Calculating Maximum 4-20mA Loop Distance for SITRANS P220

Overview of 4-20mA Current Loop Transmission

The 4-20mA current loop remains the dominant analog signaling standard in process instrumentation. Unlike voltage signals, the transmitted current is independent of cable resistance, so the loop's accuracy is preserved over distances that would devastate a 0-10V signal. The constraint is no longer signal fidelity but voltage headroom: enough supply voltage must remain to push 20mA through every series resistance in the loop while the transmitter still has its minimum operating voltage.

For a Siemens SITRANS P220 pressure transmitter installed 450m from the PLC analog input module, the question is not whether 4-20mA "works at distance" but whether the supply voltage can sustain 20mA through the copper conductor loop plus the receiver's shunt resistance plus any safety barrier or indicator in series.

Loop Load Equation and Voltage Budget

Every 4-20mA loop is governed by the same Ohm's-law relationship. The SITRANS P220 datasheet provides the canonical form:

R_total_max = (U_B - 10V) / 0.02A

Where:

• U_B = supply voltage applied to the loop (V DC)
• 10V = minimum terminal voltage the transmitter requires to operate linearly at 20mA
• 0.02A = full-scale loop current (20mA)
• R_total_max = maximum permissible series resistance across the entire loop at 24V DC

With a typical 24V DC supply:

R_total_max = (24V - 10V) / 0.02A = 14V / 0.02A = 700 Ω

This 700 Ω ceiling is the entire budget for everything in series: wire (go and return), analog input shunt, intrinsic-safety barrier, indicator, surge protector, and any field-mounted repeater burden. The TI training module on current-loop transmitters describes this exact relationship, treating the loop as a series circuit where compliance voltage is shared by the transmitter and all external burdens.

The general expression for any supply voltage:

SITRANS P220 Key Specifications Relevant to Loop Sizing

The Siemens SITRANS P220 is a compact piezoresistive pressure transmitter for gauge and absolute pressure measurement. Catalog numbers in the 7MF1567-xxx family cover ranges from 0-1.6 bar up to 0-600 bar, with a two-wire 4-20mA output and HART option on selected variants.

Receiver (Analog Input) Resistance

The PLC analog input module terminates the loop with a precision shunt resistor that converts 4-20mA back into a measurable voltage. The input resistance of this module is subtracted directly from the 700 Ω budget. Common shunt values:

For a Siemens ET 200SP AI 4xI 2-/4-wire (6ES7134-6TD00-0CA1) configured as a current input, the burden is 250 Ω nominal. After subtraction:

R_wire_max = 700 Ω - 250 Ω = 450 Ω

This is the entire conductor budget for the round trip (out to the transmitter and back).

Wire Resistance and Round-Trip Distance

Standard copper conductor DC resistance at 20°C (NEC Chapter 9, Table 8 values, AWG):

For metric installations using typical instrumentation cable (e.g., 0.75 mm² ≈ 18 AWG, 1.0 mm² ≈ 17 AWG), values from IEC 60228 apply. Loop resistance for typical screened instrument cable (e.g., LiYCY 2 x 0.75 mm²):

Worked Example: 450m Run with SITRANS P220

Step 1 - Confirm supply. Standard 24V DC control panel supply.

Step 2 - Total loop budget. R_total = (24 - 10) / 0.02 = 700 Ω.

Step 3 - Identify receiver burden. SM 1231 module (S7-1200), R_AI = 250 Ω.

Step 4 - Net conductor budget. R_wire_max = 700 - 250 = 450 Ω.

Step 5 - Convert to distance. Using 0.75 mm² screened cable with 26 Ω/km loop resistance (one-way round trip counted as 2 x one-way distance):

For a 450m one-way cable route, the round-trip conductor length is 900m:

R_wire = 0.9 km × 26 Ω/km = 23.4 Ω

23.4 Ω ≪ 450 Ω budget. The installation is comfortably within compliance with substantial margin (≈ 426 Ω unused).

Step 6 - Cross-check with the maximum one-way distance:

D_max = 450 Ω / (26 Ω/km) = 17.3 km one-way (34.6 km of conductor). The 450m cable run is well under 3% of the theoretical maximum.

HART-Enabled Variants: Reduced Headroom

When the SITRANS P220 is configured for HART communication, the modem superimposes a 1mA peak-to-peak AC signal on the 4-20mA DC. The minimum loop resistance to support clean HART decoding is 230 Ω; the maximum to support 20mA at 24V is unchanged (700 Ω). However, the effective compliance voltage of the transmitter drops to ≈12V at 20mA because HART draws an additional average current:

R_total_max(HART) = (24 - 12) / 0.02 = 600 Ω

R_wire_max(HART) = 600 - 250 = 350 Ω

This still leaves 350 Ω for the 900m of 0.75 mm² conductor (23.4 Ω), so the 450m HART installation remains compliant.

Repeater / Loop-Powered Isolator Selection

If the calculated resistance exceeds the budget - for example, if a long run is combined with multiple instruments, intrinsic-safety barriers, or an HART indicator - a loop-powered repeater (galvanic isolator or signal conditioner) is inserted. The repeater acts as a new 24V source for the field side while presenting only its own small burden to the controller side.

A single loop isolator is almost always sufficient. Two isolators are only required when the field device itself needs galvanic isolation and the panel-side PLC input must be referenced to a different ground potential (e.g., separate MCC and DCS grounds). Three or more isolators should never be stacked in a single current loop - they add series burden and can introduce noise coupling.

Intrinsic-Safety and Surge Considerations

If the loop runs into Zone 1 / Zone 0 (IEC 60079 / NEC 500), each Zener barrier or galvanic isolator adds typically 90-330 Ω. The SITRANS P220's Ex-ia variants (catalog suffix "E" in 7MF1567) are rated for barrier use, but the barrier must be included in the resistance budget:

R_total = R_barrier + R_isolator + R_wire + R_AI + R_indicator + ... < 700 Ω

Common values:

• MTL 7787+ Zener barrier: ~340 Ω
• Pepperl+Fuchs Z 728: ~290 Ω
• Stahl 9002/13-280-093-001: ~280 Ω

Surge protectors (e.g., Dehn BVT AVD 24) add ≈10 Ω. They are negligible in the DC budget but protect the loop from induced transients on long outdoor runs.

Verification and Commissioning Procedure

1. Disconnect the field transmitter and apply a precision 4-20mA source (Fluke 754, Beamex MC6, or WIKA CPB-Cal) at the panel terminals.
2. Inject 4.000mA and verify the scaled engineering value in the PLC matches the expected zero (e.g., 0.00 bar for a 0-10 bar transmitter).
3. Inject 12.000mA and verify 50% scale.
4. Inject 20.000mA and verify full scale.
5. Measure the voltage at the transmitter terminals with the loop energized at 20mA. Confirm it is at least 10V (non-HART) or 12V (HART) but not above 33V.
6. Measure the actual round-trip conductor resistance by shorting the field end and measuring resistance from the panel with the loop de-energised. Compare to the calculated value; deviation should be <10%.
7. Record the loop burden budget consumption in the project's loop sheet.

Troubleshooting Matrix

Field-Proven Heuristics

• 24V is the universal sweet spot. Below 18V the loop has insufficient headroom; above 30V the transmitter wastes power as heat.
• Round-trip rule. Always double the one-way distance before consulting the wire-resistance table.
• Twisted, shielded pair. Use instrument cable such as Belden 8761 or Lapp UNITRONIC LiYCY 2 x 0.75 mm². Ground the shield at the panel end only to avoid ground loops.
• Temperature derating. Copper resistance rises ≈0.4% per °C. A cable run operating at 60°C ambient has ≈16% more resistance than the 20°C table value. Re-compute the budget at the worst-case ambient.
• Spare pairs unused? Don't parallel them to halve resistance. The single-line ground reference and HART signalling depend on a single twisted pair; paralleling introduces imbalance.

References to Manufacturer and Standards Documentation

• TI - Introduction to 4-20-mA current loop transmitters: https://www.ti.com/video/6213906041001
• Fluke - What Is a 4-20 mA Current Loop?: https://www.fluke.com/en-us/learn/blog/calibration/what-is-a-4-20-ma-current-loop
• NEC Chapter 9, Table 8 - Conductor properties (AWG resistance and cross-section).
• IEC 60228 - Conductors of insulated cables (metric cross-section classes).

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