Crash course: Ohm's Law for electricians with photos (part 1)

Ohm's Law, the one formula you actually use

Ohm's Law is V = I × R. Voltage equals current times resistance. That is it. Every troubleshooting call, every voltage drop calc, every "why is this breaker tripping" question comes back to this.

Rearranged, you get I = V / R and R = V / I. Memorize all three forms. You will use them on service calls, on rough-ins, and when a homeowner asks why their space heater keeps popping the 15 amp kitchen circuit.

Units matter. Volts, amps, ohms. Mix in milliamps or kilo-ohms without converting and your math will lie to you.

What each variable means on a real jobsite

Voltage is pressure. It is what pushes electrons through the conductor. A 120V branch circuit has more pressure than a 24V thermostat wire, which is why one shocks you and the other barely tickles.

Current is flow, measured in amps. This is what does the work and what heats the conductor. NEC 310.16 ampacity tables exist because current, not voltage, is what melts insulation and starts fires.

Resistance is the pushback, measured in ohms. Every conductor, splice, termination, and load has it. A loose wire nut adds resistance. A corroded lug adds resistance. That is where the heat shows up on your thermal camera.

• Voltage (V): pressure, measured across two points
• Current (I): flow, measured in-line with a clamp meter
• Resistance (R): opposition, measured with power off

The power triangle, Ohm's Law's cousin

P = V × I. Watts equal volts times amps. You need this one just as often, especially for load calcs under NEC 220 and when sizing equipment per NEC 110.3(B) nameplate data.

Combine with Ohm's Law and you get P = I² × R, which is why a bad connection gets hot fast. Double the current through a resistive joint and you quadruple the heat. That is not an opinion, that is physics, and it is why NEC 110.14 is so specific about torque.

Field tip: if a terminal is warm to the touch under normal load, it is already failing. Kill the power, pull it apart, and retorque to the label spec. Do not wait for the smoke.

Working the formula on a service call

Customer says their 1500W space heater trips the 15A breaker. Quick check: I = P / V, so 1500 / 120 = 12.5 amps. That is fine on a dedicated 15A circuit, but not when the microwave and toaster share it. NEC 210.23(A) and the 80 percent continuous load rule (NEC 210.19) tell you the same story.

Another one: you read 108V at a receptacle under load when the panel shows 122V. That 14V drop across the branch circuit means resistance somewhere. Using I = V / R backward, if the load is pulling 10A and you lost 14V, you have 1.4 ohms of unwanted resistance in the run. That is a back-stabbed receptacle, a loose neutral, or a marginal splice.

NEC 210.19(A) Informational Note No. 4 recommends keeping branch circuit voltage drop under 3 percent. On a 120V circuit, that is 3.6V max. Anything worse and you are wasting energy and cooking terminations.

Series vs parallel, keep them straight

Series circuits: current is the same everywhere, voltage divides across loads, resistances add. Think of old Christmas lights where one dead bulb kills the string.

Parallel circuits: voltage is the same across each branch, current divides, resistances combine as 1/Rt = 1/R1 + 1/R2... This is how every branch circuit in a house works. Each receptacle sees the full 120V, and the total current is the sum of what each load pulls.

1. Series: same current, voltage splits, R adds up
2. Parallel: same voltage, current splits, R combines reciprocally
3. Mix of both: solve the parallel section first, then treat it as a single resistor in the series math

Voltage drop, the calc you owe every long run

For single phase, VD = 2 × K × I × L / CM. K is 12.9 for copper, 21.2 for aluminum. L is one-way length in feet. CM is circular mils from NEC Chapter 9, Table 8. Memorize copper K or keep it on your phone.

Short version for 120V copper runs: if the one-way distance in feet times amps divided by 100 is more than about 3, you need to upsize. Not exact, but close enough to flag a run before you pull wire you are going to regret.

Field tip: voltage drop does not trip breakers, but it ruins motors, dims LEDs, and makes resistive heat output fall off a cliff. Size for the load you will have in five years, not the one on the plans today.

Part 2 will cover AC specifics: power factor, true vs apparent power, and why your clamp meter reads differently on a VFD-fed motor than on a straight-wired one.