Crash course: Voltage, amperage, and resistance basics for low-voltage techs (part 2)

Picking up where Part 1 left off

Part 1 covered Ohm's law, the water analogy, and what voltage, current, and resistance actually mean on a meter. Part 2 puts those numbers to work in the field: low-voltage circuits, voltage drop, grounding, and the math you run before you pull cable.

Low-voltage means 50V or less for most Class 2 and Class 3 work per NEC Article 725. Doorbells, thermostats, access control, structured cabling, PoE. Different rules, different failure modes, same physics.

Voltage drop is your number one enemy

On a 120V branch circuit, a couple volts lost in the conductor barely registers. On a 24V thermostat run or a 48V PoE camera at the end of 280 feet of Cat6, those same lost volts kill the device. NEC 210.19(A) Informational Note 4 recommends a maximum 3% drop on branch circuits and 5% total. For low-voltage, treat 3% as a hard ceiling, not a guideline.

The formula for single-phase voltage drop: VD = (2 x K x I x L) / CM. K is 12.9 for copper, I is current in amps, L is one-way length in feet, CM is circular mils of the conductor. Memorize it or keep it on your phone.

• 22 AWG: 642 CM
• 18 AWG: 1,624 CM
• 16 AWG: 2,583 CM
• 14 AWG: 4,107 CM
• 12 AWG: 6,530 CM

Working an actual PoE example

You are running a 802.3af camera, 12.95W max draw at the device, 48V nominal. That's roughly 0.27A. Cable is 23 AWG (about 1,288 CM), run is 250 feet one way. VD = (2 x 12.9 x 0.27 x 250) / 1,288 = about 1.35V. That's under 3% of 48V, so you are fine.

Bump that camera to a 802.3at PTZ pulling 25.5W (0.53A) on the same run, voltage drop roughly doubles to 2.7V. Still inside spec, but you have lost margin. Add a heater or a longer run and you start dropping packets and rebooting.

Always size for the device's worst-case draw, not nameplate average. PoE cameras with heaters and IR illuminators pull current in bursts that swamp the steady-state number.

Grounding, bonding, and why your readings lie

If your meter shows 1.8V between neutral and ground at a receptacle under load, that is normal voltage drop on the neutral. If it shows 6V, you have a loose neutral or an overloaded shared circuit. NEC 250.4(A) lays out the performance requirements: bonding creates an effective ground-fault current path, grounding stabilizes voltage to earth.

For low-voltage techs, the trap is mixing reference grounds. A camera powered from one panel, talking to an NVR powered from another, can sit on top of a ground potential difference. That shows up as ground loops, hum bars on analog video, or dead Ethernet ports. NEC 800.100 and 820.100 require bonding of communications cable shields and shields of coax to the building grounding electrode system.

1. Verify continuity from the device chassis ground back to the panel ground bar.
2. Measure voltage between the two grounds with the system de-energized at the load. Anything over 1V is a problem.
3. Bond shields at one end only for analog runs to avoid loops.

Resistance checks that catch problems early

Before you energize anything, ring out the cable. A Cat6 run should read near zero ohms end to end on each pair, and infinite resistance pair to pair and pair to shield. Anything else is a pinched cable, a bad termination, or a staple through the jacket.

For control wiring, a 24VAC thermostat circuit should show the transformer's secondary winding resistance, typically 5 to 30 ohms, when measured across R and C with the system off. A dead short reads near zero and will cook the transformer the moment you turn the breaker on. NEC 725.121 covers Class 2 power source limitations, but the transformer fuse will not always save you fast enough.

Megger anything over 100 feet before you terminate it. A 500V insulation test catches damaged jackets that a continuity test happily passes.

The numbers you should know cold

You will not always have your phone or a calculator handy. These are the values that come up on every job, and you should be able to recite them without thinking.

• Ohm's law: V = I x R, P = V x I
• Single-phase VD constant K: 12.9 for copper, 21.2 for aluminum
• Three-phase VD: multiply single-phase formula by 0.866 (sqrt(3)/2)
• NEC 3% branch, 5% total voltage drop recommendation: 210.19(A) IN 4 and 215.2(A) IN 2
• Class 2 limits: 100VA max, typically 24V at 4.17A: NEC Chapter 9 Table 11(A)
• PoE: 802.3af = 15.4W source / 12.95W device, 802.3at = 30W / 25.5W, 802.3bt Type 3 = 60W / 51W, Type 4 = 100W / 71W

Run the math before you pull the cable. Ring out the cable before you terminate. Verify ground potential before you connect equipment across panels. Three habits that separate techs who get callbacks from techs who do not.