Crash course: Voltage, amperage, and resistance basics with calculations (part 3)

Ohm's Law: The One Equation You Actually Use

Voltage (E), current (I), and resistance (R) are locked together by Ohm's Law: E = I × R. Rearrange it three ways and you can solve almost every field problem that doesn't involve a calculator hunt. Memorize the wheel or just remember the triangle.

Voltage pushes. Current flows. Resistance fights back. Watts (P) is the work being done: P = E × I. If you know any two values, you can solve for the rest. That's it. Everything else is just unit conversion and being honest about what's actually on the conductor.

• E = I × R (volts)
• I = E ÷ R (amps)
• R = E ÷ I (ohms)
• P = E × I (watts)

Voltage Drop: When 120 Isn't 120 Anymore

NEC 210.19(A) Informational Note No. 4 recommends branch circuits not exceed 3% voltage drop, with a combined feeder and branch total under 5%. It's not enforceable, but it's the number inspectors and engineers check. Long runs to a detached garage, well pump, or sign circuit are where you'll feel it.

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 Chapter 9, Table 8. For three-phase, swap the 2 for 1.732.

If a customer complains lights dim when the well kicks on, measure voltage at the panel and at the load while the motor starts. More than a 5% sag and you're undersized, period.

Series vs Parallel: Know Which Circuit You're Looking At

Series circuits share one path. Current is the same everywhere, voltage divides across each resistance, and total resistance adds: R total = R1 + R2 + R3. Pull one device and the whole string drops. Old Christmas lights, control circuits, and safety interlocks live here.

Parallel circuits share voltage. Each branch sees the same E, current divides based on each branch's resistance, and total resistance is always less than the smallest branch: 1/R total = 1/R1 + 1/R2 + 1/R3. Every receptacle on a 120V circuit is parallel. That's why pulling one plug doesn't kill the rest.

1. Series: same current, voltage divides, resistances add.
2. Parallel: same voltage, current divides, resistances combine reciprocally.
3. Most real loads are parallel. Most controls are series.

Sizing a Conductor From the Load

Start with the load in watts or VA, divide by voltage, get amps. A 1500W bathroom heater on 120V pulls 12.5A. NEC 210.19(A)(1) and 210.20(A) require the conductor and OCPD to be sized at 125% of continuous load. Heater runs more than 3 hours? 12.5 × 1.25 = 15.6A minimum, so a 20A circuit on 12 AWG.

For motors, use NEC 430.6(A) and pull FLC from Table 430.250, not the nameplate. A 5HP, 240V single-phase motor is 28A from the table. Branch conductor at 125% per 430.22 means 35A minimum, so 8 AWG copper at 75°C from Table 310.16. The OCPD comes from 430.52, which is a different multiplier entirely.

Three-Phase Power Without the Headache

Three-phase power: P = E × I × 1.732 × PF. For a balanced resistive load, power factor is 1. For motors, assume 0.85 unless you have the nameplate. The 1.732 is the square root of 3, and it shows up in every three-phase calc you'll do.

To find current from a three-phase load: I = P ÷ (E × 1.732 × PF). A 25kW heater bank at 480V three-phase: 25,000 ÷ (480 × 1.732 × 1) = 30A. Add the 125% continuous factor and you're at 37.5A. That's a 40A breaker on 8 AWG copper, assuming standard terminations and ambient.

On three-phase, always check phase-to-phase and phase-to-neutral. A burned B-phase fuse on a 208/120 panel will give you weird voltage readings that look like a control problem but isn't.

Quick Field Checks That Save Callbacks

Before you button up, verify under load. A receptacle that reads 120V open-circuit can sag to 105V with a vacuum running if the splice is bad or the run is too long. Voltage drop hides until current shows up. Use a plug-in load tester or a known appliance and read at the device, not the panel.

For resistance checks on de-energized circuits, a megger at 500V or 1000V tells you insulation health. Anything under 1 megohm is suspect. For continuity on a ground path, NEC 250.4(A)(5) requires an effective ground-fault path of low impedance. Bond and test, don't assume.

• Verify voltage under load, not just open-circuit.
• Check both legs on 240V and all three phases on 208/480.
• Megger long runs and motor windings before energizing.
• Confirm equipment grounding conductor continuity to the panel.