Signal Splitter or Isolator Load Calculator

The Signal Splitter / Isolator Load Calculator helps you verify whether a 4–20 mA signal splitter or isolator output can safely drive multiple PLC/DCS/indicator inputs plus cable resistance, without exceeding its maximum load capability (Ω).

Enter the splitter’s max load in ohms, the burden of each connected device, and cable data, and the tool will calculate total loop resistance and show whether your configuration is OK, borderline, or overloaded.

Introduction

1. Enter the Splitter / Isolator Max Load Capability (Ω) from its datasheet (e.g., “Max load 600 Ω at 24 V”).

2. Enter the Loop Current for Check (mA) – use 20 mA for worst-case.

3. Enter the burden (resistance) of each connected PLC/DCS input, indicator, isolator, recorder, etc., in the Load 1–4 fields. Leave unused loads blank.

4. Enter Cable Length (m) (one way) and Cable Resistance (Ω/km per conductor) from cable datasheet.

5. Click “Calculate Total Load & Status”.

6. Read the results to see total loop resistance, remaining margin vs. max load, and OK / Borderline / NOT OK status.

🧭 How to Use

1. Example Input:

   • Splitter Max Load Capability: 600 Ω

   • Loop Current for Check: 20 mA

   • Load 1 = PLC AI card: 250 Ω

   • Load 2 = Indicator: 250 Ω

   • Load 3 = (blank / 0 Ω)

   • Load 4 = (blank / 0 Ω)

   • Cable Length (one way): 200 m

   • Cable Resistance: 20 Ω/km per conductor

   Manual Check:

   1. Sum of device loads:
      250 + 250 = 500 Ω

   2. Cable resistance (round trip):
      Rcable = 2 × 200 m × (20 Ω/km ÷ 1000)
      Rcable = 400 × 0.02 = 8 Ω

   3. Total loop resistance:
      Rtotal = 500 + 8 = 508 Ω

   4. Margin vs. max load:
      Margin = 600 − 508 = 92 Ω (still good margin)

   5. Loop current: I = 20 mA = 0.02 A
      Required voltage: V = I × Rtotal = 0.02 × 508 = 10.16 V

   What you should see in the tool:

   • Sum of Device Loads ≈ 500.00 Ω

   • Cable Resistance ≈ 8.00 Ω

   • Total Loop Resistance ≈ 508.00 Ω

   • Max Load Capability = 600.00 Ω

   • Remaining Margin ≈ 92.00 Ω

   • Status: OK – Total loop load is well within splitter/isolator capability.

   • Approx. Voltage Required at 20.0 mA ≈ 10.16 V

Example

If this Signal Splitter / Isolator Load Calculator helped you, explore more of our useful instrumentation tools:

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⭐ Next Steps

👉 Check out our practical ebooks and online courses designed for technicians, maintenance engineers, and students.

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👉 Explore top-rated products on Amazon (Click here).

Stay tuned for more tools to make your instrumentation work faster, easier, and error-free!

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FAQ

Q1. Why do I need a Signal Splitter / Isolator Load Calculator?

Because each PLC/indicator/isolator input adds burden (Ω) to the loop. If total resistance exceeds the splitter’s max load, the output cannot drive full 20 mA and signals become inaccurate.

Q2. What is meant by ‘Max Load Capability (Ω)’?

This is the maximum loop resistance that the splitter/isolator output can drive at a given supply voltage (e.g., 600 Ω @ 24 V).

Q3. What should I do if the result is “NOT OK”?

Reduce number of loads on that output, shorten cable length, use a lower-burden input module, or use a separate splitter output for some devices.

Q4. Does cable resistance really matter in low-current loops?

Yes, especially for long runs (hundreds of meters). Cable resistance can add tens of ohms and reduce margin.