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

Why part 3 hammers the basics

You measure these three every day. Voltage pushes, amperage flows, resistance fights back. Get the relationships wrong on a service call and you either trip breakers, cook conductors, or chase a ghost for two hours.

This installment skips theory and stays in the field. Numbers you can plug in, code articles you can cite, and the failure modes that actually show up on a Tuesday afternoon.

Voltage: the pressure behind the work

Voltage is potential difference. 120V to neutral, 240V across the ungrounded legs of a single phase residential service, 208V phase to phase on a wye, 277V to ground on a 480/277V system. NEC 220.5(A) tells you to use nominal voltages (120, 120/240, 208Y/120, 240, 480Y/277) for branch circuit and feeder calculations unless other voltages are specified.

Voltage drop is where rookies get bit. NEC 210.19(A) Informational Note 4 recommends branch circuit voltage drop no greater than 3 percent, with combined feeder and branch not exceeding 5 percent. On a 120V circuit, that 3 percent is only 3.6V. A long run on undersized wire eats that fast.

• 120V circuit, 3 percent max drop = 3.6V
• 240V circuit, 3 percent max drop = 7.2V
• 277V circuit, 3 percent max drop = 8.31V
• 480V circuit, 3 percent max drop = 14.4V

Amperage: what actually heats the wire

Current is what sizes your conductors and your overcurrent protection. NEC 310.16 gives you ampacity tables for insulated conductors in raceway or cable. Pick the right temperature column (60, 75, or 90 degrees C) based on terminations per 110.14(C). Most equipment 100A and below terminates at 60 or 75 degrees C, so even with THHN you size off the 75 degree column.

Continuous loads catch people too. NEC 210.19(A)(1)(a) and 215.2(A)(1)(a) require branch circuits and feeders supplying continuous loads to be sized at 125 percent of the continuous load plus 100 percent of the noncontinuous load. A 16A continuous load needs a 20A circuit, not a 15A.

Field tip: when a 20A breaker on a baseboard heater nuisance trips after 45 minutes, check the load math before you swap the breaker. Three hours or more of operation makes it continuous, and that 16A heater on a 20A circuit is exactly at the line.

Resistance: the part everyone underestimates

Resistance lives in connections, terminations, and conductor length. Copper at 75 degrees C is roughly 0.778 ohms per 1000 feet for #12 AWG, 0.491 for #10, 0.308 for #8 (NEC Chapter 9, Table 8). Double the run length when calculating drop, you go out and come back.

Loose terminations are resistance you can hear and smell before you see. A backstabbed receptacle pulling 15A at a half ohm of contact resistance is dissipating over 100W at the connection. That is a fire waiting on time.

• Torque every lug to manufacturer spec, NEC 110.14(D) makes it mandatory
• Anti-oxidant on aluminum to copper transitions
• Listed connectors only, no twisting AL and CU under a wire nut
• Re-torque after the first heat cycle on large feeders

Ohm's law on the truck

V = I times R. Memorize it once, use it forever. If you know any two, you get the third. The PIE wheel adds power: P = V times I, P = I squared times R, P = V squared divided by R.

Worked example: a 240V water heater element measures 12.8 ohms cold. Current draw is 240 divided by 12.8 = 18.75A. Power is 240 times 18.75 = 4500W. That matches the nameplate, element is good. If it read 5 ohms, you are pulling 48A on a 30A circuit, element is shorted internally, replace it.

Voltage drop the quick way for single phase, 2 wire copper:

1. VD = (2 times K times I times L) divided by CM
2. K = 12.9 for copper, 21.2 for aluminum
3. I = load in amps, L = one way length in feet
4. CM = circular mils from Chapter 9 Table 8

Putting it together on a real call

Customer says the lights dim when the AC kicks on. That is voltage sag from inrush current crossing the resistance of the service conductors and any loose connections. Pull the meter, check voltage at the panel under load versus no load. More than a few volts of swing on a 200A residential service points to a loose lug at the meter base, the main breaker, or the utility neutral.

Field tip: a thermal camera on the service equipment under load finds 80 percent of these in under five minutes. Hot lug, hot breaker, hot splice. Cold copper means clean copper.

The three quantities are one system. Voltage drives, current flows, resistance opposes, and power is what gets dissipated. When the math does not match the meter, something is wrong with the installation, not the law.