AC/DC in the same conduit [RE-wrenches]

[John Raynes]

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 From an electrical engineering standpoint, well... it all depends.  The 
one thing that you NEVER want to do, if at all possible, is to run high 
impedance DC SENSE or metering lines in the same conduit as 120VAC, 
definitely not without properly terminated shielding.

In the case of DC power lines running along side of AC power lines, the 
impedances of both the AC and DC lines tend to be low in comparison to 
sense circuits, so a given amount of interference energy will not induce 
anywhere near the amount of actual voltage on the other line, as it would 
in a high impedance metering circuit.

That doesn't mean that you're home free, though.  A "DC" solar array/charge 
circuit still probably uses high frequency PWM circuits to regulate charge, 
and those pulsing currents can induce small potentials on the AC 
lines.  Shouldn't be a problem unless it interferes with the sense circuits 
on the output of the inverter, or on the input circuits of an intelligent 
appliance.  I've never heard any such cases, but I could see where pulsed 
PWM noise from a charge controller could interfere (in theory) with the 
sense circuitry associated with an inverter's search function, or with 
other inverter output metering circuits.  The inverter manus would have to 
comment further as if there's any real possibility of that.

Running battery/inverter cables in the same conduit as AC lines could be a 
real mess.  There's a lot of high-energy current pulsing occurring on the 
DC lines when the inverter is powering loads, much higher than a typical 
solar charge circuit.  I wouldn't want that combination to occur in any 
circumstance, not that I could imagine that ever happening.


Conversely, AC noise, coupled onto the DC lines, could mess with the sense 
circuits on a charge controller, but I would expect that those sense 
circuits would be pretty heavily filtered in all but the lowest end 
products, since there's no reason not to.

Just my $0.02 for what it's worth,

John Raynes
RE Solar
Torrey, UT


At 08:56 AM 6/21/2005 -0700, you wrote:
>Hey Nick and Jay and all,
>
>Yes there is a provision in the code "if part of the same system" but 
>beware.  Not only can you run into interpretation issues by inspectors 
>there is a very real possibility of induced currents and noise.  I'm not 
>an expert on it but from my experience with high voltages and amperages in 
>electric vehicles, DC current can cause some serious induction 
>currents.  So you may get away with it on a code level but the physics of 
>the matter should really be checked out!  At the amperages you are talking 
>about with your PV system - they may not be a problem - but it would be 
>nice to quantify that.
>
>Anyone out there with a fundamental electrical engineering head better 
>than mine know the really issues, here?
>
>Best,
>
>Jeff C.

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