Voltage drop length of wire (times two????)

Started by pablo, October 18, 2016, 05:41:32 PM

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pablo

October 18, 2016, 05:41:32 PM
Howdy,

I was just reading  my inverter manual and in the wire sizing chart it says to multiple the length of wire x 2 for purposes of calculating wire/size and voltage drop.    It is rather poorly written and I am hoping not saying what I think it is saying, because I have never multiplied wire length x 2 in my system design!  Have I been doing it wrong?

thank you



Off grid 12 volts, 2 Trojan L16RE batts, Samlex 2K inverter, Xantrex 40 Amp charger, Outback Flexmax 60 Amp MPPT charge controller, 2 Solar World 285 W panels, Honda 1000 Watt genny.

TomW

#1
October 18, 2016, 05:47:36 PM Last Edit: October 18, 2016, 05:49:21 PM by TomW
Quote from: pablo on October 18, 2016, 05:41:32 PM
Howdy,

I was just reading  my inverter manual and in the wire sizing chart it says to multiple the length of wire x 2 for purposes of calculating wire/size and voltage drop.    It is rather poorly written and I am hoping not saying what I think it is saying, because I have never multiplied wire length x 2 in my system design!  Have I been doing it wrong?

thank you

It is simply that the run is one conductor out and one back!!

Those electrons need a complete path.

It is for ease of calculation.

Cable resistance is generally expressed as ohms per foot so you need all the feet as it were.

Not sure what you are concerned about.

This is DC electricity 101 stuff. And a well established standard for these calculations. Trust me.

Tom
Do NOT mistake me for any kind of "expert".

( ͡° ͜ʖ ͡°)


24 Trina 310 watt modules, SMA SunnyBoy 7.7 KW Grid Tie inverter.

I thought that they were angels, but much to my surprise, We climbed aboard their starship and headed for the skies

CDN-VT

#2
October 18, 2016, 06:03:17 PM
Pablo
If your inverter is 6 feet from the battery bank measuring from the out of the positive to the breaker & then to the Positive in on the inverter  And Your Negative is from the battery to the shunt & then to the inverter negative in is 7.6  feet  you would use 13 .6   feet  or round it to 14 / 15 feet to be safe.
Use that with the Max AMP draw for cabling size.
Your lucky your not asking on the long runs of solar panels to the combiner/CC ..

VT
Canadian Solar 350W 37.6 VOC  30.6 VMP 8.22 ISC 7.87 IMP ,-15 c +30c max  4 strings in 2 in Series for 24v Classic 150 -1020 Ah  Freezers & fridges ~~~ Second Array same panels of 3sx3 parallel for 24 V Classic 150 -440 Ah Outback Barns & out blds.
48Vdc almost done,11Strings up of 3s11P same panels

pablo

#3
October 18, 2016, 07:05:01 PM
Thanks guys.

So my array is 75 feet from the charge controller and I used 75 feet to calculate voltage drop and wire size.  I should have used 150 feet?  You can see my concern.  If this is true then I probably have undersized my conductors system-wide and probably have too much voltage drop not too mention possible safety issues...    :(

Off grid 12 volts, 2 Trojan L16RE batts, Samlex 2K inverter, Xantrex 40 Amp charger, Outback Flexmax 60 Amp MPPT charge controller, 2 Solar World 285 W panels, Honda 1000 Watt genny.

ClassicCrazy

#4
October 18, 2016, 08:27:31 PM
I believe some of the calculators online have the round trip distance already calculated. But if the one you used said to multiply by 2 then it probably didn't.

What voltage and amps did you use for your calculation ? What wire size ?

Larry
system 1
Classic 150 , 5s3p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system 2
 5s 135w Kyocero , 3s3p 270w Kyocera  to Classic 150 ,   8s Kyocera 225w to Hawkes Bay Jakiper 48v 15kwh LiFePO4 , Outback VFX 3648 inverter
system 3
KID / Brat portable

pablo

#5
October 18, 2016, 08:41:05 PM
Actually the calculators I used did NOT say to multiply by 2.  This is just something I noticed later in my inverter manual and made me wonder if I should have multiplied by my lengths by two. 

For the run from the panels to the house:  39 VOC, 9 amps, and 4/0 AL conductor.
Off grid 12 volts, 2 Trojan L16RE batts, Samlex 2K inverter, Xantrex 40 Amp charger, Outback Flexmax 60 Amp MPPT charge controller, 2 Solar World 285 W panels, Honda 1000 Watt genny.

ClassicCrazy

#6
October 18, 2016, 09:10:43 PM
According to the windsun calculator
Results:
A maximum distance of 556.14 feet will limit the voltage drop to 3% or less with a 4/0 AWG Aluminum conductor delivering 9 amps on a 39 volt system.

So I guess you are okay !

http://www.windsun.com/Hardware/Voltage_Calc.htm
system 1
Classic 150 , 5s3p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system 2
 5s 135w Kyocero , 3s3p 270w Kyocera  to Classic 150 ,   8s Kyocera 225w to Hawkes Bay Jakiper 48v 15kwh LiFePO4 , Outback VFX 3648 inverter
system 3
KID / Brat portable

ClassicCrazy

#7
October 18, 2016, 09:19:16 PM
system 1
Classic 150 , 5s3p  Kyocera 135watt , 12s Soneil 2v 540amp lead crystal for 24v pack , Outback 3524 inverter
system 2
 5s 135w Kyocero , 3s3p 270w Kyocera  to Classic 150 ,   8s Kyocera 225w to Hawkes Bay Jakiper 48v 15kwh LiFePO4 , Outback VFX 3648 inverter
system 3
KID / Brat portable

CDN-VT

#8
October 18, 2016, 10:43:40 PM
Quote from: pablo on October 18, 2016, 08:41:05 PM
Actually the calculators I used did NOT say to multiply by 2.  This is just something I noticed later in my inverter manual and made me wonder if I should have multiplied by my lengths by two. 

For the run from the panels to the house:  39 VOC, 9 amps, and 4/0 AL conductor.

You have only one string of panels ?
Pablo , what is your array size & wiring configuration as in series & Parallel feeding the 75 foot away or 150 foot in round trip ?

VT
Canadian Solar 350W 37.6 VOC  30.6 VMP 8.22 ISC 7.87 IMP ,-15 c +30c max  4 strings in 2 in Series for 24v Classic 150 -1020 Ah  Freezers & fridges ~~~ Second Array same panels of 3sx3 parallel for 24 V Classic 150 -440 Ah Outback Barns & out blds.
48Vdc almost done,11Strings up of 3s11P same panels

Halfcrazy

#9
October 19, 2016, 05:33:10 AM
Changing the way wind turbines operate one smoke filled box at a time

dgd

#10
October 19, 2016, 05:55:22 AM
...errr this doesn't look right.
What happens if the neg to earth tie is used in the Epanel or MNSC and the PV array negatives are tied to an earth as well, at the PV array mounting?  Is that insufficient to ensure the negs at the controller and PV array are equal?
In any case how could you ever measure the actual voltage loss in a -ve cable earth ties at both ends?
So you connect a fluke voltmeter at the PV end of both -ve and +ve bus connections eg in PV combiner and take a voltage reading.
Then you measure the voltage at the Classic PV input terminals?
So does that mean that both the -ve and +ve are voltage dropped? the +ve is lower and the -ve is higher?

dgd

Classic 250, 150,  20 140w, 6 250w PVs, 2Kw turbine, MN ac Clipper, Epanel/MNdc, Trace SW3024E (1997), Century 1050Ah 24V FLA (1999). Arduino power monitoring and web server.  Off grid since 4/2000
West Auckland, New Zealand

Halfcrazy

#11
October 19, 2016, 11:41:32 AM
Well in our case it is not code compliant to wire it like that. BUT that said If the PV - line was earthed on both ends and you did v drop calculations based on the 2 conductors you would just not have as much drop as the resistance on the negative side would be less
Changing the way wind turbines operate one smoke filled box at a time

pablo

#12
October 19, 2016, 01:49:39 PM
Ok thanks everybody.  It sounds like I am ok.  Freaked out for a minute there....I'm glad I over-sized my conductors throughout the system. 

VT: I'll let you know.  Don't have my diagrams with me at work right now.  But yes, 1 string of 2 panels at 285 watts each.  I believe wired in series.
Off grid 12 volts, 2 Trojan L16RE batts, Samlex 2K inverter, Xantrex 40 Amp charger, Outback Flexmax 60 Amp MPPT charge controller, 2 Solar World 285 W panels, Honda 1000 Watt genny.

CDN-VT

#13
October 20, 2016, 01:42:10 AM Last Edit: October 20, 2016, 11:23:06 AM by CDN-VT
Pablo 
Post 10 11 & 12  do not effect you.


VT
Canadian Solar 350W 37.6 VOC  30.6 VMP 8.22 ISC 7.87 IMP ,-15 c +30c max  4 strings in 2 in Series for 24v Classic 150 -1020 Ah  Freezers & fridges ~~~ Second Array same panels of 3sx3 parallel for 24 V Classic 150 -440 Ah Outback Barns & out blds.
48Vdc almost done,11Strings up of 3s11P same panels

pablo

#14
October 20, 2016, 09:29:17 AM
I figured.  Thanks.
Off grid 12 volts, 2 Trojan L16RE batts, Samlex 2K inverter, Xantrex 40 Amp charger, Outback Flexmax 60 Amp MPPT charge controller, 2 Solar World 285 W panels, Honda 1000 Watt genny.
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