Instrument Cable Loop Resistance Calculator
Find the loop (out and back) resistance of a copper instrument cable pair and the voltage it drops at 20 mA and at the 22 mA alarm current.
- Rconductor = ρ × L ÷ A × (1 + α(T − 20)) = 0.017241 × 500 ÷ 0.5 × (1 + 0.00393 × 10) = 17.92 Ω
- Loop = 2 × conductor = 35.84 Ω
- Drop = I × R
Cable Resistance Formula
ρ20 = 0.017241 Ω·mm²/m, α = 0.00393 /°C
Vdrop = I × Rloop
L is the one way cable length, A the conductor cross section and T the conductor temperature. ρ20 is the resistivity of annealed copper at 20 °C. Real stranded or tinned cables can be a few percent higher; use the cable datasheet maximum resistance for final design.
Worked example
A 500 m run of 0.5 mm² pair at 30 °C has a loop resistance of about 35.8 Ω and drops 0.72 V at 20 mA, small compared with a 24 V supply.
Frequently Asked Questions
What is loop resistance?
The total resistance of both conductors of a pair, out and back. It is twice the single conductor resistance.
Does temperature matter?
Copper resistance rises about 0.4 % per °C, so hot cable trays add a few percent.
Which size is common for instrument cable?
0.5 to 1.5 mm² (about 20 to 16 AWG) is typical for 4 to 20 mA and digital signals.
Learn more on the blog
4 to 20 mA current loop explained
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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