Off-grid array 250ft away - any wiring tricks?

aj164 · December 2009

I'm putting in a completely off-grid PV array using the Xantrex XW system components. The distance from the panels to the batteries and XW equipment is just under 250ft. Using BP 190W panels, four per string, I calculate that I need either four 1/0 + gnd (combining at the array) or 21 #8 + gnd (combining at the equipment room).

BP 190W: Voc = 30.7, Vmp = 24.3, Imp = 7.8A

Are there no high-voltage solutions to the distance problem? It would be wonderful if I could have string configuration at 400V or so.

Are there any other approved "tricks", such as a bi-polar arrangement (assuming multiple XW charge controllers can share a common wire)?

Thanks -

AJB

Cariboocoot · December 2009

Re: Off-grid array 250ft away - any wiring tricks?

You've done the Voltage drop calculations: http://forum.solar-electric.com/showthread.php?t=29

There's no way around the physics. Long distance = loss. Put the array Voltage up to minimize losses and use large enough wires. No tricks, no short-cuts, no easy outs. Just make sure your array Voltage won't exceed the maximum input under all conditions - i.e. if it gets cold and the panels start to super conduct. I think those BP's are "12 Volt" panels, so it's easier to add them up to an acceptable high Voltage set-up than it would be with "24 Volt" panels. Are you using the XW 60 charge controller? It's maximum input is 140 Volts. I'm not sure of the VOC on the BP's, but you could perhaps run 7 in series, maybe 8.

I'm sure others will have valuable input on this as well.

EDIT: I found the specs on the BP190's. Their VOC is 30.6, so that would limit you to four per string unless you have severe line drop and no cold temps. Maybe pick another panel.

aj164 · December 2009

Re: Off-grid array 250ft away - any wiring tricks?

Thanks for the quick reply. I think the driving factor here is the current. I'm not sure what a lower voltage panel would do except reduce the number of circuits I have to combine. I need 7600W total. Since I'm limited to 140V, I still need to figure out how to make at least 54A go 250ft with

% voltage drop.

What is the reason for the 48A NEC Isc limit, anyway? Most people have 200A service going to their homes... :grr

AntronX · December 2009

Re: Off-grid array 250ft away - any wiring tricks?

At 7.6KW you will have 78A from PVs. Can you find some surplus 4/0 bare aluminum power line wire? At scrap yard or something? I am thinking to encapsulate it in PVC pipe per each conductor and to just bury that.

BB. · December 2009

Re: Off-grid array 250ft away - any wiring tricks?

There are multiple limits based on the expectation of safety...

60 volts max is one based on the idea that anything over that voltage is inherently dangerous to touch. And that there need to be improved isolation (such as double isolated for 120+ vac appliances or grounding of metal chassis with grounded plugs) to prevent shock.

Another is derating of wiring and fuses/beakers to 80%... It is assume that the wire is safe at 100% of load, but they want a little head room if some mistakes are made (a bit more load than planned, wiring not run as planned, etc.).

And there are issues about maximum current. A residential pole transformer is rated at 10,000 amps maximum--the core of the transformer saturates and prevents any more current from flowing... A single good sized car battery/deep cycle battery can output 1,000-4,000 amps into a dead short... Larger banks can output much more than 10,000 amps--hence why fusing/breakers rated for large interruput current is important. Too small of breaker/fuse may just arc/weld the contacts and let the excessive current through (fire risk).

Sometimes, the NEC does things that I don't believe is correct... For example limiting a 60 amp charge controller to 60*0.8=48amps maximum.

For controllers like some PWM types, they don't have current limits built in--so I can understand protecting against a solar array that may in some conditions output more than maximum current rating (such as bright sun and reflective snow).

But for MPPT type controllers, they are designed to operate at 60 amps maximum output and have internal circuits that limit output current (and power). The wiring/breaker/fuses would need to be rated 60amp/0.80>75amps -- but there is no reason to "derate" the controller to 48 amp output.

We have had this conversation here before... It would be the same as requiring that my little computer cord 60 watt power supply be rated at 1,800 watts because it is fed by a 120 vac * 15 amp service. That is insane--the computer supply is rated to be save at 60 watts and I don't need to plug it into a 0.5 amp wall outlet.

Notice, however, even though the cord power supply only draws 0.5 amps, the cord is actually much larger copper wire (probably somewhere around 18-16 awg). The cord must be able to manage the 15 amp wall outlet current to blow the circuit breaker if the cord is shorted somehow. Too small of cord wire gauge would not blow the 15 amp circuit breaker and the cord would overheat and cause a fire if shorted.

Lastly, note that AC and DC ratings for switches/fuses/breakers are different. AC switches are typically rated to a much higher interrupt voltage/current than their DC rating (if dual rated). DC current is much harder to stop flowing and sustains arcs much better than 50/60 Hz AC. DC switches need to be much larger and sometimes have special designs to interrupt DC current flow vs their AC counterparts.

-Bill

BB. · December 2009

Re: Off-grid array 250ft away - any wiring tricks?

As AntronX suggested, aluminum wire with approprately rated CU/Aluminum rated swaged connectors (like used by utility companies) may be a good option.

Also, I am not sure I see the reason to run a similarly sized ground wire with the two +/- versions. I believe that can be scaled back to 6 awg copper without much issue. Others who know code better than I can comment on that. And, I would even question the need or connections made as to if a ground wire needs to be used at all.

Your other option is to place the "battery+inverter shed" at the base of the panels and run 240 VAC or split phase 120/240 VAC from the shed to your point of use.

-Bill

System2 · December 2009

Re: Off-grid array 250ft away - any wiring tricks?

BB. wrote: »

As AntronX suggested, aluminum wire with approprately rated CU/Aluminum rated swaged connectors (like used by utility companies) may be a good option.

Also, I am not sure I see the reason to run a similarly sized ground wire with the two +/- versions. I believe that can be scaled back to 6 awg copper without much issue. Others who know code better than I can comment on that. And, I would even question the need or connections made as to if a ground wire needs to be used at all.

Your other option is to place the "battery+inverter shed" at the base of the panels and run 240 VAC or split phase 120/240 VAC from the shed to your point of use.

-Bill

This would be my vote also. It solves a number of problems.

cajun666 · January 2010

Re: Off-grid array 250ft away - any wiring tricks?

i have ther same promblem like u but my run is only 30 ft got some # 8 awg

icarus · January 2010

Re: Off-grid array 250ft away - any wiring tricks?

I vote for the battery shed, then invert and send HV to the loads. Barring that, consider double or tripling up the feed conductors. There must be a calc for the ampacity of three conductors of know size. For example would 3 # 12 wires carry 60 amps?

Tony

BB. · January 2010

Re: Off-grid array 250ft away - any wiring tricks?

Paralleling insulated wires kind of violates the ability to share current in the case of a short...

Say you have 60 amps on 3x 12 awg wires... And one gets shorted to ground... Did you have 3x 20 amp circuit breakers--one on each wire? Or did you have 1x 60 amp breaker on all three?

Now--we have a shorted 12 awg wire that either has 20 amps available or 60 amps available--obviously, we don't want to run 60 amps through the 12 awg cable as it will overheat and cause a fire.

So--we put 3x 20 amp breakers at the power end... Sounds good? Aha--Trick Question!

We have the 20 amp breaker protecting the feed from the source, but now we have 20amps * 2 as a feed from the source to the destination back up the cable to the short--and we have a 40 amp short on a 12 awg wire... Nope--not safe.

So, now we go back and gang the 3x 20 amp breakers so that they all trip if one gets an over-current... Will that work? Maybe yes, maybe no--but is not "safe".

If the short has some resistance (for example shorted to another circuit)... You could have 20 amps from the source and 40 amps taking the back way around the load still. Not safe.

So--I would not recommend paralleling insulated cables. But the question about current sharing--if the cables+breakers+connections had sufficient resistance that those 3x resistances balanced the small differences in connections (i.e., 1 ohm resistance with +/- 0.1 ohm variation--you could have pretty good current balance.

If you had very heavy cable with short runs where you had 0.1 ohms of cable resistance and +/- 0.1 ohms of imbalanced resistance--then the low resistance cable would carry more than its share of current.

-Bill

icarus · January 2010

Re: Off-grid array 250ft away - any wiring tricks?

Bill,

I realize that I am way out of my depth here but how about this:

The issue here is not the ampacity of the wire per se, but the fact that we are trying to over come line loss that is exacerbated by a combination of low voltage and long wire runs.

So let's say we are trying to carry 60 amps. #6 would carry 60 amps, so fused for 60 amps, with the wires tripled, even if any wire failed, the results would be that while the wire is protected with a proper sized breaker, the voltage drop would just be bigger.

As long as the fuse is in place for the SMALLEST wire size I don't see a problem.

Tony

BB. · January 2010

Re: Off-grid array 250ft away - any wiring tricks?

Fusing at the size of one of the insulated wires would be OK.

There is a secondary issue where one of the calculations is that a long wire run needs to have low enough resistance to pop the breaker/fuse. I am not sure how NEC would see the issue--but for off-NEC installations it would work OK. Of course, if this is the panel run--the panels are going to be current limited anyway.

-Bill

niel · January 2010

Re: Off-grid array 250ft away - any wiring tricks?

bb,
if a single wire meets nec requires then one would assume that the resistivity requirements are also within the scope of those requirements and adding a second wire would only serve to lower said resistance further. the nec would probably still reject it though because, in my guessing of their reaction, this resembles circumstances that can fail if undersized for a single wire. imo, if it passes for a single wire, then doubling that same wire with the same wire or larger should be allowed. most of the time we are using wire gauges large enough in solar that for example a cb should fail to trip that the wire in many cases will not be harmed as we oversize to minimize voltage drops.

mike95490 · January 2010

Re: Off-grid array 250ft away - any wiring tricks?

At some point, all the extra breakers and convoluted thinking, is going to cost more than just getting #2 wire and doing it right, to avoid the voltage drop. Even if inverted and 240VAC was run, the inductance and resistance may trick the inverter to trip offline once in a while.
Like the hot rod folks say, there's no replacement for displacement.

halfcrazy · January 2010

Re: Off-grid array 250ft away - any wiring tricks?

I will take a stab at this. You will have 7600 watts of pv feeding 2 Xantrex XW controllers. and the panels voc is 30 volts roughly.

So 3800 watts and we will assume around 105 volts input to the mppt controller will give us 36 amps so #2 copper at 250 ft one way has about 2.9% voltage drop. You would be fine pulling 4 pieces of #2 for arrays the xw's need separate neutral conductors. then I would pull a #4 for the ground. Hope this helps some.

mike95490 · January 2010

Re: Off-grid array 250ft away - any wiring tricks?

#2, #4. Arghh. I've got to run about 400' from inverter to a 230V well pump, and I'm looking at #2 aluminum instead of #4 copper. Big price difference. But if you are already up to #2 copper, that would need #1 aluminum to replace it, and that means a larger conduit too. And something about Aluminum only gets surface oxide, and copper eventually gets oxidized all the way thru the core.

Dave Angelini · January 2010

Re: Off-grid array 250ft away - any wiring tricks?

For the CC I would wait for this summer if you can!

halfcrazy · January 2010

Re: Off-grid array 250ft away - any wiring tricks?

Yes if you can wait a while there are a Couple high voltage cc's hitting the market this summer

Off-grid array 250ft away - any wiring tricks?

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