Voltage drop musings [RE-wrenches]

Tue May 1 15:15:28 PDT 2001

Wow,  that's more thought than I have put into the total of every system I
ever put in.  Not really but I'm going for effect here.

You never mention the VD with the current wire.  Are you anywhere near 5% at
48vdc?

1% @ 48vdc is definitely way over the top IMHO.  I like to stay under 2% at
53.ish for grid tieds and for off grids then well... that's usually
determined by whatever the customer brings or has on site when it's time to
pull wire or what's on my service truck when it's time.  True "Off Grid" in
the Ozarks puts you miles away from dirt roads on two track (grass
in-between 2 foot trails) USFS roads.  Try getting a loaded service truck
down one of those roads.  I've  ripped fenders off on rock outcroppings and
been two scared to loose momentum going up the hill to stop.  Well I've
digressed.   Back to your voltage drop musing.

Yes,  I put some more thought than that into it before hand but in reality I
keep 2 - 500' rolls of #2 copper on the service truck.  That's about all I
(and me helper) care to pull though +~100' of oversized well lubed conduit
with on of  ell's off and yet still flexible enough to get the last ell's
back on.  That usually works for about any system I do and I do almost
always do 48 volt systems.  I rarely even look at the charts or bother to
figure drop anymore.  I pretty much got it all in my head.  Of couse that
comes from pulling lots of wire across factory floors over the years.

Did you mean to say  "the charge controllers you are familiar with are MPPT
based"?  MPPT voltage is going to be pretty constant (at least that's what I
think) so yes, I'd use something over 60 volts for my voltage drop calc.

Please tell me more about your spread sheet based sun angle and shadow
length spread sheets.  My wife has a bad boy Compaq's handheld for her GPS
aviation based software.  It's so cool.  She plugs a GPS into it and walla
there is her flight plan and all the spec's for the next airport.  Glide
angle, approach speed and more.  Local guy makes the software for it.

Travis Creswell

----- Original Message -----
From: "William Miller" <wrmiller at slonet.org>
To: <RE-wrenches at topica.com>
Sent: Tuesday, May 01, 2001 3:39 PM
Subject: Voltage drop musings [RE-wrenches]

> Friends:
>
> I was pondering the subject of sizing wire for PV systems.  I have a
client
> that started construction of his system before he hired me.  He installed
> some undersized wire for the DC feed from his PV arrays.  I ran the
> calculations for a 1% drop and suggested a wire size to my client.  1% is
> my standard.
>
> The client is deciding when to replace the conduit and wire.  He was
musing
> out loud himself that he should do it before winter so that he can take
> full advantage of reduced solar energy in the winter.  This got me to
> thinking that in the winter, with reduced solar energy levels contacting
> the ground, the panels will probably not reach rated power.  So the winter
> is not when maximum wire sizing is needed.
>
> I pondered this question further.  How often will a given PV system
produce
> rated power?
>
> If the system is battery based, when the batteries are near full voltage,
> the PV arrays will be tapering the current output.  My conclusion is that
> this type of system will put out rated power only when battery voltage is
> low, during the mid-summer and near solar noon (for non-tracking arrays).
>
> If the system is a non-battery line tie, the inverters I am familiar with
> are MPPT based.  This means the system voltage will be at about 17 volts
> per panel, or 68 volts total.  Can I expect the PV system to put out rated
> power only if the sun is perpendicular to the panels?  I should calculate
> wire sizing based on 68 volts, not 48, right?
>
> If the array is fixed, this will be only at noon on two days of the year.
> Then how important is it to calculate for 1% voltage drop for those two
> moments in each year?
>
> Pondering.....
>
>
> William
>
> PS:  I bought my self a palm pilot (my friend named her Rosie).  I can use
> spreadsheets in the filed now so I can calculate wire sizes, sun angles,
> shadow length, etc. without having to go back to the office.  Its great!
>
> WM
>
>
>
> __________________________________________________________________
> William Miller
> SLO Communications: Communications and Power Systems Consulting
> PO Box 50, Santa Margarita, CA 93453
> Voice :805-438-5600 Fax: 805-438-4607 VMail: 805-546-4875
> email: wrmiller at slonet.org
> License No. C-10-773985
> _____________________________________________________________
> Compatibility:
> Word processor: WP7
> Spreadsheet: Quatro Pro 7
> CAD: Microstation 95, DXF, Visio 4.1T

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