miscellaneous wiring questions

I'm putting together a small/medium sized solar system and have the following wiring questions:

1. I read frequently on this board that there is a definite need for fuse protection on any + conductor directly connected to the battery bank. I have a Midnite Solar mini box w/ 125Amp breaker between battery bank and inverter. Do I still need a fuse for this connection, or does the breaker take care of that?

2. I plan on using 2/0 welding cable for the battery to inverter connections. My inverter (Exeltech 1100 xp) has a mechanical screw-down connection for the battery input, and it doesn't look like it could accept a wire as large as the 2/0 welding cable. Can anyone give me a suggestion on how to make this connection?

3. What kind of bolt do you use to connect your battery jumpers? I thought I read somewhere that stainless steel was recommended.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,207 admin
    Re: miscellaneous wiring questions

    A couple of answers...
    1. Breakers and fuse have the same function in your setup... They both open if the current is > 125 amps. The Breaker adds the nice On/Off function (technically, there are many types of fuses and breakers--slow blow, fast blow, etc.--but won't worry about that here).
    2. Fuses and Breakers protect the downstream wiring/connections/etc. from over current. The 125 amp breaker protects your heavy gauge wire. A 15 and breaker/fuse will protect 14 gauge wire, etc.
    3. For your 2/0 cable--roughly it is capable of ~190-283 amps of sustained current. For solar and other applications where voltage drop is important--we over size the cable for low resistance. Because this large stranded cable will not fit into the recepticals for smaller gauge wire--normally just trim enough of the excess off so the rest will go in (the remaining still needs to > ~1 gauge wire to handle the 125 amps maximum.
    By the way, welding cable is very flexible because it has such fine strands. The fine strands have lots of air-gap in between them and it makes the cross section larger than a solid or standard stranded cable used in home/industrial applications.

    This larger diameter is actually a problem in getting finding standard wiring components that will work with welding cable. If this where a NEC governed installation--it may be quite difficult to find Listed components (crimp connectors, clamps, etc.) for such an installation. (it was a real pain several decades ago when we used high flex cable for telephony -48 volt power connections (which did not require UL) and we wanted to sell the same equipment to non-telephone company businesses too--was almost impossible to find a source that "met" the requirements--may be easier now).

    My suggestion is if you don't need high flex cable--that you instead use standard NEC rated wiring. NAWS sells made up (lugged) cables for inverters/battery system with proper crimp tools and lug ends.

    Obviously, if you have the cable already you probably are not very inclined to purchase new cable from the local electrical supply house. (copper ain't cheap).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • czyhorseczyhorse Solar Expert Posts: 42
    Re: miscellaneous wiring questions

    I thought I read somewhere that fine stranded wire was not recommended for mechanical connections. I think its in Appendix K of the PV and NEC publication in the FAQ section of this message board.

    Is there some kind of reducer or adapter that can reduce the wire size down to fit into the mechanical screw terminal? The NEC guideline publication listed in the FAQ section mentions a pin or pigtail adapter. I cannot find any place online where one would be available for purchase. Anyone know where I could find one for #2 awg?
  • BB.BB. Super Moderators, Administrators Posts: 29,207 admin
    Re: miscellaneous wiring questions

    Fine stranded cable is very difficult to make up properly (and you are probably correct that it cannot be done to NEC requirements unless the cable ends receive special treatment--such as crimped on adapters).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • hillbillyhillbilly Solar Expert Posts: 334 ✭✭
    Re: miscellaneous wiring questions

    Actually, I think that those Inverter cables that NAWS sells are very flexible. Having used both the stiffer 3/0 wire that you can find at Home Depot type places, and flexible 4/0 welding cable that NAWS and many other solar stores sell, I would say that the stiffer wire of that size is pretty tough to work with and I wouldn't want to use it ever again for anything other than a very straight forward and "VERY roomy" set of connections.
    Have you thought about possibly going with an inverter/charger combo, as I would guess that most (all?) of those inverters would likely accept a larger sized terminal lug such as those on the inverter cables that NAWS sells. The problem with going with trying to reduce to a smaller wire size in any way that I can think of is that you would need a smaller fuse or breaker on that wire too (the 125 amp breaker would NOT protect your wire if you reduced it to a smaller size).
  • czyhorseczyhorse Solar Expert Posts: 42
    Re: miscellaneous wiring questions

    I just had a thought...could I purchase a fuse and holder (bolt on) such as found here:
    http://store.solar-electric.com/fb-110t.html

    Mount this just below my inverter, remove the fuse and use one half of the holder to bolt on the + welding cable conductor and bolt on a smaller + THHN conductor and run into the inverter. Then do the same with the - conductor on the other half. Would there be sufficient space between the pos and neg. conductors to do this without the possibility or arcing? Any ideas?
  • westbranchwestbranch Solar Expert Posts: 5,140 ✭✭✭✭
    Re: miscellaneous wiring questions

    more simply, just cut a piece of plywood and drill 2 holes for a 1/4 inch or ? size bolt to go connect the larger to smaller cable. No need for anything fancier, it is essentially a buss bar with only one connection.

    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,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • peterakopeterako Solar Expert Posts: 144 ✭✭
    Re: miscellaneous wiring questions

    Also check that your welding cable is acid proof, yes the cheaper ones are not and the insolation drys out creating a danger for fire.

    Greetings from Greece
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