Crash course: Voltage, amperage, and resistance basics quick reference (part 2)

Ohm's Law in the Field

Voltage pushes, amperage flows, resistance opposes. E equals I times R. Memorize it, then memorize the rearrangements: I equals E over R, R equals E over I. Every troubleshooting call comes back to one of these three values being wrong.

For AC work, swap resistance (R) for impedance (Z) when reactance is in play (motors, transformers, long runs). The math still holds, but Z accounts for inductive and capacitive loads. On a pure resistive load (heaters, incandescent), R and Z are functionally identical.

Power ties it together: P equals E times I. A 1500W heater on 120V draws 12.5A. Same heater on 240V draws 6.25A. Double the voltage, halve the current, same wattage, smaller conductors, less voltage drop. That is why we run 240V for big loads.

Voltage: What You Actually See on the Meter

Nominal vs actual matters. NEC 220.5(A) standardizes calculations at 120, 208, 240, 277, 480V. Your meter will read anywhere from 114 to 126V on a 120V circuit and still be in spec. ANSI C84.1 allows plus or minus 5% at the service. Below 110V on a 120V receptacle, start looking for loose connections or undersized conductors.

Voltage drop is not an NEC requirement, it is a recommendation in 210.19(A) Informational Note 4 and 215.2(A)(1) Informational Note 2: 3% on branch circuits, 5% combined feeder and branch. Exceed it and motors run hot, electronics brown out, LEDs flicker.

Quick field check: if a 120V circuit reads 122V no-load and 108V under load, you have a 14V drop. That is over 11%. Find the loose lug or upsize the conductor before the homeowner calls back.

Amperage: Sizing and Protection

Conductor ampacity comes from NEC Table 310.16 (the table formerly known as 310.15(B)(16) before the 2020 cycle). 14 AWG copper at 60C is good for 15A, 12 AWG for 20A, 10 AWG for 30A. Those are the small conductor rules in 240.4(D), and they override the 75C and 90C columns for residential branch circuits.

Continuous loads (operating 3 hours or more) require the OCPD and conductor to be sized at 125% of the load per 210.19(A)(1)(a) and 210.20(A). A 16A continuous load needs a 20A breaker minimum and conductor rated for 20A continuous service.

• 15A circuit: 14 AWG Cu, general lighting and receptacles
• 20A circuit: 12 AWG Cu, kitchen, laundry, bath, garage receptacles per 210.11(C)
• 30A circuit: 10 AWG Cu, dryers, water heaters, small HVAC
• 40A circuit: 8 AWG Cu, ranges, large HVAC
• 50A circuit: 6 AWG Cu (or 8 AWG at 75C with proper terminations), ranges, EV chargers

Resistance: Testing What You Cannot See

Resistance tells you whether a circuit is intact, shorted, or grounded. Three readings matter on every install: continuity (near zero ohms on a connected conductor), insulation resistance (megohms between conductors and to ground), and ground path impedance (low enough to trip the OCPD).

NEC 250.4(A)(5) requires the equipment grounding path to be capable of carrying fault current and have low enough impedance to facilitate operation of the OCPD. There is no specific ohm value mandated for branch circuits, but a clamp meter ground loop test under 1 ohm on a typical 20A circuit is a solid baseline. Anything over 5 ohms, investigate.

For grounding electrodes, 250.53(A)(2) requires a single rod, pipe, or plate electrode to be augmented with a second electrode if it does not achieve 25 ohms or less to earth. Two rods spaced 6 feet apart satisfy the rule without testing.

Putting It Together: The Service Call Workflow

Dead circuit, no obvious cause. Start at the panel: voltage line to neutral, line to ground, neutral to ground. Line to neutral should match line to ground within a volt or two. A high neutral to ground reading (over 2V) means a loaded or compromised neutral.

1. Confirm voltage at the breaker, both poles if 240V
2. Voltage at the first device, no load
3. Apply load, measure voltage drop across the run
4. If drop exceeds 5%, ohm out the conductors with the breaker off
5. Check terminations at every box back to the panel

The neutral is where 80% of intermittent residential problems live. Loose neutral lugs at the panel cause flickering, dead half-circuits, and fried electronics. Torque to manufacturer spec per NEC 110.14(D), every time.

Common Mistakes That Cost Callbacks

Mixing aluminum and copper without listed connectors violates 110.14(A) and causes high-resistance joints that arc and burn. Use AL/CU rated wire nuts (purple) or listed splice devices. Antioxidant compound on aluminum lugs, every time.

Undersizing the neutral on a multiwire branch circuit. The neutral carries the unbalanced current, but on nonlinear loads (LED drivers, electronics, VFDs) third harmonic currents add on the neutral instead of canceling. NEC 310.15(E) addresses this. On heavy electronic loads, treat the neutral as a current-carrying conductor and derate accordingly.

Ignoring temperature correction. Conductors in an attic in summer, in conduit on a south wall, or bundled with more than 3 current-carrying conductors get derated per 310.15(B) and 310.15(C)(1). A 12 AWG THHN at 90C is rated 30A in free air, but in a hot attic with 6 conductors in conduit, it might be down to 18A. Run the math, do not eyeball it.