Voltage drop musings [RE-wrenches]

[William Miller]

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
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