Author Topic: Voltage drop wiring question  (Read 277 times)

kauaisolarman

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Voltage drop wiring question
« on: October 17, 2016, 02:03:22 AM »
so i am trying to choose the correct wire size for my application and i have been playing with the voltage drop calculator online to figure out my voltage drop percentages per the different wire sizes

here it is
http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.8152&voltage=109.5&phase=dc&noofconductor=1&distance=150&distanceunit=feet&amperes=37&x=53&y=21


anyway my question is what is a good/safe/happy voltage drop percentage to shoot for?

i have heard 2% or less? 
in that case would 2.87% be too much and need the next wire size up?

thanks for any input
2 solar systems in progress;
1. 24V interstate battery; 5x sanyo hit 190Watt in parallel to classic 150-SL + 8x Shell 140 watt series/parallel to classic 150-Sl (follow me).

2. 48V aquion battery; 6x panasonic HIT 325, 6x suniva 315,  CC TBD.

RossW

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Re: Voltage drop wiring question
« Reply #1 on: October 17, 2016, 04:14:45 AM »
my question is what is a good/safe/happy voltage drop percentage to shoot for?

There are so many "brain-dead" so-called professionals out there who can't count past 10 without taking their shoes and socks off that some regulations were required - at least for mains work. The average homeowner is even less informed about elecktrickery.

For your low-voltage, own-consumption PV - notwithstanding any regulations, legislation, rules or other artificial restrictions - there isn't really an answer to your question... it depends on how much power you're prepared to drop in the cables and how much you're prepared to spend.

The cable doesn't care if you're losing 10%, 20%, 50% - as long as it doesn't burn out. That's unlikely to happen below a temperature that'll burn you if you touch it...
How much the cable will drop is a function of its resistance and the current you're shoving down it.
The current you can work out from the voltage and panel wattage - near enough anyway.
The resistance is a function of the cable cross-sectional area, the conductor material, and the length. Aluminium is cheaper than copper, but has a higher resistance.
Doubling the distance is the same as using a wire that's half the CSA. So keeping your runs to the minimum length you can will save you on losses or money on cable.

Personally, I look at how much I'm dropping in the cables, and convert that to money over the life of my installation. If I'm "wasting" 20 watts in my cables at full generating capacity, by say, 3 hours a day that's roughly 20kWh/year. If that is "worth" 20c/kWh to me, that's $4/year, or $40 over 10 years. How much larger cable can I buy for $40?  (To convert to money, think "if I need to run the generator to make up the shortfall, what's it cost me?)

Others will likely have a different slant on the equation :)
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Kent0

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Re: Voltage drop wiring question
« Reply #2 on: October 17, 2016, 01:03:58 PM »
I'm basically in agreement with RossW that for PV circuits, especially at low voltage, the 2% rule should just be tossed. I do suggest a slightly different computation.

1) You don't want a dangerous situation, so finding a safe wire is the first step. Determine the minimum allowed wire size for current and the environment that the wire is in. If the wire on the roof or in a conduit with many other conductors, its ampacity should be derated as specified in the NEC.

2) Determine the %loss for this wire. The %voltage loss = the %power loss, so you can use that to determine the number of watts lost in the wire. Do the same for the next larger wire size.

3) Compute the cost increase for using the larger wire. Compute the watts saved by using the larger wire. Take the ratio of these two values to find the $/watt figure.

4) With the $/watt figure you have to decide if the larger wire is worth it. Consider the cost of a PV module in $/watt or the cost of putting in a PV system in $/watt. Using arbitrary figures here, the question becomes do you buy larger wire at $7 per watt when you can buy PV power at $3/watt?

5) If the next wire size is worth it, repeat the exercise until you eventually find a wire size that isn't worth it. My experience with low voltage wire is that 2% loss is often way too low to justify.

« Last Edit: October 17, 2016, 01:07:10 PM by Kent0 »

TomW

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Re: Voltage drop wiring question
« Reply #3 on: October 17, 2016, 01:31:04 PM »
Over time my views on the 2% "rule of thumb" has changed.

Overall, I would agree with Ross and Ken. Especially with the low cost of modules these days, you can toss another couple hundred watts of PV at a system perhaps cheaper than fatter copper to offset the loss in cable. So these "rules" vary with the timing. $5 a watt solar made copper seem cheaper not so long ago.

Staying in the safe zone for ampacity is essential, regardless.

So, it is, indeed, a question only the one asking can answer after some research or advice.  2% is a nice goal but certainly not a deal breaker if you go higher within reason. We will be up near 3% on our current planned system using #4 to carry around 50 amps over a 60 foot run. On grid, off grid change the dynamic as well.

Your Mileage may Vary and Do not attempt at home, closed course professional drivers, etc.

Tom
« Last Edit: October 17, 2016, 01:32:56 PM by TomW »
Do NOT mistake me for any kind of "expert".

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Vic

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Re: Voltage drop wiring question
« Reply #4 on: October 17, 2016, 01:50:32 PM »
I,  also,  generally agree with the above,  particularly with the further refinement,  that the above is really for PV circuits,  as stated by KentO,  especially for MPPT CCs.

For PV cables,  feeding PWM CCs,  cable voltage drop can be  a bit more of an issue,  but the target system does NOT use a PWM CC ...

And,  of course,  voltage drops in battery interconnects and inverter cables,  voltage drop can be a very important consideration.   Generally,  the lower the system voltage,  the greater the importance,  due to higher currents for a given amount of power being handled.  2-ish percent drop on interconnects,  inverter cables,  or on the CC-to-battery cable would be excessive.  For the CC to battery connection,  one should also try to add some drop that exists at current in the circuit beaker that protects the CC to battery cable,  particularly on systems that have considerable Opportunity Loads,  during Absorb.

Not to try to lecture,  too much.
FWIW,   Vic

Off Grid - Two systems -- 4 SW+ 5548s, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 3X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  Thanks MN for Great Products/Service/Support!

CDN-VT

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Re: Voltage drop wiring question
« Reply #5 on: October 17, 2016, 10:14:43 PM »
Nope no lecture, just all good views from all of the replys.

One other thinking would be if in the future , you were going to update,or build on further , undersized is a huge hamper later.
I have found WARM joints & a few wires that were poor & undersized . Needed new wire & conduit .. So skimming  to close could cost later also.


VT

Also !! The length is measured in a complete circuit , so MY panels are from combiner to  negative run is 40 feet to top panel and my positive on the same string is 55 feet from the combiner to the bottom panel &  the panel cables adding all 3 in series is 18 feet so 18+55+40 = 113 foot run .

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.

EDIT ADD !
On the posted on the first OP thread of the link he posted , They have this " Please use one-way distance to the load. Not round trip distance."  So here I would always use my longer run length if there was a difference like my panels.

http://www.calculator.net/voltage-drop-calculator.html

Some don't state if they want round trip or one way , so garbage in is garbage till you know.

VT

http://www.solar-wind.co.uk/cable-sizing-DC-cables.html
« Last Edit: October 18, 2016, 06:53:18 PM by CDN-VT »
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russ_drinkwater

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Re: Voltage drop wiring question
« Reply #6 on: January 10, 2017, 03:23:23 PM »
No matter what you decide do not be cheap with the cable you choose. Within ballpark will do, but will it last and what if you want to push more power through the length later.
Go for the slightly larger cable and save some $$$$ later one through increased life.
A bit like A/C  units when working out the size you need. Always go for a bit extra capacity justincase or forlateron.
Always worked for me.
Standalone. 20 Hyundai x 220 watts panels, 2 x classic 150's, Latronics 24 volt 3kw inverter, 8 x L16 Trojan 435 amp hour Batteries and Whiz bang Jnr, 12 Rolls surrete  4KS 25P  batteries and WBJ.
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niel

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Re: Voltage drop wiring question
« Reply #7 on: January 10, 2017, 09:14:18 PM »
just a few quick thoughts here on the subject. it is to be the best you can manage to buy that will not thwart your goals or safety. 2% is a good goal, but not etched in stone and i've often said 3% to be more realistic. future use is a big concern as copper prices won't be going down in general so the barely made it wiring will probably need replacing if there's any future expansion and even if no expansion on larger wires it will translate to more power delivered when full output is present.

that's another point as often full power is not present and most of the time losses will be lower due to lower production. this would be during sunrise/sunset and cloudy/rainy times. there is fudge room, but going too cheap can cost you more.
« Last Edit: January 10, 2017, 09:17:42 PM by niel »

TomW

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Re: Voltage drop wiring question
« Reply #8 on: January 11, 2017, 10:24:02 AM »
Like others have said.

Don't go cheap on cable. Most systems tend to grow, not shrink so having a bit of headroom on cabling is just smart.

I agree, the 2% bit is not a deal breaker if you don't get there but if you factor in the future you probably want to go with larger cable.

It is like many things in life, nobody can tell you what you need, only you can sort that out from the facts.

Best of luck.

Tom
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my t30

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Re: Voltage drop wiring question
« Reply #9 on: January 22, 2017, 10:47:46 PM »
Like others have said, It comes down to cost per watt lost. I tried to stick to the 2% on my first system with a shorter wire run. But then because of location, I put in a 750 wire run and the 2% went out the window. I'm running at about 14-16% wire loss(which was the balance between wire cost and panel cost at the time) and its been working fine. It's not ideal but it has worked in my situation. Also I'm off grid which makes a bit of difference.

Good luck and enjoy
Philip