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

The three quantities you cannot ignore

Voltage, amperage, and resistance are the foundation of every troubleshooting call, every load calc, and every conductor sizing decision. Get these wrong and you either trip breakers, cook insulation, or misdiagnose a fault. Get them right and most of the NEC starts to read like common sense.

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

Memorize the three rearrangements and you can solve almost any field problem with a calculator and a clamp meter:

• E = I x R (find voltage)
• I = E / R (find current)
• R = E / I (find resistance)

Voltage: pressure, not flow

Voltage is potential difference between two points. No difference, no push, no current. That is why a bird on a single energized line does not get cooked. There is no second reference for the voltage to act across.

Standard nominal system voltages you will see daily: 120/240V single phase residential, 120/208V wye in light commercial, 277/480V wye in commercial and industrial. NEC 220.5(A) tells you to use these nominal values when running branch circuit, feeder, and service calculations unless the AHJ specifies otherwise.

Voltage drop is the field consequence you actually feel. NEC 210.19(A) Informational Note 4 and 215.2(A)(1) Informational Note 2 recommend keeping branch circuit drop at 3% or less, with combined feeder and branch drop at 5% or less. These are recommendations, not mandates, but ignore them and you will get nuisance calls about dim lights and motors that grunt on startup.

Amperage: what actually heats the conductor

Current is what does the work and what kills the conductor when sized wrong. Ampacity tables in NEC 310.16 give you the allowable current for a given conductor size, insulation type, and ambient condition. Always start there before pulling wire.

A common field calc: a 1500W resistive load on 120V draws I = 1500 / 120 = 12.5 amps. Add the 125% continuous load factor required by NEC 210.19(A)(1)(a) for loads running 3 hours or more, and you are sizing the branch circuit for 15.6 amps. That puts you on a 20A breaker with 12 AWG copper, not 14 AWG.

Tip: when in doubt on a load that runs all day, treat it as continuous. Sizing up one trade size of conductor costs pennies. A burned-up panel feeder costs a weekend.

Resistance: the silent variable

Resistance is the part most guys never measure until something is already wrong. Conductors have it, connections have it, loads are built around it. Cold resistance and hot resistance are different, which is why a motor that megs fine cold can still fail under load.

You will use resistance most often in three places: continuity checks, ground path verification under NEC 250.4(A)(5), and insulation resistance testing. A bonded equipment ground should read near zero ohms back to the service. An insulation resistance test with a megohmmeter on a 600V circuit should typically read in the megohms, not kilohms.

Quick rules of thumb on conductor resistance:

1. Resistance goes up as conductor length increases.
2. Resistance goes down as conductor cross section increases (bigger wire, less resistance).
3. Resistance goes up as temperature rises in copper and aluminum conductors.
4. Loose terminations add resistance, which creates heat, which creates more resistance. That is the failure spiral behind most burned lugs.

Putting Ohm's Law to work in the field

Scenario one: a 240V baseboard heater pulls 10 amps. The element resistance is R = 240 / 10 = 24 ohms. If you meter the element cold and read 240 ohms, the element is open or carbonized. Replace it.

Scenario two: you read 118V at the panel and 109V at a receptacle under load. Drop is 9V on a 120V nominal circuit, or 7.5%. That is well past the 3% recommendation in 210.19(A) IN 4. Check terminations first, then look at conductor size against the run length.

Scenario three: a 5 HP single phase 240V motor with a nameplate FLA of 28 amps. Branch circuit conductors per NEC 430.22 must carry 125% of FLA, so 28 x 1.25 = 35 amps minimum ampacity. That puts you at 8 AWG copper at 75 deg C from Table 310.16, before any derating.

Test gear and what to actually trust

Your meter is only as good as your last calibration and your last test lead inspection. NEC 110.16 covers arc flash labeling, and NFPA 70E governs how you approach energized work, but neither replaces a CAT III or CAT IV rated meter on the right voltage range.

Tip: test your meter on a known live source before and after every measurement on a dead circuit. Live, dead, live. If you skip the second live test, you do not know if your meter died between readings.

Part 2 will cover power (watts, VA, and VARs), the power triangle, and how power factor changes the current you actually pull on three phase gear. Until then, keep Ohm's Law on a sticker inside your meter case and stop guessing at conductor sizes.