Crash course: Ohm's Law for electricians no jargon edition (part 5)

Ohm's Law in one line

V = I × R. Volts equal amps times ohms. That's the whole thing. Every calculation you do on the job, voltage drop, breaker sizing, heater loads, motor troubleshooting, runs off this single equation or one of its siblings.

The two siblings worth memorizing: I = V / R (solve for current) and R = V / I (solve for resistance). Power rides along with P = V × I. Four variables, three equations, and you can back into anything a meter won't tell you directly.

Stop thinking in formulas and start thinking in behavior. Push harder (more volts), more current flows. Squeeze the pipe tighter (more ohms), less current flows. That's it.

What each variable actually means on a jobsite

Voltage is pressure. The utility pushes 120V or 240V into the panel whether anything is connected or not. Current is flow, measured in amps, and only exists when a load closes the circuit. Resistance is restriction, measured in ohms, baked into every conductor, connection, and device on the line.

Power, in watts, is what you're actually paying for and what the nameplate cares about. A 1500W heater on a 120V circuit pulls 12.5 amps (1500 / 120). That's why it trips a 15A breaker the second the toaster kicks on.

• Volts (V): electrical pressure, measured across two points
• Amps (I): flow through a conductor, measured in series
• Ohms (R): restriction, measured with power off
• Watts (P): work being done, volts times amps

Sizing loads without a calculator

Most residential and light commercial load math is just P / V = I. Nameplate says 1800W, circuit is 120V, you're pulling 15 amps. On a 20A circuit that's 75% loaded, right at the continuous-load ceiling in NEC 210.19(A) and 210.20(A), which cap continuous loads at 80% of breaker rating.

For 240V single-phase, divide by 240. A 4500W water heater element draws 18.75A, which is why it lives on a 30A breaker with 10 AWG copper per NEC Table 310.16. For three-phase, the formula adds a √3: I = P / (V × 1.732 × PF). Memorize 1.732 and you're done.

Field tip: if the nameplate only gives you watts and volts, divide before you pull wire. Half the undersized feeder callbacks I see come from guessing off wattage instead of calculating amps.

Voltage drop, the calculation nobody runs until they should have

NEC 210.19(A) Informational Note 4 recommends branch circuits not exceed 3% voltage drop, and feeders plus branches not exceed 5%. It's not enforceable, but inspectors, engineers, and tenants with flickering lights all care.

The single-phase formula: VD = (2 × K × I × D) / CM. K is 12.9 for copper, 21.2 for aluminum. D is one-way distance in feet. CM is circular mils from NEC Chapter 9, Table 8. Run it before you pull a 200-foot branch on 12 AWG, because at 16A over that distance you're already past 3%.

1. Get the one-way run length
2. Get the expected current (not the breaker size, the actual load)
3. Plug into VD = (2 × K × I × D) / CM
4. If VD / V > 0.03, upsize the conductor

Troubleshooting with Ohm's Law at the meter

A circuit that should pull 10A but pulls 2A has a resistance problem. Loose lug, corroded splice, failing contactor coil, all of it shows up as added ohms. Kill power, meter resistance across the suspect section, compare to what it should be (usually near zero for conductors and connections).

A motor drawing locked-rotor current on a good supply points at a mechanical bind or shorted winding, not the feeder. Megger the windings per manufacturer spec and check insulation resistance against NEC 110.7 and NETA ATS values. If the windings read infinite ohms to ground and balanced phase-to-phase, the problem's mechanical.

Field tip: before you condemn a breaker, measure voltage at the load under load. If voltage sags more than a few percent when the motor starts, your problem is upstream impedance, not the device.

The numbers to keep in your head

You don't need a reference for the common stuff. Burn these into memory and you'll solve 80% of jobsite math without pulling out a phone.

• 120V × 15A = 1800W (standard branch circuit max)
• 120V × 20A = 2400W (small appliance circuit per NEC 210.11(C)(1))
• 240V × 30A = 7200W (dryer, water heater territory)
• 240V × 50A = 12,000W (range, EV charger, welder)
• Three-phase multiplier: 1.732
• Copper K factor: 12.9. Aluminum K factor: 21.2
• Continuous load ceiling: 80% of breaker rating

Ohm's Law isn't academic. It's the fastest diagnostic tool you own, and it works the same on a 15A lighting circuit as it does on a 4000A switchgear lineup. Get fluent with the four variables and the rest of the code starts making more sense.