Loop Power & Voltage Drop Calculator

The Loop Power & Voltage Drop Calculator helps you verify whether your 24 VDC (or other) power supply can safely drive a 4–20 mA instrument loop, including transmitter, PLC/DCS input cards, and long cable runs.

By entering the supply voltage, loop current, device load, cable length, and cable resistance, you can instantly see the cable voltage drop, total loop voltage required, and whether the loop is OK or under-powered.

Introduction

1. Supply Voltage (V) – enter your loop power supply voltage (commonly 24 VDC).

2. Loop Current (mA) – use 20 mA for worst-case check (max load).

3. Total Device Load (Ω) – sum of transmitter, PLC/DCS analog input, and any series resistors.

4. Cable Length (m) – one-way distance from power supply/PLC panel to field transmitter.

5. Cable Resistance (Ω/km per conductor) – from cable datasheet (typical: 18–25 Ω/km for 1.5 mm²).

6. Click “Calculate Loop Power & Voltage Drop”.

7. Read the result box to see cable resistance, voltage drop, loop margin, and OK / NOT OK status.

🧭 How to Use

1. Example values to test in the calculator:

   • Supply Voltage = 24 V

   • Loop Current = 20 mA

   • Total Device Load = 500 Ω (e.g., 250 Ω AI card + 250 Ω transmitter)

   • Cable Length = 300 m (one way)

   • Cable Resistance = 20 Ω/km per conductor

   Step-by-step manual check:

   • Cable resistance (round trip):
     Rcable = 2 × 300 m × (20 Ω/km ÷ 1000)
     Rcable = 600 × 0.02 = 12 Ω

   • Total loop resistance:
     Rtotal = 500 + 12 = 512 Ω

   • Loop current:
     I = 20 mA = 0.02 A

   • Required voltage:
     Vreq = I × Rtotal = 0.02 × 512 = 10.24 V

   • Voltage drop on cable:
     Vcable = I × Rcable = 0.02 × 12 = 0.24 V

   • Margin from 24 V supply:
     Margin = 24 − 10.24 = 13.76 V (good margin)

   Expected result summary in the tool:

   Results:
   • Loop Current Used for Check: 20.0 mA
   • Cable Resistance (round trip): 12.00 Ω
   • Total Loop Resistance (devices + cable): 512.00 Ω
   • Voltage Drop on Cable: 0.24 V
   • Required Supply Voltage at 20.0 mA: 10.24 V
   • Available Margin from Supply (24.00 V): 13.76 V
   • Loop Status: OK – Good margin available.

Example

FAQ

Q1. What is a 4–20 mA loop?

A 4–20 mA loop is a standard analog signal used in industrial instrumentation where 4 mA represents the zero/low range and 20 mA represents the full-scale/high range.

Q2. Why do I need to check loop power and voltage drop?

A. If the power supply cannot overcome the total loop resistance (devices + cable), the transmitter cannot drive the full 20 mA, causing incorrect readings and instrument faults.

Q3. What happens if the margin is very small?

A small voltage margin makes the loop sensitive to temperature changes, aging, extra resistance (loose terminals, corrosion), and additional devices. Good design keeps some healthy margin.

Q4. Can I use this calculator for multiple devices in one loop?

A. Yes. Just add all series resistances (transmitters, isolators, barriers, analog inputs, extra resistors) into the Total Device Load (Ω).