Voltage drop musings [RE-wrenches]

Tue May 1 16:15:57 PDT 2001

<x-flowed>
You don't need to limit voltage drop to 1%.   Various Code interpretations 
would limit it to no more than 2 or 3%, or even 5%.  This is from a Fine 
Print Note in the code.  It is a recommendation only.

The Code rulings for feeders and branch circuits would limit the drop from 
point of origin to end use to a total of 5 %.  This does not specifically 
apply to PV, but it is sometimes used as a guideline.  So far as I know, 
their is no Code limit to the VD.

Here is the NEC on the issue:

FPN No. 4: Conductors for branch circuits as defined in Article 100, sized 
to prevent a voltage drop exceeding 3 percent at the farthest outlet of 
power, heating, and lighting loads, or combinations of such loads, and 
where the maximum total voltage drop on both feeders and branch circuits to 
the farthest outlet does not exceed 5 percent, will provide reasonable 
efficiency of operation. See Section 215-2 for voltage drop on feeder 
conductors.

With a PV array without MPPT, a higher voltage drop doesn't matter 
much.  The voltage difference between battery voltage and module voltage 
will dissipate as heat.  The difference in current will not be 
great.   Current charges batteries, not voltage.

Drake

At 01:39 PM 05/01/2001 -0700, you wrote:
>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
>_____________________________________________________________
>"I stand by all the misstatements that I've made." Dan Quayle
>__________________________________________________________________
>
>- - - -
>To send a message: RE-wrenches at topica.com
>
>Archive of previous messages: http://www.topica.com/lists/RE-wrenches/
>
>List rules & etiquette: http://www.mrsharkey.com/wrenches/etiquete.htm
>
>Check out participant bios: www.mrsharkey.com/wrenches/index.html
>
>Hosted by Home Power magazine
>
>Moderator: michael.welch at homepower.com
>

- - - -
To send a message: RE-wrenches at topica.com

Archive of previous messages: http://www.topica.com/lists/RE-wrenches/

List rules & etiquette: http://www.mrsharkey.com/wrenches/etiquete.htm

Check out participant bios: www.mrsharkey.com/wrenches/index.html

Hosted by Home Power magazine

Moderator: michael.welch at homepower.com

==^================================================================
EASY UNSUBSCRIBE click here: http://topica.com/u/?bz8Qcs.bz9JC9
Or send an email To: RE-wrenches-unsubscribe at topica.com
This email was sent to: michael.welch at homepower.com

T O P I C A  -- Learn More. Surf Less.
Newsletters, Tips and Discussions on Topics You Choose.
http://www.topica.com/partner/tag01
==^================================================================

</x-flowed>