Crash course: Voltage, amperage, and resistance basics for inspectors (part 3)

Why this matters on a panel call

Inspectors and electricians who can't translate a meter reading into a code citation get stuck. Voltage, amperage, and resistance are the three numbers that drive every calculation in the NEC, from conductor sizing in 310.16 to GFCI requirements in 210.8(A). Part 3 of this crash course skips the textbook and goes straight to what shows up on the job.

If you came from parts 1 and 2, you already know Ohm's Law (V = I x R) and the power formula (P = V x I). This post connects those numbers to inspection decisions: why a 3% voltage drop matters, when a low resistance reading is a fault, and how amperage drives every breaker call you make.

Voltage: what the meter is actually telling you

Voltage is potential difference. On a 120/240V single-phase service, you should read 120V hot to neutral, 240V hot to hot, and near zero neutral to ground. NEC 220.5(A) sets the nominal voltages used for load calculations at 120, 208, 240, 277, 480, and so on. Anything more than 5% off nominal at the service is a problem, and 250.4(A)(5) requires that the bonding path keep neutral and ground within millivolts of each other.

Voltage drop isn't a code violation by itself, but Informational Note No. 4 to 210.19(A) and Note No. 2 to 215.2(A)(1) recommend keeping branch circuits at 3% or less and the combined feeder plus branch at 5% or less. On a long run feeding a well pump or a detached garage, that note becomes the difference between a motor that starts and a motor that hums and trips.

Field tip: if you read 122V hot to neutral but 6V neutral to ground, stop. You have a loose neutral or a shared grounded conductor, not a healthy service.

Amperage: the number that sizes everything

Current is what heats conductors and trips breakers. Every ampacity table in 310.16 assumes a specific insulation type, ambient temperature, and number of current-carrying conductors. Miss one of those adjustments and your 12 AWG THHN that looked good for 20A is suddenly only good for 16A after derating.

The 80% continuous load rule in 210.19(A)(1) and 215.2(A)(1) trips up more electricians than any other amperage rule. A continuous load is anything expected to run for three hours or more (Article 100). For those, the conductor and OCPD must be sized at 125% of the continuous load plus 100% of the non-continuous.

• 20A circuit, continuous load: max usable is 16A.
• 30A circuit, continuous load: max usable is 24A.
• 50A circuit, continuous load: max usable is 40A.
• 100A feeder, continuous load: max usable is 80A.

Resistance: the cheapest fault-finding tool you own

Resistance readings catch problems before they become fires. A bonding jumper should read under 0.1 ohm. An equipment grounding conductor on a 100 foot run of 12 AWG copper should land near 0.2 ohm. If you see 2 or 20 ohms, you have a loose lug, a corroded splice, or a missing bond.

Insulation resistance is the other side. NEC 110.7 requires that completed wiring be free from short circuits and from grounds other than as required by Article 250. A megohmmeter test at 500V or 1000V on a de-energized circuit should read in the megohms. Anything in the kilohm range is degraded insulation, and you note it before the panel goes hot.

Field tip: always zero your leads before a low-resistance test. Two feet of test lead can read 0.3 ohm on a cheap meter, and you'll chase a ghost fault for an hour.

Putting the three together on an inspection

The three numbers are linked. A loose connection raises resistance, which drops voltage at the load, which forces current up if the load tries to maintain power. That's how a marginal splice in a junction box turns into a melted wirenut six months later. Your job on an inspection is to spot the chain before it completes.

Walk a service this way: voltage at the main, voltage at the furthest receptacle under load, resistance from the ground bar to a known earth reference, and amperage on each ungrounded conductor under normal load. Compare against nameplate and against 220 calculations. If any of the four disagree, the install isn't done.

1. Verify nominal voltage per 220.5(A).
2. Confirm OCPD sizing against 240.4 and the 80% rule where it applies.
3. Check EGC continuity per 250.118 and 250.122.
4. Spot-check voltage drop on the longest branch circuit.
5. Measure neutral to ground at multiple points, looking for parallel paths per 250.6.

Common red flags and the article that covers them

Most field violations come back to a handful of articles. Knowing which one to cite turns a vague "this looks wrong" into a defensible correction notice. Keep these on a pocket card until they're muscle memory.

• Hot neutral, voltage between neutral and ground above 2V under load: 250.6, 250.142.
• Undersized EGC on a feeder: 250.122 Table.
• Continuous load on a non-derated breaker: 210.19(A)(1), 215.2(A)(1).
• Voltage drop over 5% combined: Informational Note to 210.19(A).
• Missing GFCI on a required receptacle: 210.8(A) and (B).
• Bonding jumper missing or reading high resistance: 250.92, 250.102.

Voltage tells you the system is alive. Amperage tells you it's loaded. Resistance tells you the path is clean. Read all three, cite the article, and move on to the next call.