Correct RS-485 cable?

[i_intub8_u]

Hello—

I can’t seem to find the correct RS-485 4 conductor stranded cable anywhere online? I’m trying to run this cable from my basement utility room to the UWC in my garage. I have a CT beast and the $2500 credit for the PowerShare gateway. I already had a UWC installed for my MYP. So I’ll be getting another UWC with the PowerShare gateway bundle. The drywall to my garage is ripped out right now due to a leaking water supply line. Before they put up all new drywall, I’m trying to run this RS-485 wire myself so I can connect the gateway to the UWC here in the future. Qmerit’s first quote was $8800 even though I already have the UWC installed. Which is insane. I’m hoping to have my local electrician do the PowerShare connection (he does generator installs all the time). But I thought while the drywall is open, I’d try to source and run this comms cable myself. Can’t seem to find it on Amazon or google when I search. I’ll probably need 40 feet total just to be on the safe side. Any suggestions? Thanks!

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[UberNoob]

Guessing here. I’d suspect they show up with a roll of wire and a baggie of these connectors. Surly China has knockoffs by the container load.

 

[i_intub8_u]

All the connections are just direct wire, no connectors. I’m just having trouble sourcing the wire. I’m trying to run it while the drywall is tore off. I’m not having the electricians come to do the PowerShare before the other construction crew redoes the drywall. So that’s why I’m looking to source the comms cable myself.

 

[cybercricket]

Something like this should work: https://www.amazon.com/LINKSQ-Conductor-Shielded-Spindle-Transmission/dp/B0DVQ5BNJS/
RS485 is relatively low speed, it's not super picky.

 

[i_intub8_u]

Here is the website Tesla has the specs for the cable. I’m still sort of confused. If I’m well under 40 feet, would 20 awg wire work? They say 16-24 awg unless you are running long distances. 18 awg for up to 82 feet and 16 awg for 150 feet run. Mine will be less than 40 feet. But then what is the second diagram? I thought the 4 wires went from the UWC to the gateway. But then the second diagram shows more connections in the gateway? I’m confused.

https://energylibrary.tesla.com/doc...UID-F8B8EBC3-ADC1-4634-AD02-96A3E0C5AE8C.html

 

[cybercricket]

Here is the website Tesla has the specs for the cable. I’m still sort of confused. If I’m well under 40 feet, would 20 awg wire work? They say 16-24 awg unless you are running long distances. 18 awg for up to 82 feet and 16 awg for 150 feet run. Mine will be less than 40 feet. But then what is the second diagram? I thought the 4 wires went from the UWC to the gateway. But then the second diagram shows more connections in the gateway? I’m confused.

https://energylibrary.tesla.com/doc...UID-F8B8EBC3-ADC1-4634-AD02-96A3E0C5AE8C.html

The wire gauge recommendations for various distances is primarily due to them sending 12V power supply over the same cable. Thicker conductors are needed to reduce the voltage drop on longer cable runs. RS485 itself doesn't really need conductors that thick regardless of distances.

You can use a voltage drop calculator to determine what percentage drop they're accounting for say when they suggest 18awg for 82ft, and then based on that determine minimum conductor size for your run of 40 feet or less.

Second diagram shows CAN wiring instead of RS485, and for a different set of devices.

 

[yohst]

Voltage drop over this distance is of little concern. You best use twisted pair or shielded twisted pair for better noice performance (in particular if its routed past electrically noisy systems). Try to avoid running long stretches in parallel with power cables and thr like.

 

[cybercricket]

Voltage drop over this distance is of little concern. You best use twisted pair or shielded twisted pair for better noice performance (in particular if its routed past electrically noisy systems). Try to avoid running long stretches in parallel with power cables and thr like.

I disagree on all points. Voltage drop can be significant over 40 feet given enough current, I will provide some calculations below. You then mention the twisted pair cable - the main purpose of twisted pair is to minimize cross-talk in full-duplex communication. RS485 is simplex, and runs on the significantly lower bitrates than Ethernet where STP is usually used. Finally, I did recommend going with shielded signal wire "just in case", even though I don't believe RS485 would be affected by the electrical noise.

Below are the calculations for a 2A load at 12V on a 40ft run :

22AWG - 13% drop, down to 10.43V
24AWG - 21% drop, down to 9.50V
26AWG - 33% drop, down to 8V

Some of these drops may be significant enough that the target device won't power on. I don't know what the actual draw of that thing is, but 2A @ 12V is pretty standard in many applications, which is why I picked that number. At higher amperage the drop will be even higher.

Finally, I do agree that a standard Cat 5E or even Cat 6 Ethernet cable can be used for this problem if there is a bunch readily available, but I would then recommend using 4 wires for power (one entire pair +12V, another entire pair GND) to reduce the voltage drop.

 

[gaspimp]

Sprinkler/irrigation control wire is a pretty good way to get quality 18/4, 18/5, and 18/7 cable locally. Sold at any home depot. Direct burial and UV outdoor rated as a bonus. Thermostat wire is another option if you need something a bit thinner and for indoor use only.

I've used sprinkler wire for RS485 based pool equipment and no issues - in fact it's far better than the trash the manufacturer supplies. You shouldn't need shielded/twisted pair at those distances, but if you think you do...CAT6 ethernet is an option too.

 

[Rohan]

Hello—

I can’t seem to find the correct RS-485 4 conductor stranded cable anywhere online? I’m trying to run this cable from my basement utility room to the UWC in my garage. I have a CT beast and the $2500 credit for the PowerShare gateway. I already had a UWC installed for my MYP. So I’ll be getting another UWC with the PowerShare gateway bundle. The drywall to my garage is ripped out right now due to a leaking water supply line. Before they put up all new drywall, I’m trying to run this RS-485 wire myself so I can connect the gateway to the UWC here in the future. Qmerit’s first quote was $8800 even though I already have the UWC installed. Which is insane. I’m hoping to have my local electrician do the PowerShare connection (he does generator installs all the time). But I thought while the drywall is open, I’d try to source and run this comms cable myself. Can’t seem to find it on Amazon or google when I search. I’ll probably need 40 feet total just to be on the safe side. Any suggestions? Thanks!

Use the Cat 5E Cable. No Connectors just bare wire.

 

[yohst]

I disagree on all points. Voltage drop can be significant over 40 feet given enough current, I will provide some calculations below. You then mention the twisted pair cable - the main purpose of twisted pair is to minimize cross-talk in full-duplex communication. RS485 is simplex, and runs on the significantly lower bitrates than Ethernet where STP is usually used. Finally, I did recommend going with shielded signal wire "just in case", even though I don't believe RS485 would be affected by the electrical noise.

Below are the calculations for a 2A load at 12V on a 40ft run :

22AWG - 13% drop, down to 10.43V
24AWG - 21% drop, down to 9.50V
26AWG - 33% drop, down to 8V

Some of these drops may be significant enough that the target device won't power on. I don't know what the actual draw of that thing is, but 2A @ 12V is pretty standard in many applications, which is why I picked that number. At higher amperage the drop will be even higher.

Finally, I do agree that a standard Cat 5E or even Cat 6 Ethernet cable can be used for this problem if there is a bunch readily available, but I would then recommend using 4 wires for power (one entire pair +12V, another entire pair GND) to reduce the voltage drop.

RS485 uses a differential voltage. Though your computed voltage drops may be correct, it is the voltage _difference_ that drives the bit stream. Many RS485 receivers tolerate absolute voltage levels between -7 and 12 volt and use just 200mV voltage _difference_ to conclude a logical one.

Because it is a _differential_ voltage scheme, keeping induced (noise) voltages as much as possible to common mode is important, hence twisted pair. Twisted pair’s pupose is not to reduce cross talk in duplex communications, it is to reduce differentially induced noise; by twisting the cable, each wire is equally exposed to a nearby noise source and thereby picks it up as common mode, not differential mode so the receiver does ‘see’ it. In duplex comms, since the two channels (receiver and transmitter) run parallel, the induced noise indeed mostly comes from cross talk. In a single channel configuration such as rs485 is, avoiding running parallel to power lines and other like sources is important.

nobody, nobody drives 2A down a 485 line. If 485 is used in a current loop (somewhat rare) 20mA is standard.

Pick up a datasheet of any rs485 receiver (maxim’s MAX485 used to be very common) for details how it works.

 

[i_intub8_u]

Thanks for all the replies. But now I’ve gotten a lot of differing opinions.
Would this work?
https://a.co/d/hoxPseK

 

[cybercricket]

RS485 uses a differential voltage. Though your computed voltage drops may be correct, it is the voltage _difference_ that drives the bit stream. Many RS485 receivers tolerate absolute voltage levels between -7 and 12 volt and use just 200mV voltage _difference_ to conclude a logical one.

Because it is a _differential_ voltage scheme, keeping induced (noise) voltages as much as possible to common mode is important, hence twisted pair. Twisted pair’s pupose is not to reduce cross talk in duplex communications, it is to reduce differentially induced noise; by twisting the cable, each wire is equally exposed to a nearby noise source and thereby picks it up as common mode, not differential mode so the receiver does ‘see’ it. In duplex comms, since the two channels (receiver and transmitter) run parallel, the induced noise indeed mostly comes from cross talk. In a single channel configuration such as rs485 is, avoiding running parallel to power lines and other like sources is important.

nobody, nobody drives 2A down a 485 line. If 485 is used in a current loop (somewhat rare) 20mA is standard.

Pick up a datasheet of any rs485 receiver (maxim’s MAX485 used to be very common) for details how it works.

The purpose of the shielding is to protect against the external sources of noise. 2A is not for RS485, it's for power supply that's being sent over the same cable.

 

[cybercricket]

Thanks for all the replies. But now I’ve gotten a lot of differing opinions.
Would this work?
https://a.co/d/hoxPseK

This one isn't a shielded cable, unlike the one I sent you earlier.

 

[i_intub8_u]

This one isn't a shielded cable, unlike the one I sent you earlier.

Oh jeez. Ok. They only have 18 awg in 30 feet. I was trying to find something longer just in case. I’d rather have excess I can trim than be too short. But I might be able to make 30 awg work.
https://a.co/d/133I9Gn

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