

• V = I x R, when you know current and resistance.
• I = V / R, when you need to predict load on a known impedance.
• R = V / I, when you are reading a meter and want to verify a winding or element.

Example: a baseboard heater element measures 12 ohms cold. On 240V it will draw 240 / 12 = 20A. That puts it right at the limit of a 20A circuit before continuous-load derating, so it belongs on a 30A circuit per NEC 424.3(B), which requires fixed electric space heating to be treated as continuous and the branch circuit sized at 125%.

Power: the practical fourth quantity

Power (P) in watts ties voltage and amperage together: P = V x I. On AC circuits with motors or electronics, multiply by power factor for true watts, but for resistive loads (heat, incandescent, water heaters) the simple form is exact.

NEC 220.12 and Table 220.12 use VA per square foot for general lighting load calculations. NEC 220.14 lists specific receptacle and appliance loads in VA. Knowing P = V x I lets you reverse any of those numbers back into amps for breaker and conductor sizing.

Field tip: a 4500W water heater on 240V is 18.75A. NEC 422.13 calls storage water heaters of 120 gallons or less continuous, so size the circuit at 18.75 x 1.25 = 23.4A. That is a 25A or 30A breaker on 10 AWG copper, not a 20A on 12.

Part 2 will cover series versus parallel behavior, how impedance differs from pure resistance on AC, and the calculations behind voltage drop on long runs.