Crash course: Voltage, amperage, and resistance basics step by step (part 4)

Why this matters on the job

Voltage, amperage, and resistance are the three values you measure, calculate, and troubleshoot every shift. If you can't move between them in your head, you'll misread a meter, oversize a conductor, or chase a ghost fault for two hours. Part 4 ties the math to what you actually see at the panel, the receptacle, and the motor.

This is not theory for an apprentice exam. It's the working version: what each value does, where it shows up in the NEC, and how to use it without a calculator when the foreman is waiting.

Voltage: the push behind the work

Voltage is electrical pressure, measured in volts. In residential, you're working with 120V line to neutral and 240V line to line. Commercial bumps you to 208Y/120V or 480Y/277V depending on the service. Industrial gets you into 600V and above, which triggers different working space rules under NEC 110.26.

Nominal voltage is what the system is rated. Actual voltage at the load is almost always lower because of drop. NEC 210.19(A) Informational Note 4 recommends branch circuit voltage drop not exceed 3%, with combined feeder and branch not exceeding 5%. It's a recommendation, not a rule, but inspectors and engineers treat it as one.

• 120V: standard receptacles, lighting, small appliances
• 240V: dryers, ranges, water heaters, well pumps
• 277V: commercial fluorescent and LED fixtures
• 480V: three-phase motors, HVAC, large equipment

Amperage: what actually does the damage

Amps are the flow rate. They're what trip your breaker, heat your conductor, and kill a person at currents as low as 100 mA across the heart. Voltage gets the headlines but amperage is what you size wire and overcurrent protection around.

NEC 310.16 is the table you'll open more than any other. It gives ampacity by conductor size, insulation type, and ambient temperature. Remember the 60/75/90 degree column rules under NEC 110.14(C): terminations on equipment rated 100A or less default to the 60 degree column unless the equipment is listed for 75. Above 100A, you get the 75 degree column.

Tip: When you're sizing for a continuous load, multiply by 125% per NEC 210.19(A)(1) and 215.2(A)(1). A 16A continuous load needs a 20A circuit, not a 15A, even though the math seems to fit.

Resistance: the part that bites you on troubleshooting

Resistance is opposition to current flow, measured in ohms. Conductors have a small amount, loads have a designed amount, and bad connections have way too much. Most of your service calls trace back to a resistance problem somewhere: loose lug, corroded splice, undersized neutral, failing element.

You measure resistance with the circuit dead. Always. A meter on ohms with voltage present will give you garbage readings and possibly a blown fuse. NEC 110.12 covers neat and workmanlike installation, which directly affects long term resistance at terminations. A torque wrench on every lug is not optional anymore under NEC 110.14(D).

Ohm's Law without the calculator

V equals I times R. Memorize the wheel and you can solve any single phase problem in your head. The three forms you'll actually use:

1. V = I x R, when you know current and resistance and need voltage
2. I = V / R, when you're checking expected current draw on a known load
3. P = V x I, when sizing for wattage and you need amps

Example: a 1500W heater on 120V pulls 12.5A. Same heater on 240V pulls 6.25A. Half the current, quarter the heat loss in the conductor, which is exactly why the NEC pushes higher voltages for larger loads. Power loss in the wire scales with the square of current, per the I squared R rule.

For three phase, multiply by 1.732 (square root of 3). A 480V three phase motor at 50A draws roughly 41,500 VA. Keep that conversion in your phone if you don't trust the mental math under pressure.

Putting it together at the panel

When you walk up to a panel with a tripping breaker, you're solving for one of three things: too much current (load problem), low voltage (supply or drop problem), or high resistance (connection problem). Your meter tells you which one in under two minutes if you check in order.

• Voltage at the breaker, hot to neutral and hot to ground
• Voltage at the load with everything energized, looking for drop
• Amp clamp on the conductor, compare to nameplate and breaker rating
• Resistance check on the disconnected load and the equipment ground

Tip: If your hot to neutral reads 120V but hot to ground reads 108V, you've got a neutral problem, not a load problem. Walk the neutral path back to the panel before you touch anything else.

NEC 250 covers grounding and bonding, and most of the weird voltage readings you'll chase as a journeyman trace back to a bond that was never made or was loosened during a remodel. Verify your grounding electrode conductor and your main bonding jumper before you blame the utility.