Using two smaller diameter wires instead of one?

johnelarue
johnelarue Solar Expert Posts: 33
Is it possible to use two smaller diameter wires for each positive and negative from panels to controller? For example, if I need 3.5 mm dia. Wire , can I use two 2.0mm wires that are separate for each lead? Would this "split" the amps so as not to incur losses? I came across some used wire free and am trying to be cheap. Or should I just forget it and buy the larger diameter wires? Thanks much.

Comments

  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Using two smaller diameter wires instead of one?
    johnelarue wrote: »
    Is it possible to use two smaller diameter wires for each positive and negative from panels to controller? For example, if I need 3.5 mm dia. Wire , can I use two 2.0mm wires that are separate for each lead? Would this "split" the amps so as not to incur losses? I came across some used wire free and am trying to be cheap. Or should I just forget it and buy the larger diameter wires? Thanks much.

    For the record, it's a violation of code. But there are some tricks that would make it totally safe and arguably legal.

    First of all you must deal with ampacity of the conductors... Each of the thin wires must be able, by itself, to safely handle the maximum possible current. In other words the reason to 'parallel' your wires is to reduce voltage drop, NOT to increase ampacity.

    The trick to make it legal is to put an appropriately sized fuse or circuit breaker in each of the thin wires. Technically, that means they are not in parallel... to be in parallel they would have to be joined to each other (not to a fuse) by a connector at each end.

    The reason you can't use 'parallel' small wires to increase ampacity (even with fuses in each wire) is because the current sharing will not be equal. That is, more current will flow in one wire than the other. As soon as one fuse blows, all the current will flow through the other wire and blow that fuse also.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • TucsonAZ
    TucsonAZ Solar Expert Posts: 139 ✭✭
    Re: Using two smaller diameter wires instead of one?
    vtmaps wrote: »
    For the record, it's a violation of code. But there are some tricks that would make it totally safe and arguably legal.

    First of all you must deal with ampacity of the conductors... Each of the thin wires must be able, by itself, to safely handle the maximum possible current. In other words the reason to 'parallel' your wires is to reduce voltage drop, NOT to increase ampacity.

    The trick to make it legal is to put an appropriately sized fuse or circuit breaker in each of the thin wires. Technically, that means they are not in parallel... to be in parallel they would have to be joined to each other (not to a fuse) by a connector at each end.

    The reason you can't use 'parallel' small wires to increase ampacity (even with fuses in each wire) is because the current sharing will not be equal. That is, more current will flow in one wire than the other. As soon as one fuse blows, all the current will flow through the other wire and blow that fuse also.

    --vtMaps

    Good comment, makes sense, I was actually just wondering this and have a quick question before this thread get derailed into a tangent about the wire coating composition of x versus y wire gauges and how the plastic softener for 4 gauge wire comes from china and is inferior to the stuff from india used in 2 gauge.

    I'm running 4 gauge wire for now, from all of the specs I could find including my inverter manufacturer that's what they suggest to use. My battery interconnects are 6" long from positive to negative and 12" from -/- and +/+ with a 3' lead to the breaker and another 3' lead to the inverter. I could chop a foot off of each of those if I really wanted but liked that ability to move things around a little. That should leave me with a voltage drop of well under 2% which should be completely fine. The question is, would I see and advantage to doubling up (or even replacing) any of that wiring or is the voltage drop low enough to not be cause for concern? Like the OP, I have A TON of 4 gauge wire in both fine strand and stranded so I would rather use that.

    To the OP, the calculators account for paralleling and drop the loss in half so I'm guessing it's common practice.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Using two smaller diameter wires instead of one?
    johnelarue wrote: »
    Is it possible to use two smaller diameter wires for each positive and negative from panels to controller? For example, if I need 3.5 mm dia. Wire , can I use two 2.0mm wires that are separate for each lead? Would this "split" the amps so as not to incur losses? I came across some used wire free and am trying to be cheap. Or should I just forget it and buy the larger diameter wires? Thanks much.

    Don't do this. Use the right size wire.

    NEC actually allows parallel conductors of very large size. You're looking at about 7 AWG here so as vtMaps said that would not be allowed.

    There is at least one maker of poor quality inverters which habitually promotes huge capacity 12 Volt units that utilize multiple parallel input wires in order to handle the ridiculous currents involved. It's a bad idea.

    If you decide to do this anyway put over-current protection on both wires so that if one should fail the other isn't subjected to the full current and burns up. Again as vtMaps said doing it for reducing V-drop is a different matter (much safer).

    People who try to make the wrong thing do the right job usually are disappointed with the result.
  • ZoNiE
    ZoNiE Solar Expert Posts: 100 ✭✭
    Re: Using two smaller diameter wires instead of one?
    TucsonAZ wrote: »
    ... how the plastic softener for 4 gauge wire comes from china and is inferior to the stuff from india used in 2 gauge.

    Huh? Is this a joke or something to look for?
    Attachment not found.
  • johnelarue
    johnelarue Solar Expert Posts: 33
    Re: Using two smaller diameter wires instead of one?

    Thank you all for your help, it is greatly appreciated as always.

    FWiw, I used parallel wires on my two batteries for 4 years without incident.
  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    Re: Using two smaller diameter wires instead of one?

    This is a plea for using the right cable. Work out your voltage drop, and order the cable.

    For cables between batterys to busbars to controllers, err on the bigger is better principle. This is for lots of reasons:

    - you dont want any measurable voltage drop between the controller and the battery, otherwise your setpoints will be up the wazoo.
    - voltage sag between battery and inverter will kill your surge capability
    - these cables are (or darn well should be) short, hence the money is trivial

    zb.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • johnelarue
    johnelarue Solar Expert Posts: 33
    Re: Using two smaller diameter wires instead of one?

    Than you zone blue,

    I've since replaced all cables to large gauge on the battery and controller side of things.

    When I get some larger cable for the PV side will do too ,
  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    Re: Using two smaller diameter wires instead of one?

    Pv is different. Its long, expensive and always an exercise in dimishing returns in terms of the cost trade off between copper and pv. Choose a compromise, but be sure to work out how mcuh the pv would cost to make up for cable losses as well.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar