When it comes to deciding just how far you can run conduit and cable, there are a couple of things you must consider: the power supply and the communication signal. Resistance, electrical noise, and the output signal limit the length of your cable runs.
Here’s how it all works:
Power Supply & Voltage Drop
Where is your power supply and how much power is it providing? This is a key consideration when deciding how far you can run your wire. Too far and you may not have enough juice left to power up your sensor.
The first consideration is how much you are using to power your sensor. Most of our sensors are powered with between 12 and 24 VDC. Using 12 VDC will result in a shorter cable run than using 24 VDC because of the voltage drop you will experience in the cable. The longer the cable, the more the voltage drops.
Thankfully this can be easily calculated using Ohm’s Law. We discuss this in more detail in our article, “How Ohm’s Law Affects Level Sensors and Pressure Transducers”. As a rule of thumb, make sure you’re using at least 24 AWG wire.
Basically you can determine the voltage needed to power through the resistance in your line by multiplying voltage and output current, or V=IR. Luckily most of us will not have to worry about that math as most manufacturers have already done it and provide specs to keep you straight. In many cases this distance will be between 25 and 100 ft., though some can reach a few thousand ft.
Just as your power supply will drop or weaken over distance so too will your output signal. What type of output is your sensor using? A sensor that has a voltage output, 0-5 VDC for example, will not be able to travel as far as Modbus RS-485 or even 4-20 mA outputs. You need to know how far a given output signal can travel.
As every conductor has at least a little resistance, voltage drop is going to happen.
Basically the amount of signal loss is determined by the characteristics of the conductor – in this case, the cable. The two determining factors are the length and the gauge of the cable. As stated, use an appropriate gauge – we recommend at least 24 AWG.
A shorter length of cable will equal a smaller voltage drop. This means the longer the cable, the greater the voltage drop. This can alter the signal coming from the sensor and give you a false reading or create a loss of signal.
So how far is too far? When dealing with voltage outputs, you should generally try to keep the cable to 25 ft. and no longer than 100 ft. This distance will be determined by the voltage output. A higher voltage will be able to travel a bit farther than a lower voltage.
A voltage output is also prone to electrical noise. This affects the length in noisy environments. Use shielded cable and ground your sensor properly. For more tips on grounding and how it affects noise, visit our article, “4 Critical Tips For Stopping Electrical Noise”.
4-20mA outputs are less susceptible to voltage drop, but it is still a problem if the voltage supply isn’t high enough. Most of our sensors work on 12-24 VDC. Drop below 12 VDC and you likely won’t have enough power to operate a sensor, much less drive a signal through the wire.
Unlike voltage outputs, 4-20mA outputs can travel long distances – over 1000 ft. But we generally recommend you keep it under 2000 ft. If you go farther than this, you could begin to see power loss due to higher resistance in a long wire. Once your drop below the requisite 12 VDC, your signal will drop off as well.
However the bigger challenge at these distances is the price you will pay for that much conduit and cable. At this point you may be better off considering a wireless option.
Modbus RS-485 and other digital outputs are not susceptible to the same issues as voltage outputs or even 4-20mA outputs. They can travel long distances because of the nature of the signal. Similar to 4-20mA, they are generally not susceptible to voltage drop as long as the power supply is adequate. Generally speaking, they follow a different set of rules than analog outputs.
The industry standard for digital outputs limits the distance to 4000 ft. RS-485 travels over 2 wires in a balanced signal. Simply put there are two wires that transmit the signal.
The signal is called balanced because the signal on one wire is ideally the exact opposite of the signal on the second wire. In other words, if one wire is transmitting a high, the other wire will be transmitting a low and vice versa. This balance means RS-485 has good noise rejection thus enabling it to travel long distances.
If you need to run your cable as far as these distances it may merit a conversation about wireless options. It could save you time and money depending on your needs.
To review, the critical factors are the signal type, the power supply, the cable gauge (resistance), and electrical noise. These work together to determine how long you can run your cable.
Of course, in the real world, cost is one the most important considerations.
Still have questions? We’re happy to help! Let’s talk about your application needs and how to get the most out of your wiring scheme.
top photo credit: U.S. Navy Seabee Museum via flickr cc