Crash course: Voltage, amperage, and resistance basics for inspectors (part 2)

Why Part 2 Matters

Part 1 covered the definitions. This installment puts voltage, amperage, and resistance to work on the inspection side. You are no longer asking what these values are. You are asking whether the installation in front of you respects them.

Inspectors who treat Ohm's Law as a field tool catch problems that pass a visual walkthrough. Loose lugs, undersized conductors, and shared neutrals all show up as numbers before they show up as smoke.

Voltage Drop in the Real World

NEC 210.19(A) Informational Note 4 and 215.2(A)(1) Informational Note 2 recommend a maximum 3% drop on branch circuits and 5% combined on feeders and branches. These are not enforceable mandates in most jurisdictions, but they are the bar a competent install meets.

Run the math before you reach for a meter. For a single-phase circuit, voltage drop equals 2 times length times current times conductor resistance, divided by 1000. A 100 amp feeder pulled 150 feet on #2 copper drops roughly 4.6 volts on a 240 volt system. That is under 2%. Push the same load on #4 and you are over the recommended limit.

• Branch circuits: target 3% or less
• Feeder plus branch: target 5% or less
• Long runs over 100 feet: upsize one trade size as a default
• Motor loads: factor in starting current, not just FLA

Amperage and Conductor Sizing

Ampacity tables in NEC 310.16 are the starting point, not the finish line. Termination temperature ratings under 110.14(C) almost always pull you back to the 60 or 75 degree column regardless of what the conductor insulation is rated for. THHN at 90 degrees does not mean you get to use the 90 degree column for sizing.

Continuous loads under 210.19(A)(1) and 215.2(A)(1) require conductors rated at 125% of the continuous load plus 100% of the noncontinuous load. Lighting, signs, and most commercial HVAC fall under this rule. Miss it and the conductor runs hot, the breaker nuisance trips, and the install fails.

Field tip: when you see a 100 amp breaker on #3 copper feeding a continuous load, walk it back. The conductor needs to handle 125 amps of ampacity at the terminal rating, which #3 copper at 75 degrees does not provide.

Resistance, Bonding, and Ground Faults

Resistance is the value most inspectors skip and the one that bites hardest. A bonding jumper that reads 2 ohms instead of 0.1 ohms will not clear a fault fast enough to trip the OCPD. The breaker needs to see a low impedance path back to the source, and resistance is what determines whether that path works.

NEC 250.4(A)(5) requires the effective ground fault current path to be permanent, electrically continuous, and capable of safely carrying fault current. This is not a paperwork requirement. Test it. A clamp meter on the EGC during a load test, or a dedicated low resistance ohmmeter on the bonding path, tells you what the eyeball cannot.

• Main bonding jumper: verify per 250.28
• Equipment bonding jumpers: 250.102, sized per Table 250.122
• Grounding electrode conductor: 250.66, sized per service conductor
• Ground rod resistance target: 25 ohms or less, or install a second per 250.53(A)(2)

Using Ohm's Law on the Job

V equals I times R. Memorize the two rearrangements: I equals V divided by R, and R equals V divided by I. On a service call or an inspection, you will use these to validate readings against expected values rather than trusting the number on the meter.

If a 20 amp circuit is reading 14 amps under load and the homeowner says the receptacle feels warm, calculate the implied resistance at the connection. A 2 volt drop across a backstabbed receptacle at 14 amps is roughly 0.14 ohms of added resistance. That is enough heat to char a device over time. The math caught it before the infrared camera did.

Field tip: a clamp meter that only reads current is half a tool. Carry one that does true RMS voltage and resistance, and use all three modes during rough and final.

What to Flag on Inspection

Voltage, amperage, and resistance issues rarely announce themselves. They hide behind labels, behind panel covers, and inside terminations that looked fine at rough. Use the values, not the appearance, to make the call.

1. Conductor sized to insulation rating instead of termination rating, violating 110.14(C)
2. Continuous load circuits sized at 100% instead of 125%, violating 210.19(A)(1)
3. Long branch runs with no upsize for voltage drop, missing the 210.19 informational note
4. Bonding jumpers undersized or omitted, violating 250.102 and Table 250.122
5. Parallel conductors not identical in length, material, and termination, violating 310.10(G)

The inspector who carries Ohm's Law in their head walks panels faster and writes corrections that hold up. Part 3 will move from these basics into single phase versus three phase calculations and where the math changes on you.