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Enter the supply, transmitter, cable and everything wired in series to see whether voltage is left up to the alarm current, how many receivers still fit and what each mA of signal is worth.
Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.
Supply voltage at minimum tolerance minus the drops in the cable, receivers and indicators. The loop works while the line stays above lift-off.
Calibrated range between 4 and 20 mA, with the NAMUR NE 43 failure zones. Point: current read.
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With a 24 V supply, a 10.5 V transmitter and a check at 22 mA, maximum load is (24 − 10.5) ÷ 0.022 ≈ 614 Ω. Allowing 10 to 20 Ω of cable, two 250 Ω receivers fit. With a −5% supply tolerance, load falls to about 559 Ω.
Under NAMUR NE 43, the transmitter signals failure by going to 21 mA or more, and many go to 22 or 22.5 mA. If the loop cannot carry that current, the high alarm never reaches the control system.
It is 50% of span: 200 °F. For flow measured by ΔP without root extraction in the transmitter, 12 mA is 50% of ΔP but 70.7% of maximum flow.
With 18 AWG copper at 104 °F, each conductor has about 6.9 Ω per 1,000 ft. Over 1,000 ft (2,000 ft of conductor both ways), resistance is about 13.8 Ω, small next to a 250 Ω receiver.
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