Voltage drop from Array. How much would you tolerate? MPPT?

solorone · June 2013

So my array is 220' feet from my home. I have not used a MPPT during my absence from selling photovoltaic gear, and am totally having to learn the ins and outs of MPPT.
It all makes sense and seems to be a great step forward in efficiency.
I am not too concerned with the cost of MPPT, though I would have to have a back up on hand, and that is my concern. Right now there is nothing between the sun and my inverter, yes there would be a controller of sorts, but if it there was a failure, you could always go around the controller and watch/manually deal with the issue.

So how much would I hurt myself by NOT using a MPPT controller? Just sounding out the options. The other side of this is I could salvage 900' of 0000 wire, a lot of work, as the conduit has the 0000 and a pair of #2 and # 8 and 6 pair of #10, hey this was 1983, I was looking to the further.

solar_dave · June 2013

Re: Voltage drop from Array. How much would you tolerate? MPPT?

The beauty of MPPT is you can wire your panels to a higher voltage and use a much smaller wire size to control the voltage drop. Many of the controller can take 150V and one Xantrex limits is 600V. This also means less strings to deal with.

solorone · June 2013

Re: Voltage drop from Array. How much would you tolerate? MPPT?

Thanks, Yes I understand I can string together 480 V. DC and run it down my #2 pair wire. I could salvage the 0000 for a nice $$ pay day, but I just wanted to discuss the pros and cons of NOT going MPPT and how much power would I loose, how would it hurt me. Will MPPT setup produce more power on a cloudy day. Just considering options. Seems 24V array would be easier to wire together.

Cariboocoot · June 2013

Re: Voltage drop from Array. How much would you tolerate? MPPT?

Well I tried your numbers (220' of 4/0 @ 24 Volts & 128 Amps) and got some ugly results:
V-drop: 2.837 Volts
Percentage: 11.82

In other words "no go".
I think you may have made a mistake putting it in as "4 conductors per phase in parallel". DC doesn't have phases, and as far as 4 conductors in parallel on DC that could be interpreted as 4 wires total or 4 wires positive and 4 wires negative. This is the same as dividing the current. In fact using 1/4 the current I get V-drop percentage @ 2.95 - much closer to your results.

Another note: no single charge controller could handle 128 Amps anyway, so it would be impossible to run all on one set of wires & controller.

Now let's up to MPPT input using just double the Voltage (220', 4/0 wire, 48 Volts, 64 Amps):
V-drop: 1.419 Volts
Percentage: 2.96

If you up the Voltage to 120 (5X base) the current drops to 25.6 and the wire size can be reduced to 4 AWG to get:
V-drop: 2.877 Volts
Percentage: 2.4

I'm just doing this to show what happens when you change the Voltage and current (but retain the same power) over the same distance. Using the same calculator of course.

solorone · June 2013

Re: Voltage drop from Array. How much would you tolerate? MPPT?

Yes I see my error, I knew the numbers did not FEEL right from using Ohms law 30 years ago, I should have done it "long hand" LOL. I now see 1,38 V drop for one pair of conductors at 64 A. I recall a a controller that would handle that awhile back, but with the MPPT that may have eliminated the manufacture of that unit.. so it is now looking like I have wasted a bunch of space on an ill thought out post. sorry, guess I will plan on shooting 480 V to house, maybe go ahead and salvage the 4/0. Thanks for the head clearing.

NorthGuy · June 2013

Re: Voltage drop from Array. How much would you tolerate? MPPT?

By not using MPPT, you're losing some 10-20% of power just because it's not MPPT, and in addition to this you restrict yourself to he battery voltage.

I think you underestimate the benefits of a high voltage. If you go from 24V to 480V, your 128 amps become 6 amps. With such low current, you can use really thin wire (400 times thinner than for 24V to be axact). And you do not need to worry about maintaning the exact voltage because MPPT controller can accept a wide range of voltages. The only concern is that thin wire will have some heat loss. At 6A it'll be:

#14 - 40W
#12 - 25W
#10 - 16W

At 3A (panels producing at 50%), the loss will be 1/4 of this.

Even 40W is nothing for your 3kW array, so #14 will do.

solorone · June 2013

Re: Voltage drop from Array. How much would you tolerate? MPPT?

No, I understand the benefits of high voltage, I have been off line for 30 years, and dealt with voltage loss as a sacrifice to to keeping my air conditioning, a total canopy of hardwoods, I have trees that rub the house in a good wind. thus the long run to the panels. I have #2 wire in the conduit, and this should have practically no noticeable voltage drop. i simply made an error that lead me down the wrong path. I am all about not having to many things between me and the sun, I like the fact that the sunlight comes out my lights with no interference of things that can fail.

solorone · June 2013

Re: Voltage drop from Array. How much would you tolerate? MPPT?

Cariboocoot wrote: »

Well I tried your numbers (220' of 4/0 @ 24 Volts & 128 Amps) and got some ugly results:
V-drop: 2.837 Volts
Percentage: 11.82

In other words "no go".
I think you may have made a mistake putting it in as "4 conductors per phase in parallel". DC doesn't have phases, and as far as 4 conductors in parallel on DC that could be interpreted as 4 wires total or 4 wires positive and 4 wires negative. This is the same as dividing the current. In fact using 1/4 the current I get V-drop percentage @ 2.95 - much closer to your results.

Another note: no single charge controller could handle 128 Amps anyway, so it would be impossible to run all on one set of wires & controller.

Now let's up to MPPT input using just double the Voltage (220', 4/0 wire, 48 Volts, 64 Amps):
V-drop: 1.419 Volts
Percentage: 2.96

If you up the Voltage to 120 (5X base) the current drops to 25.6 and the wire size can be reduced to 4 AWG to get:
V-drop: 2.877 Volts
Percentage: 2.4

I'm just doing this to show what happens when you change the Voltage and current (but retain the same power) over the same distance. Using the same calculator of course.

Not that anyone would need it today, but I now remember selling Morningstars. think they were Pro star 30, in series I believe they could handle 72 A at 24 V. Yep looks like all this is moot, and it is a MPPT for me, no other way, but thanks for bouncing my ill prepared thoughts about.

solorone · June 2013

Re: Voltage drop from Array. How much would you tolerate? MPPT?

Wow a rough week for an older fellow, new batteries are in and 9980 # left today with the re-cycler for a heck of a nice payday. Time for a bit of rest. Thanks all.

BB. · June 2013

Re: Voltage drop from Array. How much would you tolerate? MPPT?

Measure/log (against each cell/battery) resting voltage, electrolyte levels, temperature corrected specific gravity (ideally before you even accept the shipment). If anything happens down the road, knowing the condition the batteries arrived in may help.

-Bill

Voltage drop from Array. How much would you tolerate? MPPT?

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