Wire help: Little 4=Big 2?

Just wiring up some panels, I've got some 4 wire cable and am wondering if I join 2 strands to make a 2 wire cable will it double with no ill effect?

Comments

  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Wire help: Little 4=Big 2?

    Don't do this.

    Whereas technically two smaller gauge wires in parallel can carry 2X their rated current it is not a good practice. In fact NEC does not allow it on smaller wire gauges. The reason being if one of the wires gives out the other one will have to carry all the current itself. The only safe way to undertake it is to put a fuse on each wire to protect it for its rated capacity and no more. Then you have the possibility that one could fail, the other would pop its fuse, and you'd have no circuit.

    Use the correct wire size for the job.
  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    Re: Wire help: Little 4=Big 2?
    8lander wrote: »
    Just wiring up some panels, I've got some 4 wire cable and am wondering if I join 2 strands to make a 2 wire cable will it double with no ill effect?

    It all depends on why you are doubling up the strands. As Cariboocoot mentioned, do NOT double the strands as a way to carry more current.

    I paralleled two strands of #6 wire between my combiner box and my charge controller, but that was to reduce voltage drop on a long cable. My combined panels can never put out more than 16 amps and a SINGLE strand of #6 wire can handle 60 amps. Therefore if one of my parallel strands of wire drops out, the other strand can very safely handle the current.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • 8lander8lander Posts: 18Registered Users
    Re: Wire help: Little 4=Big 2?

    Yes it's more for voltage drop.
    I've got 4 wire 8 awg sitting in my garage (so free) and need to wire 2 panels on RV which are 235w / 24v.
    Probably no more than a 30ft run oneway to the controller.
    I could join each panel wire with an individual wire from the same cable and join them at the 15a breaker in the baby box?
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Wire help: Little 4=Big 2?

    235 Watt panels are probably actually 30 Vmp @ 7.8 Imp or near. Wired in parallel that would be 15.6 Amps max. Across 30 feet one pair of 8 AWG wire would give a 2.5% Voltage drop.

    Running one set of the wires per panel and combining at the controller would work even better.
  • 8lander8lander Posts: 18Registered Users
    Re: Wire help: Little 4=Big 2?

    Excellent, thanks.
    Saves me a few hundred bucks.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Wire help: Little 4=Big 2?

    While we're at it, shall we have the stock conversation about panel Vmp vs. system Voltage? It would go something like this: What is the system Voltage and actual panel Vmp? If it is 12 Volts and 30 Volts you need to use an MPPT type charge controller or else you are throwing away much of the panels' power. And if you are using an MPPT type controller you may be able to wire the panels in series and reduce the Voltage drop even further.

    These details can make a big difference in performance.
  • 8lander8lander Posts: 18Registered Users
    Re: Wire help: Little 4=Big 2?

    The panels are as shown. The controller is an outback mppt 60.
    I am losing marks for having 4 x 12v 150ah AGM's in parallel to feed a 12v inverter.
    A 24v inverter would have been a better match.
    Although I recently bought an amp clamp to keep an eye on them.
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  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Wire help: Little 4=Big 2?

    Because you have the MPPT controller you're good to go with the two panels in parallel at Vmp 29.8 Imp 7.89. You can do this either with both on one set of 8 AWG or with individual 8 AWG wires per panel joined at the controller (you could actually use two sets of 10 AWG) or even with both panels in series on 14 AWG wire. The last choice is least attractive as the higher array Vmp will reduce controller efficiency slightly.

    Your four parallel batteries is something of a potential problem. I hope you have them connected to bus bars with equal length wiring. That is the best way to reduce the current sharing trouble. You should also have a fuse on each one for safety.
  • BB.BB. Posts: 27,575Super Moderators admin
    Re: Wire help: Little 4=Big 2?

    And, just to address the original question--Adding two wires in parallel reduces the AWG by 3 steps (doubling of wire cross section/area). Two 10 awg wires in parallel are equivalent to a 7 AWG wire.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 8lander8lander Posts: 18Registered Users
    Re: Wire help: Little 4=Big 2?

    Thanks for all the help so far guys.
    I've only got one 15A breaker.
    Will it support the two panels in parallel?
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Wire help: Little 4=Big 2?
    8lander wrote: »
    Thanks for all the help so far guys.
    I've only got one 15A breaker.
    Will it support the two panels in parallel?

    Two panels in parallel don't actually need any circuit protection on them.
    As far as being able to handle it, if the combined current of the two panels is <15 Amps it would work.
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Wire help: Little 4=Big 2?

    That's based on Isc btw. In this case the Imp of the two panels will exceed 15 Amps so the breaker may trip anytime maximum current is achieved.
  • 8lander8lander Posts: 18Registered Users
    Re: Wire help: Little 4=Big 2?

    Ok so I hooked them up on the roof tonight, wire to wire, so I'll try them in parallel but if it don't work I can always flip it at breaker box.
    Suppose I should only do the disconnecting / connecting at night?
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  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Wire help: Little 4=Big 2?

    Cover the panels with cardboard or a thick blanket and you'll block enough light to where the output is safe for connection changing.

    It looks like you cut off the MC connectors on the panels. That may not have been a good idea. Normally you'd just make "pigtails" out of an extension cable and leave the panels intact.
  • 8lander8lander Posts: 18Registered Users
    Re: Wire help: Little 4=Big 2?

    So I did another step the hard way, that's good I'm right on schedule. &#55357;&#56841;
    I sure will have learned a lot once I'm done.
    Did I increase the resistance of the connection, using red marrettes?
    Or are the MC's just more convenient?
  • SkiDoo55SkiDoo55 Posts: 414Solar Expert ✭✭✭
    Re: Wire help: Little 4=Big 2?

    On cutting off the MC4 connectors is mainly convience and it may have "VOIDED" the warranty on the panels?
    GT3.8 w/4600W Trina 230W, TX5000 w/5000W ET-250W, XW4024 w/1500W ET-250W, 4 L16, 5500W Gen. (never had to use) Yet!!
  • 8lander8lander Posts: 18Registered Users
    Re: Wire help: Little 4=Big 2?
    That's based on Isc btw. In this case the Imp of the two panels will exceed 15 Amps so the breaker may trip anytime maximum current is achieved.

    So should I use a 30A breaker for two panels in parallel?
    Also made up a couple busbars. With equal length cables.
    Also ordered 150A breakers for each battery.
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  • inetdoginetdog Posts: 3,121Solar Expert ✭✭✭✭
    Re: Wire help: Little 4=Big 2?
    8lander wrote: »
    So I did another step the hard way, that's good I'm right on schedule. ��
    I sure will have learned a lot once I'm done.
    Did I increase the resistance of the connection, using red marrettes?
    Or are the MC's just more convenient?
    The MCs are safer too since when you unplug them both wire ends are touch safe. That does not happen for either end when you take the marrette off.
    That could save you from having to work "hot" on a high voltage DC string or else having to cover the panels or work at night. :)
    SMA SB 3000, old BP panels.
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