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What is the maximum wire length for digital inputs and outputs on Powerlogic meters?

Issue
Customer would like to determine the maximum distance between a meter and a digital input source or an output signal receiver 

Product Line
ION and Powerlogic meters with digital I/O options

Resolution
There is no absolute maximum distance of I/O wiring and actual length depends on several factors including:
  • The field voltage source
  • Wire AWG and manufacture specs
  • Wire type
  • Electrical noise and environment 
  • Meter specs 
* The calculations below are meant to be a rough estimation and they are not accurate unless all the factors are considered *

Digital Input Example:

The PM800 equipped with a PM8M2222 Digital I/O extender with field source of 120 Vac. The wire to be used is a 16 AWG with a resistance of 4.016 ohm per 1000 feet. The PM800's digital input's turn on voltage is 20 Volts and the maximum current draw is 2 mA.

Vdrop= Current X R (per1000 feet) *2*length
Vinput= Vsource - Vdrop

The Vinput needs to exceed the turn on voltage in order for the meter to recognize the signal. In this example, the theoretical maximum would be in the hundreds of thousands of feet. Needless to say, noise as well as other factors will affect the quality of the signal and practically limiting the cable run.

Digital Output Example

A similar calculation can be established for digital outputs once the specifications are known. For example, the ION 7650 has a maximum load of 3 A @ 380 VAC, hence the rated burden is 126.666 ohm. The total loop resistance should be less than the rated burden.  A 16 AWG wire has a resistance of 4.016 ohms per 1000 feet and the maximum theoretical distance that would sustain current without significant voltage drop would be 31,500 feet. Moreover, Form A digital output have a similar calculation given the maximum load voltage and current as well as the input source. For example, if the turn on is 20 volts on a PLC and the ION 7650's form A relay is rated at 80mA @ 30 VDC then :
Vdrop= Current X R (per1000 feet) *2*length
Vinput= Vsource - Vdrop
The theoretical maximum attainable distance would be around 15,600 feet for a 16 AWG wire originating from the ION7650's I/O output.
 
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