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

Bill Brooks bill at brooksolar.com
Tue Jun 21 15:30:26 PDT 2005 

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Nick,

I don't think the code cares about electronic noise issues when it says no
dc and ac in the same raceway. I think the code is focused on safety. The
concern is is getting dc and ac connected together and causing saturation of
ac transformers and other electrical hazards. If dc and ac of the same
system (a la multiple inverters with a single gutter), then the conductors
must all be rated for the highest voltage of either system (normally 600Vdc
is the highest voltage for single-phase inverter systems). If the dc and ac
wires in a system like this get crossed, it will clear all the fuses and
breakers and should leave the system dead and open to the ac side (dc side
may still have voltage and/or current), but it is unlikely that dc will ever
make it to the ac system. I hope this adds a little to the understanding.

Incidentally, all sensing and communication wiring, if installed in the
gutter, must be rated at the maximum voltage. This can be a real hastle with
various Cat5 and similar cables.

Bill.

-----Original Message-----
From: Nick Lucchese [mailto:nickl at sierrasolar.com] 
Sent: Tuesday, June 21, 2005 11:08 AM
To: RE-wrenches at topica.com
Subject: Re: AC/DC in the same conduit [RE-wrenches]


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Thanks John, Jeff, Dan and Jay,
  Yes, to what degree that this may be a problem sounds like is still 
somewhat of a mystery and probably won't be a problem from what you 
all have shared. Unfortunately the customer's mind will most likely 
not be at ease unless I can prove that it surely won't be a problem 
for him. The MX-60 is at the end of the line so hopefully there won't 
be any PWM noise on the AC line. If the wire is re-pulled at least 
the twisted Tri-Metric wire and Mate cable can be placed in the 
conduit alongside the DC solar wires where the undersized #6 AC wires 
are now. Luckily they did wire the battery/inverter cables alone 
although they only hooked up one set and stole/took the other set 
home to hide so the customer would not know they were left over 
-similar to the Tri-Metric and GFP/2 that was pushed aside in the 
"parts left over" area.

  I would still like to hear more on the fundamental electrical 
engineering issues as Jeff stated if anyone has anything to add. 
Thanks again for all the responses,

Nick A Lucchese



>
>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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