AC/DC in Same Conduit? [RE-wrenches]

Thu Jul 11 14:00:26 PDT 2002

David/Bill:

I don't think the Harmonic coupling is an issue for the inverter because
of all the dc input filtering.  Dc does not couple to ac and any ac
noise is filtered by the caps. 

Generally we suggest keeping the DC run short for two reasons:

1) Sometimes the inspector will require a dc disconnect at the array and
a second one at the inverter if they are too far apart.  This adds
significant extra cost.  If you have a long ac run you can use a single
2 pole breaker in a box (ala Square D AC disconnect) at the inverter and
then the utility required disconnect at the service location.

2) If you have a long dc run then the inverter has a longer "antenna" on
the dc sides to pick up lightning.  With short dc runs we normally do
not recommend any lightning protection on the dc side but with longer
run it   >> might << be a good idea.  Since I don't think there are UL
Listed arrestor at 600 Vdc (Delta LA 600 series is as close as you get -
RU for ac only) this can be a little bit of an inspector problem.

On the wiring cost issue:  The currents on the ac and dc side are
similar so it probably isn't really an issue.  One could argue that
voltage drop on the dc side is slightly better since this just reduces
performance a little bit but, IMHO, you would almost the same amount on
the ac side anyway.  

On the ac side however, voltage drop becomes voltage rise which can
narrow the IEEE 929 window (from the inverter perspective).  This is
usually only an issue if you have a utility voltage that is running high
to begin with.  Lately several installers have been reporting that PG&E
is running at up to 250 Vac.  Add a little voltage rise to drive the
current back into the grid and a little bit more for voltage drop in the
wiring and you are at the IEEE 929 limit of 264 (we trip off at about
262 to 263 by the way).  PG&E will tell you tough luck since their spec
is 252 or less.

If you have any questions, or if I can be of any further assistance,
please do not hesitate to contact me.

Best Regards,

John Berdner

SMA America, Inc.
20830 Red Dog Road
Grass Valley, CA  95945
530.273.4595 (voice)
530.274.7271 (fax) 

-----Original Message-----
From: Bill Brooks [mailto:billb at endecon.com] 
Sent: Thursday, July 11, 2002 12:29 PM
To: RE-wrenches at topica.com
Subject: RE: AC/DC in Same Conduit? [RE-wrenches]

David,

The only concern I would have with a 500' run of DC and AC is the
potential
for harmonic coupling of the AC with DC circuit. I've never tried it so
I
can't say it would even happen, but it seems questionable at best and it
seems that it could cause the inverter to operate in a funky manner. The
other option would be to put the inverter out at the array so there is
only
AC in the conduit. Would this work for you. I know the DC side is higher
voltage and would be better to run the longer distance, but this seems
like
an exception to that rule.

Bill.

-----Original Message-----
From: David Blecker [mailto:blecker at earthsys.org]
Sent: Thursday, July 11, 2002 11:12 AM
To: RE-wrenches at topica.com
Subject: Re: AC/DC in Same Conduit? [RE-wrenches]

Hmmm...so the question then becomes, is there a safety or performance
reason
not to mix voltages?  I assume Martha Stewart would not approve of such
things unless the conductors were color coordinated.

Or, is this an NEC conspiracy to sell more conduit?  I'm doing an PV
install
in remote Alaska for an locally owned utility.  No code inspectors nor
NEC
deities to frown upon our work.  Its a long run from the array to the
building (500 feet) and the thought of installing 1000 feet of conduit
for
an AC and DC run doesn't make my newt within very joyful.

Thoughts?

-David

At 10:23 AM 7/11/2002 -0700, you wrote:
>Wrenches:
>
>Now that I know there are NEC Goddesses, I'll have to pay more
attention!
>(Not entirely off-topic, Michael :-) )
>
>Mixing of "separately derived" power sources within a single raceway,
>without a physical separation, to my knowledge has always been
prohibited.
>This includes 480 VAC in the same conduit as 120 VAC, etc.
>
>I don't have time to dig out the reference(s) right this second, but,
as
>Bill B said, "Do it and become a Newt".  Do Newts still exist?  I
thought
>the last one retired a couple of years ago.....
>
>-Matt Lafferty
>pvpro at attbi.com
>
>David asks:
>
>> Can 120 VAC and high voltage (about 350 V) power cables be run in the
same
>conduit without offending the NEC gods and goddesses?
>>

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