Crash course: Ohm's Law for electricians 2023 NEC update (part 3)

Ohm's Law in the Field

V = I × R. Three variables, one relationship, and the entire trade leans on it. Voltage in volts, current in amps, resistance in ohms. Rearrange as needed: I = V/R when you need current, R = V/I when you need resistance. Memorize the triangle or skip it, just know the math cold before you open a panel.

Power rides alongside: P = V × I, or P = I²R, or P = V²/R. Watts tell you heat and load. Heat is what trips breakers, cooks insulation, and ruins splices. Every calculation you run on a job connects back to one of these four equations.

The 2023 NEC did not rewrite physics, but it tightened several load calculation and GFCI rules that lean directly on these numbers. If your math is sloppy, your compliance is sloppy.

Voltage Drop and Conductor Sizing

NEC 210.19(A) Informational Note 4 and 215.2(A)(1) Informational Note 2 still recommend a maximum 3% drop on branch circuits, 5% combined feeder and branch. Informational, not mandatory, but most AHJs treat it as gospel and most specs require it. Calculate it or eat the callback.

Single-phase voltage drop: VD = (2 × K × I × L) / CM. K is 12.9 for copper, 21.2 for aluminum at 75°C. L is one-way length in feet. CM is circular mils from Chapter 9, Table 8. Three-phase swaps the 2 for 1.732.

Long run to a detached garage? Run the drop calc before you pull wire. Upsizing from #12 to #10 on a 100 foot 20A circuit costs less than pulling it twice.

Load Calculations and the 2023 Update

Article 220 is where Ohm's Law meets the permit desk. The 2023 cycle restructured Article 220 for clarity and added 220.87 for determining existing loads using maximum demand data. If you have 30 days of recorded data from the service, you can use it, multiplied by 125%, as the calculated load.

For dwelling units, 220.82 optional method still applies: 100% of the first 10 kVA, 40% of the remainder, plus the largest of HVAC loads per 220.82(C). Watts divided by voltage gives you amps, and amps drive your service size.

• General lighting: 3 VA per square foot (220.41)
• Small appliance branch circuits: 1500 VA each, minimum two (220.52(A))
• Laundry branch: 1500 VA (220.52(B))
• Fixed appliances over 1/4 HP or 500W: nameplate rating
• EV charging: per 625.42, now explicitly included in service calcs under 2023 changes

GFCI, AFCI, and Resistance to Ground

NEC 210.8(A) and 210.8(B) expanded GFCI coverage in 2023, including single-phase receptacles 150V to ground, 50A or less, and three-phase receptacles 150V to ground, 100A or less, in the locations listed. The device works by detecting a current imbalance as small as 5 mA between hot and neutral. That 5 mA is Ohm's Law in action: a human body at roughly 1000 ohms across 120V passes 120 mA, which is lethal.

Ground fault paths depend on low-impedance bonding per 250.4(A)(5). If fault current cannot return to source fast enough to trip the overcurrent device, the fault burns instead of clearing. Run the math: a 20A breaker needs roughly 100A to trip instantly on its magnetic curve. At 120V, that means the total fault loop impedance must be under 1.2 ohms.

Conductor Ampacity and Temperature

Resistance rises with temperature. Table 310.16 ampacities assume a 30°C ambient. Hotter ambient, higher resistance, more I²R heat, lower allowable current. Correction factors in 310.15(B) are not optional on attic pulls, rooftop conduit, or boiler rooms.

The 2023 NEC clarified 310.15(C) adjustment for conduit exposed to sunlight on rooftops, reverting some of the more conservative 2017 rooftop adders. Still check the table. A run of #12 THHN in a rooftop EMT at noon in July is not a 20A circuit anymore.

Before you derate, count the current-carrying conductors in the raceway. Neutrals on a 3-phase wye carrying nonlinear loads count. That triplen harmonic current is real amps, real heat.

• 4 to 6 CCCs: 80% ampacity
• 7 to 9 CCCs: 70%
• 10 to 20 CCCs: 50%
• Ambient above 30°C: apply 310.15(B)(1) multiplier

Quick Field Checks

Keep a mental checklist when something does not add up. Low voltage at the load with correct voltage at the panel means resistance in the conductor, a connection, or the device. Measure voltage drop under load, not open circuit. An open neutral will read 120V everywhere until you pull a trigger.

High amp draw on a known load points to low voltage, shorted windings, or a mechanical bind on a motor. A motor pulling locked rotor amps continuously is a fire waiting for an ignition source. Clamp it, read it, compare to nameplate FLA, and act on the number.

1. Verify voltage at source and load
2. Measure current under actual operating conditions
3. Calculate expected resistance, compare to measured
4. Check terminations before blaming the conductor
5. Document readings for the next tech and for the AHJ

Ohm's Law does not care about your schedule. Run the numbers, size the wire, torque the lugs, and the 2023 NEC takes care of itself.