Wire Run & Voltage Drop Electrical
A long home run costs copper resistance, and what that resistance does depends entirely on the signal riding it. Pick the signal type, the gauge, and the one-way length — the verdict speaks that signal’s language: loop supply margin for 4-20 mA, real-versus-imagined error on 0-10 V, and degrees of lie on a temperature sensor.
Input
The transmitter minimum is on its datasheet — 12 V is a common floor for 2-wire transmitters. The sense resistance is what the AI burns to read the current; 250 Ω (1–5 V) is typical, some inputs use 100 Ω.
Output
Worked example — the 4-20 mA case for long runs
A loop-powered pressure transducer sits 500 ft from the panel on 18 AWG, fed from 24 VDC through a 250 Ω sense resistor.
- Round-trip copper:
R = 2 × 500 × 6.385 ÷ 1000 = 6.4 Ω. - Worst case is full-scale, 20 mA:
drop = 0.020 × (6.4 + 250) = 5.1 V. - At the transmitter:
24 − 5.1 = 18.9 V— comfortably above the 12 V floor.
That margin is why 4-20 mA owns long runs: current is the signal, so copper resistance costs supply headroom instead of accuracy, and there’s thousands of feet of headroom to spend.
Copper resistance reference
| AWG | Ω per 1000 ft (one conductor) | Ω per 100 ft round trip |
|---|---|---|
| 14 | 2.525 | 0.51 |
| 16 | 4.016 | 0.80 |
| 18 | 6.385 | 1.28 |
| 20 | 10.15 | 2.03 |
| 22 | 16.14 | 3.23 |
Solid copper at 25 °C, per NEC Chapter 9 Table 8. This tool is US-native (AWG, Ω/1000 ft) — a metric mm² / Ω-per-km option is a tracked follow-up. Copper runs ~0.4 %/°C hotter-is-more, so a ceiling-plenum run in summer reads a touch higher than the table.