The short answer
Voltage drop is the difference between the voltage at the source and the voltage at the load, caused by the resistance of the conductors carrying current between them. It is usually quoted as a percentage of the supply voltage, because 3 V lost from 120 V matters far more than 3 V lost from 480 V.
Two things follow immediately from that definition, and both are worth holding on to:
- No current, no drop. The drop is proportional to the current flowing. An idle circuit reads full voltage at the far end no matter how bad the wiring is.
- The lost voltage becomes heat. It is not stored anywhere and it is not recoverable. Every volt dropped is a volt you paid for and did not use.
Why it happens
Copper and aluminium conduct electricity well, but not perfectly. A conductor is a lattice of metal atoms with electrons moving through it, and those electrons collide with the lattice as they go. Each collision transfers a little energy to the metal, which shows up as heat. The bulk effect of all those collisions is what we call resistance.
Ohm’s law says the voltage across a resistance carrying a current is V = I × R. Apply that to a cable run and the drop is set by how much current is flowing and how much resistance the run has. That is the whole mechanism.
On AC there is a second effect. The alternating current sets up a changing magnetic field around each conductor, and that field induces a voltage opposing the change: inductive reactance. It does not dissipate energy the way resistance does, but it does add to the voltage drop, and it matters most for large conductors, inductive loads and steel raceways. The AC calculator models both parts.
The four things that control it
Voltage drop responds to exactly four inputs. Every fix is one of these four moved in the right direction.
| Factor | Effect | Relationship |
|---|---|---|
| Current | More current, more drop | Directly proportional: double the current, double the drop |
| Length | Longer run, more drop | Directly proportional: double the distance, double the drop |
| Conductor size | Bigger conductor, less drop | Inversely proportional to area: double the area, halve the drop |
| Material | Aluminium drops more than copper | About 1.64× at the same size, from the conductivity difference |
Temperature is a fifth, smaller lever. Conductor resistance rises with temperature, so a hot conductor drops more than a cold one carrying the same current, about 4.8% more going from 75 °C to 90 °C in copper. You cannot usually choose the temperature, but you can decide what to assume, and assuming the conductor runs cool is optimistic.
What it does to equipment
A load designed for a nominal voltage generally tolerates some variation. What excessive drop does depends on what is at the far end.
Motors
A motor’s torque falls roughly with the square of the applied voltage, so a 10% drop costs about 19% of available torque. To deliver the same mechanical output at lower voltage the motor draws more current, which heats the windings and, because current has increased, makes the voltage drop worse. Starting is the hardest moment: inrush current is several times running current, so the drop during a start is several times the steady-state figure.
Resistive heating
Power scales with the square of the voltage, so a 5% drop costs about 10% of the heat output. The element still works; it just takes longer, and the appliance appears underpowered rather than faulty.
Lighting
Incandescent lamps dim and shift warmer. LED drivers and electronic ballasts hold output steady until their input falls below the regulation range, and then either drop out or start flickering. The failure is abrupt rather than gradual, which makes it hard to diagnose from the symptom.
Electronics and controls
Switch-mode supplies are constant-power devices: as input voltage falls they draw more current to hold output, which is the opposite of what a struggling circuit needs. Contactors and relays can chatter or fail to pull in when the voltage at the coil sags during a start.
Voltage drop vs voltage loss
In everyday use the two phrases mean the same thing and nobody is confused. If you want to be precise:
- Voltage drop is the potential difference across the conductors, measured in volts. It is what a meter reads between the two ends of the run.
- Power loss is what that drop costs, measured in watts. It is
I² × Rsummed over the current-carrying conductors, and it is the figure that appears on your electricity bill.
The two are linked but not interchangeable. A 20 A circuit on 100 ft of 12 AWG copper drops 7.92 V and loses 158 W as heat. Halve the current and the drop halves, but the loss falls to a quarter, because it goes with the square.
In a cable vs across a resistor
Both are V = I × R, but they are opposite in intent, and the calculators for them are different tools.
| In a cable | Across a resistor | |
|---|---|---|
| Intent | Unwanted side effect | The component’s purpose |
| Resistance | Set by material, size and length | Chosen by the designer |
| Depends on distance | Yes, directly | No |
| System multiplier | 2 or √3, for the return path | None: two terminals |
| Design goal | Make it smaller | Make it exactly what you need |
| Calculator | Voltage drop calculator | Resistor calculator |
Questions
What causes voltage drop?
Conductor resistance. Every metal conductor opposes the current flowing through it, and by Ohm’s law that opposition produces a voltage difference between the two ends of the run. The energy that difference represents leaves the circuit as heat in the conductor. On AC there is a second contribution, inductive reactance, which comes from the magnetic field the alternating current sets up around the conductor.
Is voltage drop the same as voltage loss?
In everyday use the two terms are used interchangeably and both mean the volts that do not arrive at the load. If you want to be precise, voltage drop is the potential difference measured across the conductors, and the associated power loss is what that difference costs you in watts: drop is in volts, loss is in watts. Some equipment makers also use "voltage loss" for the drop inside a device rather than in the wiring.
Does voltage drop waste energy?
Yes, and it is measurable. Power lost in the conductors is I² × R for each current-carrying path. A 20 A circuit on 100 ft of 12 AWG copper loses about 158 W as heat, a permanent 158 W penalty for as long as the load runs, paid for at the meter and never delivered to anything useful. Larger conductors cut that loss in proportion to the resistance reduction.
Can voltage drop be zero?
Not in a real conductor. Any current through any resistance produces some drop. It can be made small enough to ignore (a short run of large conductor at modest current may drop a few hundredths of a percent), but it never reaches zero outside a superconductor.
Does voltage drop happen with no load connected?
No. Voltage drop is proportional to current, so with no current flowing there is no drop and the far end of the run reads full supply voltage. This is exactly why measuring voltage at an idle outlet tells you nothing about whether the circuit has a voltage drop problem: the drop only appears under load. It is also the basis of the voltage drop test.
Next: how to calculate voltage drop walks through the method, and how to measure voltage drop covers the meter test that finds a bad connection.