Low-voltage wire resistance and voltage drop by gauge
DC resistance of 8 to 22 AWG copper and twisted-pair cable, with the voltage lost per 100 feet of run at common lock and reader currents.
Voltage lost per 100 feet of run
Solid copper at 20 °C. A run of 100 feet is 200 feet of conductor, out and back, and that is what the table counts.
| Gauge | Ω per 1,000 ft | At 250 mA | At 500 mA | At 1 A | At 2 A |
|---|---|---|---|---|---|
| 22 AWG | 16.140 | 0.81 V | 1.61 V | 3.23 V | 6.46 V |
| 20 AWG | 10.150 | 0.51 V | 1.02 V | 2.03 V | 4.06 V |
| 18 AWG | 6.385 | 0.32 V | 0.64 V | 1.28 V | 2.55 V |
| 16 AWG | 4.016 | 0.20 V | 0.40 V | 0.80 V | 1.61 V |
| 14 AWG | 2.525 | 0.13 V | 0.25 V | 0.51 V | 1.01 V |
| 12 AWG | 1.588 | 0.08 V | 0.16 V | 0.32 V | 0.64 V |
| 10 AWG | 0.999 | 0.05 V | 0.10 V | 0.20 V | 0.40 V |
| 8 AWG | 0.628 | 0.03 V | 0.06 V | 0.13 V | 0.25 V |
To use it: multiply the figure by the run length in hundreds of feet, and subtract from the supply voltage. A 12 VDC magnetic lock drawing 500 mA at the end of 250 feet of 18 AWG loses about 0.64 V × 2.5 = 1.6 V, and sees roughly 10.4 V.
Twisted-pair cable, per conductor
| Cable | Ω per meter | Ω per 100 ft |
|---|---|---|
| Cat5e — 24 AWG solid | 0.0842 | 2.57 |
| Cat6 — 23 AWG solid | 0.0668 | 2.04 |
| Cat6A — 23 AWG solid | 0.0668 | 2.04 |
| Cat6 — 24 AWG solid | 0.0842 | 2.57 |
| CCA 24 AWG (not standards compliant) | 0.1400 | 4.27 |
Rules of thumb that hold up
- Check the voltage at the device against its minimum operating voltage, not against an arbitrary percentage.
- Recheck at the battery's end-of-discharge voltage. A 12 V system on battery may start near 10.5 V before any drop at all.
- Going to 24 VDC halves the current for the same power and cuts the drop to a quarter in percentage terms. Use it for long lock runs where the hardware allows.