Loop power budget mō ngā loop 4-20 mA

How the tool checks the loop#

A 4–20 mA loop is one series circuit. The loop supply drives the current through the transmitter, the cable and the receiver input. The transmitter needs a minimum voltage at its terminals to work.

1. Conductor resistance. The tool reads the resistance of a copper conductor for the size you pick. It can also take the value from the cable data.
2. Cable resistance. The current goes out on one conductor and comes back on the other. So the tool doubles the one-way length.
3. Loop resistance. It adds the cable resistance to the input resistance, the HART resistor and any other series parts.
4. Voltage at the transmitter. It multiplies the loop current by the loop resistance and subtracts that drop from the supply voltage.
5. Voltage margin. It subtracts the transmitter minimum voltage. A margin of 0 V or more passes.
6. Maximum length. It gives the longest one-way run that keeps the margin at 0 V or more.

Worked loops#

Each line opens the tool with its values. Every line uses a 24 V supply, a 12 V transmitter minimum voltage and a 250 Ω receiver input.

• Pressure transmitter, 500 m of 1 mm² at 20 mA. The loop passes with a margin of 6.48 V.
• HART loop, 400 m of 1 mm² at 22 mA. A 250 Ω HART resistor sits in the loop. The loop passes with a margin of 0.55 V.
• HART loop, 500 m of a 36 Ω/km pair at 22 mA. The resistance comes from the cable data. The loop passes with a margin of 0.21 V.
• The same HART loop at 700 m. The loop fails with a margin of −0.11 V. Shorten the run, use a larger conductor or raise the supply.

New Zealand and Australia#

The Standard select picks the part of AS/NZS 3008.1 the tool reads for the conductor resistance. Part 1.2 is the New Zealand part, 2017 edition. Part 1.1 is the Australian part. For Australia, the Edition select picks 2025 or the superseded 2017 edition. All three print the same resistance values. The impedance table entry says how AmpSize checked them.

Related checks#

• The comms cable selector checks the cable construction and the run length for the interface.
• The DC cable size calculator sizes the 24 V d.c. supply cable to the transmitter panel.
• The segregation checker checks the spacing between the loop cable and power cables.