what % voltage drop is safe?

solarvic
solarvic Solar Expert Posts: 1,071 ✭✭✭✭
in General Solar Power Topics #1
What percentage voltage drop is still safe before wire overheats? :Dsolarvic:D
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
    Re: what % voltage drop is safe?

    I think it may help to think more along the lines of how much current is flowing through the wire. Look at an ampacity table. Voltage drop in a wire may have consequences, but overheating the wire is more a function current, temperature, insulation, etc.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    #3
    Re: what % voltage drop is safe?

    I already looked at an ampacity chart. I figured I would have a little over 7% voltage drop and still be below the ampacity of the wire. The wire is already run and I was thinking about taking out the gti inverter and connecting to a classic 150. So my question is still how much percentage voltage drop is safe, not what is more efficient. Since I already have the classic 150 I don,t want to buy the 600 vdc scniender charge controler. solarvic
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #4
    Re: what % voltage drop is safe?
    solarvic wrote: »
    So my question is still how much percentage voltage drop is safe, not what is more efficient.

    I understand your question. To answer it you need to know the resistance of the wire and its length. Then you can use Ohm's law to calculate the current from the voltage drop and the resistance. Then you can look up the ampacity of the wire and see if the calculated current exceeds the ampacity. If it exceeds the ampacity it is not safe.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    #5
    Re: what % voltage drop is safe?

    I just ask a simple question in the general sence. Was hoping for a simple answer in a general sence. I just ask what is an acceptable percentage voltage drop that is safe. The chart here gives a 5 percent voltage drop. Says you double it for 10 percent voltage drop. So in that sence my 7 percent is safe.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #6
    Re: what % voltage drop is safe?
    solarvic wrote: »
    I just ask a simple question in the general sence. Was hoping for a simple answer in a general sence.

    OK, I will try again :D

    What makes a wire unsafe is too much heat. The heat produced is proportional to the square of the current passing through the wire. Other factors, such as ambient temperature, the insulation on the wire, conduit, affect the current that may safely flow through a wire. Ampacity tables tell you what is safe.

    As far as voltage drop is concerned, if you lower the percentage of voltage drop by raising the voltage applied across the wire, you will be increasing the current through the wire, and making it hotter.

    Wires are made safe by limiting the current through them with fuses. A wire that can handle 20 amps at 12 volts can also handle 20 amps at 120 volts.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #7
    Re: what % voltage drop is safe?

    As vtMaps said this is not a question of saftety.

    If efficency is not important, then any voltage drop is acceptable if voltage is still high enough for the controller to function (usually 5-10% above highest battery voltage, e.g. 35V for 24V system).
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #8
    Re: what % voltage drop is safe?

    Part of the reason is that if you drop 5% over a 100 foot cord (10 amps at 120 volts * 5% voltage drop = 60 watts) stretched out--No problem.

    Wrap the cord it a tight coil and put it under a blanket--It will probably catch fire. Similar if that 60 watts is a drop over 2' of cables (60 watts on 2' of cable in conduit vs 100' of cable).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #9
    Re: what % voltage drop is safe?
    solarvic wrote: »
    What percentage voltage drop is still safe before wire overheats? :Dsolarvic:D
    Your question does not have a deterministic answer because ampacity and voltage drop are only sort of related. For a given amount of current and resistance, a wire of a certain length will have a calculable voltage drop. Holding everything else constant, doubling the length of the wire will double the voltage drop but the heating of the wire won't change.
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  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    #10
    Re: what % voltage drop is safe?

    Safety is best determined by ampacity not voltage drop. Voltage drop is pertinent when efficiency is the concern. Electrical designs typically use 2% voltage drop, however in solar designs where expensive PV panels are involved and current flows are continuous, much less than 2% is justified.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #11
    Re: what % voltage drop is safe?

    nec likes to use 5%, but i doubt you will have a safety issue with 7%. in solar the biggest problem is a lack of efficiency which wastes power.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #12
    Re: what % voltage drop is safe?
    niel wrote: »
    nec likes to use 5%, but i doubt you will have a safety issue with 7%. in solar the biggest problem is a lack of efficiency which wastes power.
    Voltage drop in and of itself has very little direct impact on safety. I can design a connection with 50% voltage drop which is perfectly safe, or if I ignore ampacity, one with 0.01% voltage drop that will burn up the instant the switch is closed.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: what % voltage drop is safe?

    Off-grid wire-sizing rule: handle the current first, the Voltage second. :D
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #14
    Re: what % voltage drop is safe?
    Cariboocoot wrote: »
    Off-grid wire-sizing rule: handle the current first, the Voltage second. :D
    That's the rule everywhere where electricity is concerned. Ampacity trumps Vd every time.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #15
    Re: what % voltage drop is safe?

    i agree with you ggunn. the vdrop does influence the degree of heat losses are in a wire, but if the wire is rated at the proper ampacity then there's no problem even if the wire would be extra long (high vdrop) as the heat would be properly sunk into the mass of the wire.
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  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    #16
    Re: what % voltage drop is safe?

    I thank everyone for thier replys to my question. After all my calculations I decided what I wanted to do won,t work unless I buy a 600 vdc charge controler. So am just going to leave the panels previously mentioned be connected to my gti inverter. I have 2 other arrays that are closer to my house so might change one of them to my offgrid system. solarvic
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #17
    Re: what % voltage drop is safe?
    solarvic wrote: »
    I thank everyone for thier replys to my question. After all my calculations I decided what I wanted to do won,t work unless I buy a 600 vdc charge controler. So am just going to leave the panels previously mentioned be connected to my gti inverter. I have 2 other arrays that are closer to my house so might change one of them to my offgrid system. solarvic
    Live long and prosper.
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