user11174: Let's say someone has four CAT5 cables that are running along the ceiling for the length of a room (30 feet), of which most of the twisted pairs are connected to Arduino input/output pins on one end. Would it be okay to keep these wires bundled together when running along the ceiling? Or is it safer to keep them separated by a few inches? Is interference usually a problem only in very specific situations, or should it be considered for all long wire runs?
How about if there is a telephone cable (hooked up to a landline telephone) running along the same length? Is there any chance of interference if this is too close to the Arduino cables?
How about if there are 120v mains lines running the same length? Should these be kept at a distance from the Arduino cables?
On the other end of the cables that are hooked up to the Arduino, some of the following are connected: temperature sensors, light sensors, servo motors, LCDs, relays, and push-buttons.

user7856: It depends on the frequency/frequencies you care about transmitting on your cables. One simple approach is to put on both ends a filter that lets pass only those frequencies you care about.
You can also consider using some error detection/correction encoding or check if there is some existing library that suits your needs. This last approach will work if you do not get too many interferences and your application can tolerate that data is re-transmitted every now and then, making the communication timing not 100% deterministic.

user14231: A benefit of using CAT-5 is that the pairs are twisted.  You would want to look at how you are using the pairs so that you're not introducing cross talk inadvertently, but the twisted pairs tend to resist RFI which is why its used.  Telephone wire, however, can be a completely different story.  You may find some that is twisted pair, but most if it is simply flat cable.  Worse, much of it is very cheap flat cable with what amounts to a piece of thread with a very thin foil wrapped around it.  This is very high loss and very vulnerable to RFI.

11174: <p>Let's say someone has four CAT5 cables that are running along the ceiling for the length of a room (30 feet), of which most of the twisted pairs are connected to Arduino input/output pins on one end. Would it be okay to keep these wires bundled together when running along the ceiling? Or is it safer to keep them separated by a few inches? Is interference usually a problem only in very specific situations, or should it be considered for all long wire runs?</p>

<p>How about if there is a telephone cable (hooked up to a landline telephone) running along the same length? Is there any chance of interference if this is too close to the Arduino cables?</p>

<p>How about if there are 120v mains lines running the same length? Should these be kept at a distance from the Arduino cables?</p>

<p>On the other end of the cables that are hooked up to the Arduino, some of the following are connected: temperature sensors, light sensors, servo motors, LCDs, relays, and push-buttons.</p>
 7856: <p>It depends on the frequency/frequencies you care about transmitting on your cables. One simple approach is to put on both ends a filter that lets pass only those frequencies you care about.
You can also consider using some error detection/correction encoding or check if there is some existing library that suits your needs. This last approach will work if you do not get too many interferences and your application can tolerate that data is re-transmitted every now and then, making the communication timing not 100% deterministic.</p>
 14231: <p>A benefit of using CAT-5 is that the pairs are twisted.  You would want to look at how you are using the pairs so that you're not introducing cross talk inadvertently, but the twisted pairs tend to resist RFI which is why its used.  Telephone wire, however, can be a completely different story.  You may find some that is twisted pair, but most if it is simply flat cable.  Worse, much of it is very cheap flat cable with what amounts to a piece of thread with a very thin foil wrapped around it.  This is very high loss and very vulnerable to RFI.</p>
