Crash course: Voltage, amperage, and resistance basics for low-voltage techs (part 1)

Voltage: the pressure behind the work

Voltage is electrical pressure. It's the potential difference between two points that pushes electrons through a conductor. No pressure, no current, no work done. On a low-voltage job, you're typically dealing with anything 50V or under per NEC Article 725 for Class 2 and Class 3 circuits, though "low-voltage" in the trade often stretches up to 100V depending on who you ask.

The thing to remember: voltage exists whether current is flowing or not. A 24VAC transformer sitting on a thermostat wire still has 24V across its terminals with the load disconnected. That's potential. Connect the load and you get current.

Common low-voltage values you'll meet in the field:

• 24VAC: HVAC controls, doorbells, irrigation valves
• 12VDC / 24VDC: access control, security panels, LED strips
• 48VDC: PoE devices, telecom equipment
• 50V and under: NEC Class 2 limit for most signaling and control

Amperage: what actually does the work

Amps measure current, the rate of electron flow past a point. Voltage pushes, amperage moves. When something gets hot, trips a breaker, or melts a connector, amperage is the culprit, not voltage. A 9V battery can't hurt you. A car battery at 12V can weld a wrench to a frame because it dumps hundreds of amps on a dead short.

For sizing conductors and overcurrent protection on low-voltage circuits, NEC 725.121 sets the power source limits for Class 2 and Class 3. NEC Chapter 9 Table 11(A) and 11(B) give the actual VA and current limits per voltage class. Memorize the ones you use, the rest you can look up.

If a Class 2 transformer keeps tripping its internal fuse, stop replacing it. You have a short or an overload downstream. The transformer is doing exactly what it's listed to do.

Resistance: the brake on the system

Resistance, measured in ohms, is opposition to current flow. Every conductor, connection, and load has it. Copper has very little, your terminations should have almost none, and a load like a relay coil or a speaker has whatever it's designed to have.

The hidden killer on low-voltage runs is unintended resistance: corroded splices, loose terminals, undersized conductors over long pulls. A 24VAC valve at 500 feet on 18 AWG sees real voltage drop, and the valve drops out even though the transformer reads 24V at the panel.

Quick reference for solid copper resistance per 1000 ft at 75C:

• 22 AWG: about 16.5 ohms
• 18 AWG: about 6.5 ohms
• 16 AWG: about 4.1 ohms
• 14 AWG: about 2.6 ohms
• 12 AWG: about 1.6 ohms

Ohm's Law: the only formula you can't skip

V = I x R. Voltage equals current times resistance. Rearrange for whatever you're missing. It's the foundation of every troubleshooting decision you make on a low-voltage system, and it's the difference between guessing and knowing.

Three forms, all the same equation:

1. V = I x R (find voltage if you know current and resistance)
2. I = V / R (find current if you know voltage and resistance)
3. R = V / I (find resistance if you know voltage and current)

Practical example: a 24VAC zone valve pulls 0.5A holding current. R = 24 / 0.5 = 48 ohms. If you measure the coil cold and get 200 ohms, the coil is open or the winding is partially burnt. You didn't need a manual, you needed Ohm's Law and a meter.

Voltage drop: where theory hits the field

NEC 210.19(A) Informational Note 4 recommends keeping branch-circuit voltage drop under 3%, with 5% total for feeder plus branch. It's not enforceable as code, but it's the industry standard and inspectors and engineers will hold you to it on spec jobs.

For low-voltage, the rule matters more, not less. A 5% drop on 120V is 6V and most loads tolerate it. A 5% drop on 24V is 1.2V, and a contactor rated for 24V with a 20% pickup tolerance is now operating right at its margin. Add a hot day, a slightly weak transformer, and you have a nuisance call.

On any run over 200 feet at 24V or below, calculate voltage drop before you pull wire. Re-pulling 500 feet of 18 AWG because you should have run 16 AWG is an expensive lesson.

Putting it together on a service call

Voltage tells you if the source is alive. Resistance tells you if the path is intact. Current tells you if work is actually happening. A complete diagnosis touches all three, in that order, and most failed troubleshooting comes from skipping a step because the symptom "looked obvious."

Standard sequence at the panel or device:

1. Measure source voltage with the load connected and energized, not just open-circuit
2. De-energize and measure resistance of the load and the conductor path
3. Re-energize and measure current draw against the nameplate or spec

Part 2 covers AC vs DC behavior, series and parallel circuits, and how transformers change the math on the secondary side.