Voltage drop calculations

Steve961
Steve961 Solar Expert Posts: 93 ✭✭✭✭
in Solar Beginners Corner #1
I have been reevaluating my choice in charge controllers recently and was looking at the Morningstar SunSaver MPPT. Part of my decision is based on the distance my mount is from the charge controller and the high cost of wire at this time. I was looking at the wire size charts in the Morningstar manual (see pic below) and comparing it to the Voltage Drop Calculator I got through the forum here. I can't seem to get the numbers to agree between the two and was wondering if I did something wrong.

For example, the Morningstar manual shows a ONE WAY wire length of 90 feet for 24 volts, 10 gauge wire, 8 amps, and a 3% loss. The Voltage Drop Calculator from this forum requires a TWO WAY length, and when I enter 180 feet for 24 volts, 10 gauge wire, and 8 amps, I get a 6% drop. Since the calculators voltage drop is exactly double from the chart, it seems as if the one way/two way thing is messed up on one of these.

Any ideas here?

Attachment not found.

Thanks.

Steve
· Share on Twitter

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Voltage drop calculations

    Morningstar seems to have miscalculated here. I tried another V-drop calculator and got 6.16% drop with 10 AWG @ 90 feet one-way (180 total distance) 24 Volts and 8 Amps.
    · Share on Twitter
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #3
    Re: Voltage drop calculations
    Cariboocoot wrote: »
    Morningstar seems to have miscalculated here. I tried another V-drop calculator and got 6.16% drop with 10 AWG @ 90 feet one-way (180 total distance) 24 Volts and 8 Amps.
    My guess is that since Morningstar asked for the one-way wire length, what they gave was the voltage drop in that one wire.
    Which is not the most helpful thing in the world unless you plan to use different size wires for + and - or for Hot and Neutral.
    I would drop an email to Morningstar asking about it!
    SMA SB 3000, old BP panels.
    · Share on Twitter
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Voltage drop calculations
    inetdog wrote: »
    My guess is that since Morningstar asked for the one-way wire length, what they gave was the voltage drop in that one wire.
    Which is not the most helpful thing in the world unless you plan to use different size wires for + and - or for Hot and Neutral.
    I would drop an email to Morningstar asking about it!

    Yep. Not much point in calculating the drop for halfway, eh? That current's got to come back some day! :roll:
    · Share on Twitter
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #5
    Re: Voltage drop calculations
    Cariboocoot wrote: »
    Yep. Not much point in calculating the drop for halfway, eh? That current's got to come back some day! :roll:

    But if it's Alternating Current, doesn't it come back on the same wire? :-)

    More seriously, if you have a 240 volt center-neutral circuit, two balanced 120 volt loads will each have only one wire's worth of voltage drop, since no current will flow in the neutral.
    If you consider them as one 240 volt load, then you have twice the voltage drop, but the percentage stays the same since it is measured against twice the line voltage, and it all comes out even. (Have to think about that one some more....)
    SMA SB 3000, old BP panels.
    · Share on Twitter
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Voltage drop calculations

    It's those "two balanced 120 Volt loads" that are so hard to find.
    BTW, that practice was not allowed where I used to live in NY and I found it is allowed here in BC.

    Good thing we're talking about solar panels and not AC wiring.
    · Share on Twitter
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #7
    Re: Voltage drop calculations
    Cariboocoot wrote: »
    It's those "two balanced 120 Volt loads" that are so hard to find.
    BTW, that practice was not allowed where I used to live in NY and I found it is allowed here in BC.

    Good thing we're talking about solar panels and not AC wiring.

    Some places still allow "Edison" circuits with a common neutral shared between opposite phases, but boy are you in for a surprise when you interrupt the neutral after turning off just one circuit breaker! I found out the hard way when doing some rewiring in our basement. They were common in Illinois before 1940 and are still seen in older houses. The architect/electrician sometimes put some effort into trying to run the circuits to get a balance (all built-in lighting load on both legs, for example.)
    Voltage drop calculations are just as important when you put the inverter close to the panels but have to run the AC a long way. Its just nice that you will then usually be working with a higher voltage (in an off-grid system). In a grid-tied system the panel string DC voltage may well be higher than 120 volts.
    SMA SB 3000, old BP panels.
    · Share on Twitter
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #8
    Re: Voltage drop calculations
    inetdog wrote: »
    But if it's Alternating Current, doesn't it come back on the same wire? :-)

    More seriously, if you have a 240 volt center-neutral circuit, two balanced 120 volt loads will each have only one wire's worth of voltage drop, since no current will flow in the neutral.
    If you consider them as one 240 volt load, then you have twice the voltage drop, but the percentage stays the same since it is measured against twice the line voltage, and it all comes out even. (Have to think about that one some more....)
    That is correct. By the same token, you can figure the voltage drop in a 480V three phase balanced circuit, like the output of an inverter, as either a one wire problem, figuring the current in one wire and comparing the drop against 277V, or as a three wire problem and comparing the drop to 480V. It works out the same.
    · Share on Twitter

Categories