Low-Voltage DC Voltage Drop Chart

Maximum one-way run by gauge and load at the 10% drop convention, computed from standard solid-copper resistance at 20°C (18 AWG = 6.385 Ω/kft) — the security industry's basis, verified against two published worked examples. For lock circuits designed to the manufacturers' 5% target, halve these distances.

12 VDC — max run (ft) by gauge and load

COMPUTED
One-way run length at a 1.2 V (10%) drop. Counts floored. Solid copper at 20°C; stranded wire runs 2–8% shorter.
Gauge0.25 A0.5 A1 A2 A
22 AWG148 ft74 ft37 ft18 ft
20 AWG236 ft118 ft59 ft29 ft
18 AWG375 ft187 ft93 ft46 ft
16 AWG597 ft298 ft149 ft74 ft
14 AWG950 ft475 ft237 ft118 ft
12 AWG1,511 ft755 ft377 ft188 ft

24 VDC — max run (ft) by gauge and load

COMPUTED
One-way run length at a 2.4 V (10%) drop — exactly double the 12 V distances at the same current, and remember a same-wattage device draws half the current here.
Gauge0.25 A0.5 A1 A2 A
22 AWG297 ft148 ft74 ft37 ft
20 AWG472 ft236 ft118 ft59 ft
18 AWG751 ft375 ft187 ft93 ft
16 AWG1,195 ft597 ft298 ft149 ft
14 AWG1,900 ft950 ft475 ft237 ft
12 AWG3,022 ft1,511 ft755 ft377 ft
Basis note: the NEC Chapter 9 Table 8 resistance values (used for power work) are given at 75°C and run ~20% higher than these 20°C figures — correct for their context, shorter distances if applied here. State the basis when comparing tables.

Using the chart

Find the device draw on its data sheet (or the access control chart’s typicals / the camera chart’s power rows), pick the gauge column that covers the run with margin, and remember loads add: two 500 mA locks on one homerun is the 1 A column. For an exact answer on a specific run, the voltage drop calculator does the arithmetic with your real numbers.

Common questions

How far can you run 18 AWG wire for a 12 V camera?

At a 500 mA draw, 187 ft at the 10% drop convention — the number the table computes from 18 AWG's 6.385 Ω per 1,000 ft. Published vendor tables cluster around the same math (one widely-used table prints 173 ft, computed at ~9%; lock-manufacturer charts built on 5% give about half). Know which convention a table uses before comparing.

What is the voltage drop formula for DC?

Vdrop = 2 × length(ft) × current(A) × resistance(Ω/kft) ÷ 1,000 — the 2 is the round trip, out on one conductor and back on the other, and it is the term a surprising number of published guides omit. Acceptable drop is conventionally 10% of supply (1.2 V at 12 V, 2.4 V at 24 V); lock manufacturers design at 5%.

Why does 24 V go so much farther than 12 V?

Twice for the volts and twice for the amps. The allowed drop doubles (10% of 24 vs 12), and a same-wattage device draws half the current at 24 V — so the same lock or camera on 24 V power reaches roughly 4× the distance on the same wire. The table shows the voltage half (same current = 2×); the current half comes from the device spec.

RUN THE NUMBERS