voltage drop and amp increases

Re: voltage drop and amp increases

to make sure i understand the circumstance does each pv have their own wire run to the cc with #6 or did both pvs get combined to travel through 1 pair of #6 wires of 25ft for each polarity? in other words how many 25ft lengths of #6 is there?

it can get complex as multiple pvs and their individual wire runs should be calculated separately. in other words if you have 4 pvs then pick the farthest run as a worst case scenario and figure the v drop and percent up to the combiner, but the wire run that has all of the currents from all of the pvs has to include the voltage drop with the most current flow and not just 1 pv. that is what i had difficulty explaining to the guy that put all of the formulas and data into the spreadsheet.

lay it all out for me and i will report on what i find.

Re: voltage drop and amp increases

well i'm ignoring the extra #6 that interties the pvs and i got 3.15% v drop.

now feeding pvs to that #6 wire with #10 will increase the v drop in that section and add to the already present 3.15% in the 25ft homerun. this is before we even address adding the extra pvs. now if you were able to just have 1 big pv for example that comes up to the full imp of the individual pvs and ignor the intertie wires to the combiner we get 7.5a + 7.5a + 7.63a + 7.63a = 30.26a. putting that into the calculator i get 6.35% and your finding 6% would be about correct as you have a bit more than doubled the current going through that same #6 wire. this does not yet include the voltage drops you'll see from the pvs to the combiner and i suspect you will be close to the 7% area in total.

this makes you fairly correct in your rough calculations that the v drop is out of line with those 4 pvs. #2 could bring it into the 3% area you had it at before depending on the length of the individual pv leads to the combiner box for whatever v drop that occurs on the pv leads to that combiner will add to what you get with the homerun from the combiner. 10ft total of #10 will add another .2% to the homerun v drop.

now i don't recall what kind of cc you have there, but if it is a high end mppt then there is the possibility of putting the pvs all in series to overcome the v drops. i got about .4% for a nominal 48v pv array at about 7.56a. now you see why we recommend using higher voltage when it's possible and mppt controllers that down convert are a blessing in this area of losses.

if you played with the calculator you would also see that doubling the distance also doubles the v drop %, but cutting the distance in half also cuts the v drop in half. you already learned that doubling the current will also double the v drop and as i pointed out cutting the current in half cuts the v drop in half too.

i can't tell you the best way to reconfigure your setup, but now you know what it is you are up against. if you wanted an even better v drop % even bigger than #2 would be needed for paralleling those 4 pvs. you aren't missing something after all and do note that from the cc to the batteries is even more critical if the v drop is fooling the cc into seeing the batteries as being charged. for instance if one were lax on that wire and it's distance it could show a .3v drop making a cc see 14.4v and in reality it is at 14.1v and is not reaching a proper charge.

now i believe you have a better understanding as to why we put importance on the v drop as our pv power is expensive, but so are our batteries that may not receive a proper charge if this isn't addressed there too. so called minor changes can have large impacts and your connections, switches, and even the fuses or circuit breakers can add a tad of resistance and increase the v drop. play with the calculator to see your options.

Re: voltage drop and amp increases

westbranch wrote: »

Niel, can you explain this. Is it in the resistance of the connection?

thanks,
Eric

i was only addressing part of the v drop losses just to illustrate what he was looking at as i gave the example of a hypothetical single large pv with the capabilities of his 4 pvs he wishes to use and no interconnections would be needed to attach it to the main run of #6 wire for it would connect directly. in the real world the wiring from pvs to their combiners does add to the v drop, but selectively. if for example you have 3 identical pvs and they connect to a common connection point or combiner with the following lengths of wire all of the same gauge,
pv1 has 5ft total
pv2 has 10ft total
and pv 3 has 20ft total
people should note that these are separate circuit branches and the v drop %s presented by each branch shall not be added together with each other.
the losses in the wires for pv1 are lower than the other 2. pv2's wiring will have twice the voltage drop as pv1 and pv3 will have twice the voltage drop as pv2, but 4x as much as pv1. the tricky part is that the voltage drop for pv2 does not figure into the voltage drop for pv1 or pv3 as it is a branch circuit that is on its own until it hits the combiner where the currents of all 3 pvs combine. so what do you do? figure for the worst case which is pv3 and take the v drop % that the wires from pv3 present and add it to the v drop % for the homerun and include the v drop % from wires to the batteries from the cc to get the total v drop %. to make matters more complex is that in the case of an mppt controller downconverting from say 24v nominal pvs to a 12v battery bank the v drop shall be calculated for 12v from the cc to the batteries while the rest are calculated at 24v. remember the v drop has a lesser effect at a higher voltage and in this example i presented the 24v pvs and mppt cc may have been opted for because of the lower v drop % present for the homerun to the cc. this will be a similar case for bobdog, but he will have 4 12v pvs acting as 1 48v nominal pv lowering his v drop % to the cc. now the cc to battery wires will be figured at 12v and will be somewhat critical as i explained before because a loss there can stop the battery from getting properly charged.

note here that you will not need the combiner or fuses if you are placing all 4 pvs in series.8) btw bobdog, if you expand your system in the future you can still do so using that same #6 wire and still be under 2% or 3% depending upon the v drop %s you have for individual strings tied and the cc to battery v drop %. like i said downconverting mppt controllers are a blessing.:D

Re: voltage drop and amp increases

you are a bit unfamiliar with mppt controllers i see. maybe this will help you.
http://www.windsun.com/ChargeControls/MPPT.htm
basically i can say in summary that it will convert the roughly 48v and 7.56a to a lower voltage and in doing so it will up the current proportionally minus the power used by the mppt cc and other efficiency losses. you will have more power and amps getting to the batteries than with a non-mppt type controller. it does not create this extra current or amps, but is more efficient at delivering the power and therefore more of the initial power is reaching the battery. even if you don't understand it, trust me, it'll work great for you. the only thing people complain about is the higher price tag involved with them.

Re: voltage drop and amp increases

bobdog wrote: »

It's not that I don't trust you at all. In fact I trust you guys very much, and I can grasp (I think) that it is far more efficient at delivering the amps to the batteries, but (here's the caveat) it still sounds like I would be sending 7.56 amps to the batteries instead of close to 30 amps. Am I understanding that correctly? That's a big difference in amps. OR is it like this...as the voltage drops from 48 volts down to 12 volts, the amps increase to above 7.56 amps? Please be patient with me as I'm still a bit confused.

Edit: I went to the manual for the Tristar MPPT 45 which is what I'm thinking about getting, and one figure showed that V x A in = V x A out. So, 48v x 7.56a = 362 watts = 12v x 30a. So, if I'm doing this correctly, then my 30 amps will be going to the batteries. Is this math correct?

Tim


Thanks for the article, it does help me understand, though I'm still wondering how I make up for 30 amps if I were to parallel the panels

It's not about Amps (current), it's about Watts (power). Power is voltage multiplied by current. If, for example, you add modules and reconfigure a system and doing this drops the current by half but multiplies the voltage by four, then you have doubled the power.

Re: voltage drop and amp increases

do note that the output level of the tristar45 is rated for 600w so you would not be able to expand much on the pvs with you being at 530w in pv. going to a 60a mppt would allow for any future expansion with the tristar60 being at 800w. if your battery bank were a higher voltage this would change what the mppt cc can handle on its output. you are already at a 12v battery bank and inverter i presume so unless you plan major changes in the battery bank and inverter the 12v output required will not change.

Re: voltage drop and amp increases

no problem as i only wish to inform you and i understand about the $. the wire is fine for an expansion at 48v with the low v drop %, but would require a much larger mppt cc at 12v out for adding more pvs. even the tristar 60 allowing for the 800w would translate roughly to only 4 67w 12v pvs in series. the wire is the least of your problems for any expansion with a 12v output as you see as any changes in the future i'd recommend upping the battery voltage and change the inverter to suit both the higher voltage battery bank and the highest loads that may be demanded of it. you are at a kind of plateau with the tristar 45 at 12v out. at 24v that same mppt controller will output 1200w and 2400w at 48v so the future changes might be in the form of the higher voltage batteries (and probably ah capacity too) as well as a new inverter in addition to more pvs.

as it is with the mppt cc it should work great for you. just be careful of going up in your power consumption needs that exceed your present upgrade.