Wire size substitions

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Dabbler
Dabbler Registered Users Posts: 22
Sorry if this has been covered elsewhere, there's so much good stuff to read here!

I was wondering if there's an easy way to compare multiple runs of smaller wire size, versus one run of a large size.

For example, I'm looking at making an 80' run (one way) that needs to be 1/0 gague to keep voltage drop under 3%. I have a good low price source for 6 gague, but I'm not sure how many I'd need to run in parellel to be equivalent to 1/0 gague. If I knew that, then I could compare prices. (plus, I'm thinking that dealing with several 6 gague wires might be practically easier than a couple stiff 1/0s, given that it will be buried in conduit and has to make several bends)

Is there any easy rule of thumb for this comparison? I have the diameters for the various wire gagues, would it be the ratio of their cross sectional area?

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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
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    Re: Wire size substitions

    An old eletrician friend told me that the rule of thumb (RoT) is 3 gauges and the wire size is doubled or halved...ie 7 is double the size of 10, 13 is half of 10. May be a bit wrong but for your application ... 6 is less than 1/4 of O(t)

    Cheers
    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
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    West Chilcotin, BC, Canada
  • morrisrv
    morrisrv Registered Users Posts: 13
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    Re: Wire size substitions

    If the NEC matters where this installation will be, 310.4 does not allow wire smaller than 1/0AWG to be paralled. there are some exceptions, but I don't think they apply here.
    Bob
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Wire size substitions

    You can make the single/parallel wire run based on solar panels (one group of four, 2 groups of 2, 4 groups of one, etc.)... Because each panel (group) is the only source of current for each circuit, you are safe running parallel runs (for what ever reason) to a common destination as there is no issue of improper current sharing.

    Just use the Wire Calculator in the sticky and just allocate the current to the wire run (1x4 vs 4x1, etc.).

    Personally, I too do not expect parallel wires/contacts to properly share current (from a common source to a common destination). I frequently had to figure out internal wiring for computer systems, and many time it would have been really nice if I could have just used one ribbon cable to bring signal and power to a daughter card of some sort.

    From my experience, you can only guarantee current sharing if you have a calibrated resistor (or resistance) in series with each run--that is significantly higher resistance than that of the conact+wire run resistance.

    Of course, nobody does that and very frequently I would see systems with (for example) 5 wires/contacts rated at 1 amp per set, running 4 amps, and after a year or three-there would be cascading burned contacts (first one would fail, then heating the next, causing it to fail, etc.)... The problem was that the contacts generally had the highest and most variable resistance--so there was always one contact with a bit higher resistance than the others (but the rest of the contact/wire run averaged low resistance--enough to start heating, but not high enough reduce current flow significantly). So, typically (IMHO) the weak point in paralleling is where the wires join (wire nuts, binding screws, etc.).

    My personal design rule was I would limit using parallel wire current increase by a 1/n type series:

    1/1 + 1/2 + 1/3 +1/4 +... (first wire capacity + second + third +...)

    So, 10 amps for n=1, 15 amps n=2, 18.3 amps n=3, etc...

    I would never exceed this ratio and I would almost always way over design parallel connections (try to limit where max current is only slightly over the rating of one circuit in the first place)--and if I could, I would use another solution instead of paralleling in the first place.

    -Bill

    By the way, the relationship between wire gauge, current, and heating is not a simple/linear relationship... Because:

    1. The surface area of wire is proportional to the diameter, but the current conductor is proportional to the square of the diameter (a wire with twice the diameter has 4x the conductor area, but only 2x the surface area--so it can run hotter).

    2. Conduit fill affects capacity ratings. A conduit can only safely dissipate so much heat--the more current carrying conductors you stuff into one conduit, the more heat generated--the hotter the wires will run.

    3. Where you run the wire/conduit will affect how much heat can be dissipated--Running bare wires through the air--cool well. Buried in dirt, will not cool nearly as well.

    Your best bet is to use the NEC (or equivalent for your locality) to calculate safe current carrying (and voltage) capacities for your installation.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
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    Re: Wire size substitions

    Nec Table 310-16
    1/0 wire = 150 amps
    #6 wire= 65 amps
    #8 wire= 50 amps
    All for copper wire.
    2 each of #6 at the same length would be 130 amps.
    3 each of # 8 would give you 150 amps.

    I run parallel runs all the time here at work but nothing that small.
    Go ahead and bite the bullet and do it right with the 1/0. Better safe than sorry, especially when your talking copper.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: Wire size substitions

    the nec asside, 2 #6 copper wires paralleled is about equal to 1 #3 copper wire. to make #1/0 you need to double the #3 so that means 4 #6 copper wires paralleled to about equal 1 #1/0.
  • Dabbler
    Dabbler Registered Users Posts: 22
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    Re: Wire size substitions

    Thank you all for the all the great information, opinions and advice.

    Based on this, I think I'm looking at three options:

    1) Bite the bullet and run the larger gague wire

    2) Run individual panels through smaller gague, combining at destination, so there's no load sharing issues

    3) I'm sort of stuck on staying with a 12 volt battery bank, because of existing installation and inverter, charger, etc. I'm attempting to "rationalize" that investing in an MX60 will let me run panels in series, upping the voltage, using smaller gague wire for the run, and still charge a 12 volt bank. The savings in wire cost may pay for part of the MX60.

    Thanks again for all of your thoughts and advice
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
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    Re: Wire size substitions

    2) Run individual panels through smaller gague, combining at destination, so there's no load sharing issues
    One way to do this is to use a PV combiner from OutBack or Midnite. See: http://www.outbackpower.com/pspv.htm and http://www.midnitesolar.com/mnpv6%20cheers%20lo%20res.pdf

    3) I'm sort of stuck on staying with a 12 volt battery bank, because of existing installation and inverter, charger, etc. I'm attempting to "rationalize" that investing in an MX60 will let me run panels in series, upping the voltage, using smaller gague wire for the run, and still charge a 12 volt bank. The savings in wire cost may pay for part of the MX60.
    There are technical pros and cons to this approach. The pro’s include higher voltage and lower current from the array (which allows for smaller wire and/or lower voltage drop), and the controller’s MPPT feature. Con’s include slightly reduced controller efficiency as the input voltage increases (all else being equal).

    The MX60 would provide a wide range of options for a 12 V system. Depending on your module quantity and specs, you could run up to 60 V nominal (five “12 V” modules wired in series: ~85 Vmp) from the array to the controller. A higher voltage could be a problem if you live in a cold location.

    HTH,
    Jim / crewzer