Voltage Drop

The formula

Exactly what the calculator computes, in the order it computes it. Highlighted terms are your inputs.

// Both conductors carry the current, so the length doubles
loopResistance = 2 × (runLength(ft) ÷ 1000) × ohmsPer1000ft

voltageDrop    = current × loopResistance          Ohm's law
voltageAtDevice = supplyVoltage − voltageDrop
percentDrop     = voltageDrop ÷ supplyVoltage

// Longest run before the device minimum is reached
maxLength = (supplyVoltage − deviceMinimum) × 1000
            ─────────────────────────────────────────────
                    2 × current × ohmsPer1000ft

PASS when voltageAtDevice ≥ deviceMinimum

What it assumes

Each of these is a way the number can be wrong for your job. Read them before you buy hardware.

1. DC only. This is a pure resistance calculation for DC circuits. It does not apply to AC runs, where inductive reactance and power factor matter.
2. Solid uncoated copper at 20 °C. Resistance values follow the standard AWG table. Copper resistance rises roughly 0.4% per °C — a run through a 50 °C plenum carries about 12% more drop than calculated. Stranded conductor is marginally higher again.
3. Enter the measured supply voltage, not the nominal rating. This is the most common source of error. A "12 V" power supply typically floats at 13.6–13.8 V, and a "24 V" supply at 27.2–27.6 V. But on battery standby that same supply falls toward 10.5 V as the battery discharges — and that is exactly when the device still has to work. Run the calculation twice: once at float, once at end-of-discharge.
4. Steady-state current only. Inrush is not modelled. Electric strikes, maglocks, sirens, and motorised devices draw substantially more on energisation, and the momentary drop is larger than shown. Use the worst-case continuous draw and treat the result as optimistic for inrush.
5. One device at the end of the run. For multiple devices tapped along a run, the drop is not uniform — calculate segment by segment, or as a worst case put the whole load at the far end.
6. Perfect terminations assumed. No allowance for splices, terminal resistance, corrosion, or a poorly seated connector. In the field these are a frequent cause of drop beyond calculation, particularly in exterior and below-grade work.
7. Aim for margin, not a bare pass. A result that clears the minimum by a tenth of a volt is a callback waiting to happen. Common practice targets no more than 10% drop, and leaving a volt in hand costs very little at installation.