[RE-wrenches] DC conductor line loss numbers

Mon Jul 27 08:34:22 PDT 2015

Hi:

When deciding on voltage drops and otherwise working out the design of a 
solar system, one thing that needs careful attention is to make sure that 
the array voltage does not drop below a level at which the inverter cannot 
produce full power.

This voltage will occur when the inverter is working at full input current 
such as a hypothetical 3000 W inverter which can handle a maximum of 10 
Amps of input current to produce its full rated 3000 Watts (10Amps @ 300 
Vdc). 

At any voltage less than 300 Vdc, the power will be limited by the maximum 
10 Amps.  For example at 290 Vdc you get 2.9 kW, 280 Vdc you get 2.8kW, 
and so on.

Even if the inverter can operate down to 100 Vdc, you don't want to be 
anywhere near there, especially not during the midday hours as that will 
hurt peak power production, a lot.  In this case -0.3% loss for every volt 
below 300Vdc.

Identifying this voltage for a particular inverter may not however be 
straightforward due to wide variety of wording used in specifying ratings. 
 Wording such as,

1. operating voltage range
2. full power operating voltage range
3. MPPT operating voltage range
4. minimum operating voltage
5. minimum MPPT operating voltage
6. rated operating voltage range
7. Rated MPPT operating voltage range
8. starting voltage
9. nominal operating voltage range
10. maximum input current
11. minimum input voltage
12. minimum DC voltage
13. nominal DC input current
14. maximum operating DC input current
15. maximum continuous operating DC input current
17.........

When looking for the voltage below which you don't want go, look for the 
following:

1. The maximum operating input current
2. The minimum MPPT operating voltage

Then make sure the array can produce at least that much voltage and 
current when its really hot when the resistance of all the wires is more 
than at room temperature.

JARMO
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From:
Glenn Burt <glenn.burt at glbcc.com>
To:
RE-wrenches <re-wrenches at lists.re-wrenches.org>, 
Date:
07/25/2015 12:46 PM
Subject:
Re: [RE-wrenches] DC conductor line loss numbers
Sent by:
"RE-wrenches" <re-wrenches-bounces at lists.re-wrenches.org>

You would want to confirm hot temp PV circuit values with chosen Vdrop to 
ensure the irrelevant value doesn't drop you out of the inverters' 
operational limit, however.

Sincerely,
Glenn Burt
Sent from my 'smart' phone so please excuse grammar and typos.
From: Chris Mason
Sent: ‎7/‎25/‎2015 15:20
To: RE-wrenches
Subject: Re: [RE-wrenches] DC conductor line loss numbers

It should be noted that the NEC recommendations for feeder circuits are to 
do with acceptable voltages at the load, i.e., you do not want your 208V 
equipment running on 200V. PV circuits are not feeder circuits, voltage 
drop is irrelevant. Power loss may be relevant, but only in terms of 
economic value. If it is preferable to add more PV and lose 10% of the 
power, as long as there are no issues of heat and potential fire, why 
shouldn't you be able to make that decision? As PV gets cheaper, replacing 
cables costing $10,000 to save $10 a year does not make sense. The NEC 
rules on voltage drop were never designed for PV source circuits.

On Fri, Jul 24, 2015 at 11:52 PM, Jerry Shafer <jerrysgarage01 at gmail.com> 
wrote:
Wrenches
I have 3 engineering firms and one in-house engineer that only use 1% 
because in the NEC it is stated as a suggestion and not a requirement but 
they take this as a must not exceed instead, 
I cant change CC as it will be remotely monitored via the Outback 
connection, the Engineers refuse to make any changes to the 1% and still 
stamp the drawings. This is what I have found in my research it came from 
Mike Holt

Contrary to common belief, the NEC generally doesn't require you to size 
conductors to accommodate voltage drop. It merely suggests in the Fine 
Print Notes to 210.19(A), 215.2(A)(4), 230.31(C), and 310.15(A)(1) that 
you adjust for voltage drop when sizing conductors. It's important for you 
to remember that Fine Print Notes are recommendations, not requirements 
[90.5(C)].
The NEC recommends that the maximum combined voltage drop for both the 
feeder and branch circuit shouldn't exceed 5%, and the maximum on the 
feeder or branch circuit shouldn't exceed 3%. This recommendation is a 
performance issue, not a safety issue.
Jerry

On Wed, Jul 22, 2015 at 11:28 AM, Starlight Solar Power Systems <
larry at starlightsolar.com> wrote:
Jerry,

A long distance wire run is practical now days using a high voltage 
controller. Have a look at Schneider and Morningstar 600Vdc controllers. 
Not sure what you mean "by nothing can be changed but wire size” but you 
will have to rewire the strings into series and protect the wire run.

Larry Crutcher
Starlight Solar Power Systems

On Jul 22, 2015, at 10:37 AM, Jerry Shafer <jerrysgarage01 at gmail.com> 
wrote:

Wrenches
Some time back there was discussion on the conductor size and efficiency 
rating requirement for long DC runs.
What I am looking at is this, 400 feet of MCM 400 to keep the line loss at 
or below 1% per NEC code for an off grid application, cost vs return is 
not acceptable. 2/0 is less than 2.5% and the cost is far less. Specs are 
4 strings of 3, 250 watt modules feeding one Outback FM 80 charge 
controller. There are lots of things I can do like SMA instread, or 200 
VDC charge controller but nothing can be changed except the wire gauge. 
Does anyone recall a thread with this topic.

thoughts ??
Jerry
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-- 
Chris Mason
NABCEP Certified Solar PV Installer™
Solar Design Engineer
Generac Generators Industrial technician

www.cometsolar.com

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