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DC String conduit planning question

TacoMeat

TacoMeat

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Jun 1, 2024
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Working on conduit planning for my PV wires, while I've looked at the NEC code on conduit this isn't my day job so I'm SURE I've missed something.

Question I have is, if I run 1" EMT from the inverter up into the attic is it allowed to run to a metal junction box (not hidden) then branches out into 2x 3/4 EMT runs to the East and West sides of my roof penetrations.

I'm running the Tigo TS4-A-O's and need to run 2 Tigo TAP units which you run in series, doing this split through a junction box will save me a TON of 600v communication cable, plus it'll be a lot easier to run all the wires through the 1" conduit.

Is this even something I'm allowed to do or does anyone have a better idea?
 
As long as the DC wires are in conduit or metal junction boxes you can split or run them where needed. The are not allowed in the same conduit as AC wire.
 
Kinda what I thought as I didn't see anything saying you couldn't but figure why "guess" when I'm sure someone who "knows" will be able to confirm.

Thank you!
 
Does TIGO/NEC allow the comms cables to be run in the same conduit as DC conductors? Note that those are Class 2 (low voltage limited power) so they can go outside the conduit. This seems smarter to do.

Also you need to check whether the comms cable can be forked. There will be an impedance bump at that point which will affect signal integrity
 
That's what I used to think, but https://forums.mikeholt.com/threads...-charger-amperage-control-725-26-b-1.2577048/
and

Wire insulation code question; VFD/Chillers and RS485 = 600v??

Location is North Carolina and we have some basic RS485 BACnet MS/TP communication going to VFD/Chiller/RTU etc etc. Inspector sees this equipment is wired for 480v and says every wire in the system needs to be 600volt rated. Problem is the RS485 cable sold everywhere is only 300v rated. Seems...
forums.mikeholt.com


requires class 1. Which I believe that the #18 TC-ER satisfies (I didn't fully check, though the catalog I just looked at for that cable model number does say it's for Class 1 applications). But it does not say it is a RS485 spec cable, so your signal integrity will be worse.

While RS-485 wire generally does not. And the Tigo Left hand (your video) does not agree with Tigo right hand :


Also, it sounds like you're stacking going non-RS485 of a21804 (which will degrade some SNR), with adding the impedance discontinuity in the middle with the East West branch (which will degrade SNR some more)

A -> B -> C

Easy to see that the impedance along the whole length is the same

A -> B
-> C

(with your fork in the middle)

Dunno about this.

So to be extra redundant and specifc, to keep the topology as a line you have to do CCA, East TAP, West TAP, in a sequence. If you use a separate control cable you could just pull that directly from East to West as the crow flies, without conduit.
 
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This one is 600V TC (eligible for Class 1 AFAIU), RS485


But I don't see that it is TC ER which is the type I used under the array for AC tray cable. ER = Exposed Run.

It is... Ahem... expensive.

So I dunno, maybe just ziptie cheaper cable to the outside of the conduit. And then check the rules on sharing Class 2 with power conductors inside your transition JB.

Sounds like supplementing flexible tubing (presumably 600V) around the Class 2 conductors might be OK.

Class 2 wiring systems with lighting power

Has anyone had issues with running a Class 2 0-10 dimming conductors (600v rated) in the same raceway as the power conductors supply the fixture. Looking at Art. 725.136, I could not find an application there that supported the concept of 600v making a difference.
forums.mikeholt.com
 
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If your cable is properly shielded twisted pair and has a sheath that is the same or better than the DC cable sheath you can run it in with the DC cables.... the two parallel DC cables should have magnetic fields equal and opposite in direction to cancel each other out. Would be better if the DC cables were twisted around each other as well, but that seems like overkill to me and all but impossible to achieve unless you lay it out and twist it then pull it as a unit with the rs485 alongside.

The RS-485 cable I have is 2 pairs with 1~2 twists per inch (125ohm) and has a thick dielectric foam around the wires, then foil, then braid, then the outer sheath.

Both the pv wires and the rs-485 would be class 2 wiring - neither relies on an earth ground for safety -- unless you have one of those HF AIO that puts AC on the PV wires. The rub only comes about when mixing class 1 and class 2 circuits.

In most stationary installs the PV wires are floating and not tied to battery negative or ground. If they are tied then the DC ground fault doesn't work and arc detection doesn't work.
 
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The runs I'm doing will not be over 100ft total, MAYBE 50ft run from gateway to East TAP, then maybe 20ft to the West TAP.

When the 1" ends it will be at the rafter where the roof penetrations will be, from there it should be pretty short runs. Doing a separate run is possible but the idea of adding another hole in the roof is less than ideal.

If really needed I'm sure I could twist the 5 PV run wires before feeding into conduit, then pull everything in one go.
 
twisting is just overkill - they are both class 2 wires - if it was a single DC line - the RS485 would pick up a DC reference from the steady magnetic field. With both in there it cancels out. Same reason your clamp meter doesn't work if you put it around both wires.

I would just pull it all together and see if it is a problem...if it is, then you can always pull a separate wire and do a second roof penitration.
 
robbob2112 said:
If your cable is properly shielded twisted pair and has a sheath that is the same or better than the DC cable sheath you can run it in with the DC cables....
Click to expand...
I was speaking to NEC, not noise.

I spammed a lot of NEC references and it's definitely more requirements than 600V. HOWEVER, 600V itself is correlated with being the right cable type (IE, 300V seems to tend to be lower class cable types, while I found several TC cable).
 
robbob2112 said:
Both the pv wires and the rs-485 would be class 2 wiring - neither relies on an earth ground for safety -- unless you have one of those HF AIO that puts AC on the PV wires. The rub only comes about when mixing class 1 and class 2 circuits.
Click to expand...
That's not what Class 2 means. Class 2 is voltage and power limited. With PVDC You're slightly above 60VDC and 100VA or so of Class 2. LOL.
 
TacoMeat said:
When the 1" ends it will be at the rafter where the roof penetrations will be, from there it should be pretty short runs. Doing a separate run is possible but the idea of adding another hole in the roof is less than ideal.
Click to expand...
This is why I suggested going outside the conduit then using the "plastic separation tube" clause for the final 12".

FWIW this is well trod/argued about in the 0-10V world, per one or two of the Mike Holt references I spammed.
 
robbob2112 said:
I would just pull it all together and see if it is a problem...if it is, then you can always pull a separate wire and do a second roof penitration.
Click to expand...
I would include the wiring detail in the plans.

And then ask the AHJ how they interpret the particular clause.

TacoMeat said:
The runs I'm doing will not be over 100ft total, MAYBE 50ft run from gateway to East TAP, then maybe 20ft to the West TAP.
Click to expand...

If you're not using 100% of the signal preserving recommendations from TIGO, then I'd personally make sure whatever is done, is changeable , and without wasting using a lot of expensive cable to do it.
 
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