4–20 mA Loop & Signal-Cable Sizing

Will that instrument actually work at the end of the cable? Voltage-drop budget for two-wire (loop-powered) transmitters, load-capability check for self-powered 4–20 mA outputs, the HART 230–600 Ω window, IS-barrier budget, and copper cable sizing with tropical temperature correction. Built for Southeast-Asian plants.

1Instrument & run

Output type^[8]

Instrument / output rating^[8]

HART / smart comms^[1][7]

Cable temperature^[3][4] (20–75)°C

One-way run L^[3] (10–5000)m

Cable cross-section^[5] (0.5–2.5)mm²

2Loop loads & supply

Receiver (AI card) R_recv^[1][7] (100–500)Ω

IS barrier / isolator^[4][3] (0–300)Ω

Supply V^[1][4] (19–30)V DC

Tx terminal min V^[1][3] (8–17)V

Other series drop^[1] (0–5)V

Vendor-convention converter — "16.6 V incl. 250 Ω" vs terminal voltage

Enter the number printed on your datasheet; the tool converts it so the two conventions never get double-counted.

Datasheet value V

Means (loop with 250 Ω AI)

—

Results — is the loop viable?

Cable R (2L)

—Ω

Total loop R

—Ω

V at instrument

—V

Item	Value	Unit

Given this cable, the loop is viable to:

Cross-section	0.5	0.75	1.0	1.5	2.5 mm²
Max one-way run (m)	—	—	—	—	—

Instrumentation this loop settles on

Duty	Instrument	Basis

Quote this loop's instruments at factory price

Analysers, the 24 V supply and cable at the manufacturer's ex-works price — our margin comes solely from the export tax rebate. Signal cable is normally bought locally; we size it here so your local purchase has a number behind it. RS-485 surges on this loop are covered by the LW-10 (Unit 2 of the I&C manual).

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Sources & parameter basis

[1]	Emerson/Rosemount load limitation — R_max = 43.5·(Vsupply−10.5); HART ≥250 Ω; a 250 Ω loop needs ≥16.6 V. Terminal minimum 10.5–12 V.
[3]	Chinhui Automation cable guide — EJA "16.6 V incl. 250 Ω" convention; I_check 24.4 mA; ρ 0.0188 @35 °C, 0.0217 @75 °C; 300 Ω load ceiling.
[4]	Multicalci instrumentation calculators — ρ_20 0.0168 Ω·mm²/m, α_Cu 0.00393 /K, R_max formula, 1.3× safety on V_tx_min.
[5]	IEC 60228 conductor resistance maxima at 20 °C (Ω/km).
[7]	HART Foundation / Endress+Hauser — total loop load window 230–600 Ω for HART signalling.
[8]	Jiuyuhub published product data (jiuyuhub.com) — 4–20 mA outputs rated into loads <750 Ω (DO6000Y / MS6000SG / PH6000C); SE10A 24 V DC, HART/Modbus.

Full derivation, worked examples and the two-vendor convention note: Instrument-Loop-Power-Cable-Calculation-Sheet.md (calculation book, Unit 1 of the I&C manual).

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