Crash course: Voltage, amperage, and resistance basics for journeymen (part 5)

Part 5: Tying it all together on the job

By now you know the trio: voltage pushes, amperage flows, resistance fights back. Part 5 is where the math meets the panel. This is the stuff you actually use when troubleshooting a dead circuit, sizing a feeder, or arguing with a GC about why his 100-foot extension cord keeps tripping.

If you skipped parts 1 through 4, the short version: V = I x R. Power in watts equals volts times amps. Everything else is bookkeeping.

Voltage drop is not optional

NEC 210.19(A) Informational Note No. 4 recommends branch circuit voltage drop stay under 3%, with combined feeder and branch under 5%. It is not enforceable as written, but inspectors and engineers treat it like gospel on long runs. Ignore it and you get nuisance trips, dim lights, and motors that hum instead of spin.

Quick field formula for single phase: VD = (2 x K x I x 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. Memorize it or tape it inside your meter case.

• 120V circuit, 3% drop = 3.6V max
• 240V circuit, 3% drop = 7.2V max
• 480V circuit, 3% drop = 14.4V max

If a run is over 100 feet, upsize one wire gauge before you pull it. Cheaper than coming back to repull #12 because the kitchen island GFCI keeps dropping out.

Reading a panel like a meter

A clamp meter on the feeder lugs tells you what the load is actually pulling, not what the breakers are rated for. Per NEC 220.87, you can use 12 months of demand data plus 125% to size service for an existing building. No data? You fall back to the standard calc in 220.40.

When you clamp each leg of a 200A service and see 70A on A phase, 65A on B phase, you have plenty of headroom. See 180A on one leg and 40A on the other, and you have a balance problem before you have a capacity problem. Move circuits, do not upsize the service.

Resistance tells you what is failing

Loose connections, corroded lugs, and damaged conductors all show up as added resistance. Resistance turns into heat under load. Heat turns into failure. A 0.5 ohm bad splice carrying 20A dissipates 200 watts at that single point. That is a soldering iron buried in your wall.

Use a low-resistance ohmmeter or a megger appropriately:

1. De-energize and lock out per NEC 110.25 and your site LOTO procedure.
2. Test phase to phase, phase to ground, phase to neutral.
3. For motors and feeders over 600V, megger per the limits in NEC 110.14(C) and the equipment nameplate.
4. Anything under 1 megohm on a 480V motor is suspect. Under 100k ohm, do not energize.

Putting Ohm's Law to work in the field

Three scenarios you will hit this month:

Scenario one. A 240V baseboard heater nameplate says 1500W. Expected current = 1500 / 240 = 6.25A. You clamp it and read 4A. Element is partially open, resistance went up, current went down. Replace the element.

Scenario two. A 20A circuit keeps tripping with a 1500W space heater plugged in. 1500 / 120 = 12.5A. Should be fine. You measure voltage at the receptacle under load and see 108V. Voltage drop on a long run means the heater pulls more current to make rated wattage, breaker sees the excess. Fix the run, not the heater.

Scenario three. Three-phase 480V motor, 10 HP, FLA from NEC Table 430.250 is 14A. You read 22A on one leg, 14A on the other two. Single phasing or a high-resistance connection on that leg. Shut it down before the windings cook.

When numbers do not match the nameplate, the wire, the connection, or the load is lying. Voltage and current never lie together.

The habits that separate apprentices from journeymen

Anyone can read a meter. The difference is knowing what the reading means before you take it. Predict the value, then measure. If the meter agrees, you understand the circuit. If it disagrees, you just found the problem.

Build the habit of mental math on every job:

• Watts divided by volts gives you expected amps.
• Volts divided by amps gives you load resistance.
• Amps squared times resistance gives you heat dissipated, in watts.
• Anything off by more than 10% from nameplate, investigate before you energize.

That is the whole crash course in five parts. Voltage, amperage, resistance, and the NEC articles that keep them in line. Tape the formulas to your meter, keep your code book in the truck, and trust the numbers over the story the customer tells you.