DC signal vs. DC power in a 4" PVC conduit

Use a SWA cable is the only advise I can give.

Is that because SWA cable has exceptional shielding? That is not one of the properties listed under SWA, also it is geneally used as power transmission, and my signal will be on the order of .250 miliamps. (tiny)

No, armored cable is not permitted in damp or wet locations! NEC code 320.12.

PVC/SWA/PVC is used all over the UK as buried cable. BS7671.

A perfect example then that one must know and use the local code and not a code from anywhere. From the NEC 2008:

ARTICLE 320

Armored Cable: Type AC....

II. Installation

320.12 Uses Not Permitted. Type AC cable shall not be permitted as follows:

(2) In damp or wet locations

Maybe a good place for fiber-optic signaling?

Can the signal be converted to something more rugged, like 4-20mA at the source? If so, then the proverbial "two bits of wet string" could be used to get from one side of the road to the other, perhaps?

"If it could be converted to something more rugged"

My OP, will the DC signal wire survive inside a 3" conduit along side of a high voltage DC cable?

Is this answer saying that it would survive if it were higher current? What is 4-20 mA?

BTW I'm sorry I don't know the "two bits of wet string proverb" Could you post it?

Thank you for responding

4-20mA, where 4mA = 0%, 20mA = 100% and 0mA = cable or transmitter fault, is such a rugged signal that nothing much will interfere with it. It can travel down any-old-lump-of-cable, as what goes in one end comes out the other. Hence the proverbial "two bits of wet string" as loop signal conductors.

My capricious view without knowing all of the circuitry tells me that there should not be a problem. I would've preferred using THHW cable below ground to meet the possible water concerns. You do meet NFPA code with the damp rating of THHN cable. Just be sure to use a similar moisture grade for the 5V signals. I would choose to use a multiconductor cable to minimize pick up loop area. A simple grounded shield in the multiconductor will be even better. One thing that is very important to know is how many cables already exist in this pipe. The maximum fill area standard for multiple cables in PVC is 40%. (Trust me, 40% is tough to pull through.)

Well after reading all that was said , i see no one has said anything about the 12" rule that is the code and out jackets having the same voltage , i see that you will have to have 2 pipes and i would run one for power and the other for what ever signal your choice is . i also see you are wanting to use THHN , why not look at XLPE as it is more for underground use and has a greater strength on water and coldness... Hope this helps ....

Read again the OP's situation. It certainly is preferred to separate different voltage cables in two paths but for one low voltage signal cable this is not practical here. There is only one conduit pipe. This is a common problem that the NEC recognizes happens and as such they do not ban multiple voltage cables in a conduit.

Again, thanks so much for the input. There are three #6 THHN in a fully sealed 3" conduit. I should have run a low voltage direct burial outside of the conduit, but I did not. (I was going to actually monitor system from the end, so I did not know that I was going to need the info at the other end. The conduit fill is well within code. The consensus seems to be that pulling a good three conductor shielded cable will do, as in, not be wacked by the high voltage DC within the same conduit.

The big question. Do you think a 3 con. 20G shielded sensor wire in PVC jacket would work, pulled inside the existing conduit?

I see no problem provided that the shield is grounded and that the cable insulation and jacket meets the standard for a damp environment. If you have yet to purchase this cable though I'd purchase a higher number of conductors than just the number of signals you know that you need now. One shielded six conductor cable will be a smaller cable than two three conductor cables.

Another option might be a radio link, perhaps?

Under the Canadian code... you would be allowed to do what you wish as long as the voltage rating of the second cable is equal to or better then the voltage rating of the highest voltage on a conductor in the raceway. See CEC rule 12-904(2)(c). While not exactly the same as the NEC, there are many many similarities.

I was going to add, but ran out of time for the edit, the cable must also be rated for a wet location.

How is that being allowed to do what I wish, since the only thing in the conduit is (3) #6 THHN cables with High DC voltage. Are you suggesting that in order to use this conduit, I must use (3) more #6 cables to move 60 mA? seems a bit like overkill, but I am most interested in being safe, and legal. I do not see an explicit NEC rule, but I've missed them before.

I said voltage not current. The insulation rating on your small cable must be at least good for the highest voltage present. Which in this case is 550 volts.

Yes of course. Thank you. I will have to check that, because the sensor wire doesn't usually have that rating.

Not a bad idea but I could not find this in the NEC standard I have either. I wonder if it's really necessary though. The cable insulation standard is for the cable residing in a conductive, grounded conduit. The insulation leakage current of the THHN wires is sufficient at 600V RMS AC. The added insulation of the low voltage cable sheath will just add insulation to what is sufficient already. I 'd be certain that this would not be a problem if the conduit was conductive and not PVC. It certainly would not hurt though using 600V rated instrumentation cable.

Hey there RF...

The reason for the requirement that the smaller cable has the same, or higher, voltage rating is to protect the circuits contained in the smaller cable. If the 550v cable faulted and the energized conductor were to contact the smaller cable, with a lower insulation rating, the insulation on the smaller cable could fail under the pressure of the higher voltage source and therebye connect that circuit to the 550 volt source.

This is especially so if the smaller cable and the higher voltage circuit share a ground connection. Hence the requirement for the insulation value on all conductors in a raceway of being equal to or higher then that of the highest voltage value contained in the raceway.

I immediately understood this fault scenario concept, which is why I said that it's not a bad idea. I was fishing for somebody to either identify this requirement in the NEC code book (good god is this thing cryptic) or to have somebody explain why it is not in the NEC code.

Apparently I also did not make my proposed reason that it might not exist in the NEC code book clear. To meet the NEC code standard there's more to any circuit than just using the correct cables. There must also be the appropriate circuit protection devices that respond to predictable faults. This provides the standard safety paradigm of requiring two things (insulation breakdown and current limitation) to concurrently fail for a disaster to happen. So if the 550V circuit breaks through the 600V rated insulation of the THHN wire and the nominal 300V insulation rating of most instrumentation cable sheaths then the 5V circuitry may get a damaging amount of current applied to the grounded shield. This increase in current draw should trip the 550V circuit protection to prevent an insulation fire. Likely this failure scenario will damage the low voltage circuitry but the 550V power will not be sustained into the system. The NEC protection is not to prevent the 5V circuitry from being damaged. That requires a considerable redesign of the whole circuitry that exceeds the scope of the NEC standard.

With a six decade electrical/electronic background, mostly in non-code, research instalations, I have confidence that this needs to be looked at as an information-transfer problem.

First, what accuracy and bandwidth is needed to transfer the signals?

Second, how do you provide the skill?

The fibre-optic route suggested is probably on the list, and may be best.

A two core (maybe coax or shielded) copper cable , bonded to one of the conductors at one end, and optically isolated at each end may meet the cost/time/skills limitations better. 5 kV is fairly easy to isolate with this approach. Good luck.