# Voltage Drop On 0.75mm2 Instrumentation Cables

#### Kim Jeck

I have a VEGABAR 82 pressure transmitter (operating voltage=9.6 to 35Vdc) that connected to the Control Panel with a cable length of est. 150meter. The cable used is a single run of 1x2x0.75mm2 instrument cable. I would like to know if the volt-drop along the cable will affect the operation of the transmitters?

From what i understand from the previous topics over here, the volt drop can be calculated using V=2xIxRxL.
What is the current i should use for the calculations as i can't find any current consumption of the datasheet of the transmitter? Should i use the output current 4-20mA?

#### David_2

The quick answer - there's no problem using roughly 18g wire for a 2 wire, loop powered transmitter over 150m.

Yes, the current used for the calculation is the loop current, but 4-20ma covers only the measurement range without taking into account the fault indication current which can be as high as 22.8mA (those complying to NAMUR standard use 21mA).

But to do a calculation you also need to know what the load resistance is. The analog input used in the current loop has a shunt/dropping resistor to develop an IR drop. I've seen probably a dozen different resistances used by different AI's, but 250 ohms is widely used in the DCS and PLC world. If HART communications is to be used, then HART requires a minimum of 230 ohms to develop the HART signal.

I didn't look up what the Vega's fault current is but I'll use 23 mA as an example.

A rule-of-thumb chart I have shows 0.75mm/18g wire has a resistance of 23 ohm per 1000m. Your wire run one way is 150m for a total of 300m out and back. 300m is 30% of 1000m, 30% of 23 ohms is about 8 ohms for 300m.

The loop voltage needs to drive 23ma through the load resistance, say 250 ohms and a wire resistance of 8 ohms, a total of 258 ohms.

E = I*R; 0.023A * 258 ohms = 5.9V, the IR drop at the maximum output of 23mA.

If you use a 24V dc power supply then the transmitter will 'see' a little over 18V (or higher), the other 5.9V being dropped across the wiring and load at the max 23mA output). 18V+ is well within the operating range spec of 9.6V to 35Vdc. Good to go.