24VDC distribution to remote I/O panels from central control panel

radfahrer

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Nov 2006
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I'm looking for any tips, gotchas, or examples people might have related to 24VDC distribution over longer distances to power a remote I/O panel.

Distances are ~200ft and my estimated load at the remote I/O panel is ~7A of 24VDC (my rough estimate to power some bigger solenoids, plus ethernet switch, Eth I/P adapter and Point I/O, then the 4-20mA circuits and discrete I/O)

Brief overview of my system, will have one main control panel with Controllogix and 3 remote I/O panels with AENT adapters to Point I/O. To simplify conduit runs and reduce shock hazard at the remote panels, the plan is to have the 120VAC to 24VDC power supplies in the main panel and then run ethernet and 24VDC in same conduit out to each remote I/O panel. I figure since it's low voltage DC, then it should be OK to run those two lines in the same conduit.

My main concern is if I will have to worry about voltage drop at the remote panels. I used an online drop calculator, and keeping it <2V over 200ft looks like I need at least 8AWG wire, which seemed a bit big to me. Figure I could goose the 24VDC up a bit if I have issues, plan to use AB 1606-XL series and I think you can adjust those up to as high as 28V if you derate the current a bit.

Has anyone done something similar? Would it be better just to have them pull 120VAC to each remote I/O panel in a separate run?

Any advice appreciated,

-radfahrer
 
For those distances, I would place the power supplies in the remote panels and pull 120vac. As you pointed out the voltage drop is too big and the signal-to-noise ratio of anything it "picks" up will be big. Unfortunately, you'd have to separate it from the ethernet pull.
 
Either way, to be NEC compliant, you would have to run the power and network circuits separately.

Per NEC 725 your regular (CMP, CMR, CM, etc... type insulation) network cable "generally" falls under the Class 2 (possibly Class 3) Circuit category and as such can "generally" only be run in the same conduit or raceway with other Class 2 or 3 Circuits. At an estimated minimum load of 7 Amps (assuming that's only one of the circuits on your 24VDC bus), your 24VDC power supply cannot possibly be rated as Class 2 or 3 since there's a 100 VA limit on those. See Chapter 9, Table 11(B).

I've noticed over the years that there seems to be a general consensus that "low-voltage" is "low-voltage", and as such 24 Volt circuits can be run on "low-voltage" cables and intermixed with network cables and such regardless of that "low-voltage" power supply's rating. The real concern only seems to be in isolating "low-voltage" and/or analog circuits from 120+ VAC circuits for the sake of minimizing interference on the analog circuit.
However, when I read article 725 of the NEC, I find a discrepancy between the "consensus" and what I'm reading (my argument above being an example of that). I'm genuinely interested in finding out what others understand the "right way" to be as I often find myself to be "the only one who sees it that way".
 
Thanks for the feedback...good info all around.

I'm going to see if the client will go with the 120VAC route with separate conduit. Just seems like the simpler option at this point.

-radfahrer
 
Late to the party, but +1 on local power supplies for each remote panel. You'll spend more in copper than it will cost to buy the power supplies.


-rpoet
 
And also, running 24VDC in the same conduit as ethernet is probably OK, but if you're running it 200ft (or 61 metres as we'd say everywhere else in the world ;) ) and then turning on and off a big solenoid with an inductive kick, well, you may start to see niggling issues anyway. I'd be separating the 24VDC and the ethernet for that reason alone, and then if you're going to separate them, you might as well run 120VAC (or 230VAC as we'd say everywhere else in the world ;) ) and put in a local PS instead :)
 
I worked on one plant that was significantly larger than any my company had done before and we ended up having to parallel up some #14 cores or we found that that we couldn't even deliver enough current to trip a 10 A supplementary protector breaker, using a central 24 V DC battery bank.

When using 24VDC power supplies instead of battery banks I think you need to pay attention to what short circuit current it can deliver to make sure it has enough oomph to pop fuses, trip breakers, and energize inductive loads like solenoids.

Now we try and get as many loads on to the 120V battery bank as possible.
 

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