Copper Bus Bar Design

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Comments

  • mtdocmtdoc Solar Expert Posts: 600 ✭✭
    Re: Copper Bus Bar Design

    Chris see my post below. I honestly don't know if it makes a difference if the fuse holder is rated for higher voltage. I only know that the standard auto type ones are rated at 32 V.
    ChrisOlson wrote: »
    The only way I could see that anything could arc in there is if you put in too short of a fuse and the springs didn't hold enough tension to keep the brass button in contact with the end of the fuse.
    --
    Chris

    That's what I thought too. But when the white plastic case melted after the fuse had blown and it shorted, I changed my mind. Of course I was pushing 30-40 amps through it and the fuse itself I believe was also rated at 32V. But I'm not sure - If the filament on the fuse blows, why it would be different?

    Anyhoo - never mind if my concern is not justified..:blush:
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: Copper Bus Bar Design

    Stand by - I'm going to go and blow one and see what it does.

    Edit:
    Well, I had to pull the cover of the bus and the cover off my 100 volt meters. I hooked a clip-on jumper to the ground bar in the bus and touched the "hot" stud on one of the meters with it. The fuse simply popped and nothing bad happened.

    I looked for the package that the fuse holders came in because there was four of them in the package and I still got one someplace. Can't find it right now. But I THINK it said on the package 20 amp max fuse in those holders. I don't think at 1 amp fuse it's going to have much of a problem, as they're only there to protect some very low draw circuits for gauges, two relays and our MidNite Solar battery meter - all fuse at one amp.

    --
    Chris
  • BB.BB. Super Moderators, Administrators Posts: 31,765 admin
    Re: Copper Bus Bar Design

    40 amps through a 14 AWG wired holder is just going to generate a lot of heat--And to blow a 40 amp fuse is going to generate more heat than a 5 amp fuse.

    Using the Power=I2R:
    • Ratio of 40amp/15 amp rating for 14 awg wire = 402/152 = 7.1 times more heat at 2.7x more current

    So, driving a lot of current through less capable wiring gets much worse heating that one would expect as the waste heat goes up with the square of the current.

    Typically, I would not expect a holder to fail from voltage--unless the voltage was high enough to sustain and arc in the holder.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mtdocmtdoc Solar Expert Posts: 600 ✭✭
    Re: Copper Bus Bar Design

    Thanks for the good info Bill.

    FWIW - in my case it was 12 gauge wire on the fuse holder and a 30 amp fuse. According to my Watt's Up meter I was drawing somewhere around 35 amps when the fuse blew. I lost power briefly, then gained it again, then lost it for good. My bike stopped (I was going up a steep hill and pedaling some) and there was a burnt smell from my battery bag where the fuse was. Opened it up to find the white plastic melted and burned.

    Anyways - Point taken on the heat increasing with the square of the current. I guess Chris's low amp circuits will probably be fine...
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: Copper Bus Bar Design
    mtdoc wrote: »
    in my case it was 12 gauge wire on the fuse holder and a 30 amp fuse.

    30 amps is pushing AWG 12 pretty hard. Even in 12V automotive applications 30 amp inline fuses no longer use the brass button contact on the fuse, but instead have a socket that the entire head of the fuse fits into. I'm guessing that at 30 amps the contact point between the brass button and fuse head got really hot and melted the holder, and it had nothing to do with what voltage it was running at.
    --
    Chris
  • mtdocmtdoc Solar Expert Posts: 600 ✭✭
    Re: Copper Bus Bar Design
    ChrisOlson wrote: »
    30 amps is pushing AWG 12 pretty hard.

    Yep, I agree. I knew I was pushing it but had hoped the 30 amp fuse would protect the wiring.

    It's interesting that these J-Case Fuse holders I'm using now have 8 AWG wire but are rated for 60 amps at 58V - since 60 amps is really pushing it for 8AWG wire.
    I'm guessing that at 30 amps the contact point between the brass button and fuse head got really hot and melted the holder, and it had nothing to do with what voltage it was running at.

    Yeah, you're probably right. What was weird was the brief, sputtering return of power after it first cut out. I guess I assumed that was because of some arcing - but maybe not.
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: Copper Bus Bar Design
    mtdoc wrote: »
    It's interesting that these J-Case Fuse holders I'm using now have 8 AWG wire but are rated for 60 amps at 58V - since 60 amps is really pushing it for 8AWG wire.

    Yeah, they do that a lot in automotive stuff because they don't really care about voltage drops over short wire runs in a car. Look at the size of wire going to a starter motor, for instance, that can easily draw 400 amps turning over an engine at zero F.
    --
    Chris
  • ouelloouello Registered Users Posts: 21
    Re: Copper Bus Bar Design

    We have these awesome fuse holder at a local store that are meant for car audio:
    http://www.canadiantire.ca/AST/browse/4/Auto/CarAudioVideo/AutomotiveAudioAccessories/PRD~0350081P/E2+Single+AGU+Fuse+Holder.jsp?locale=en
    And the wire size seams acceptable... I just need to find out if the fuse capacity is what I'm looking for...
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Copper Bus Bar Design
    BB. wrote: »
    40 amps through a 14 AWG wired holder is just going to generate a lot of heat--And to blow a 40 amp fuse is going to generate more heat than a 5 amp fuse.
    True dat. Even with 90 degree insulation the non-derated ampacity of #14 Cu is only 25A and the most code compliant OCP you can put on a #14 copper conductor is 15A. (Article 240.4(E)(3)).
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Copper Bus Bar Design
    ggunn wrote: »
    True dat. Even with 90 degree insulation the non-derated ampacity of #14 Cu is only 25A and the most code compliant OCP you can put on a #14 copper conductor is 15A. (Article 240.4(E)(3)).

    There is more than one way to blow a fuse, and there lies part of the problem:

    If you apply a really solid short circuit, and it is fed by a battery or other source which is not inherently current limited like a solar PV panel, you will explode the fuse wire. There will not be much time for the heat to spread to the fuse holder and surrounding wiring. But the vaporized metal from the fuse wire may sustain an arc inside the fuse tube until something worse happens.

    If you apply a more moderate overload, everything around the fuse will heat up to some extent as the fuse wire temperature rises to its melting point. But when the wire finally melts, it may just fall away leaving an open circuit. There may be some arcing across the gap, but it can chew away the ends of the fuse material without necessarily generating enough heat and ionization to sustain the arc.

    In between the two extremes, all sort of other things can happen, usually not as bad as either extreme.

    A high voltage DC fuse will have material inside the fuse capsule along with the wire which acts to extinguish the arc, up to whatever the rated voltage is, and it will not matter whether the fuse blows slowly or quickly.
    SMA SB 3000, old BP panels.
  • railcon56railcon56 Registered Users Posts: 3
    Found This Online FROM

    http://nordkyndesign.com/assembling-a-lithium-iron-phosphate-marine-house-bank/

    Eric,

    Thank you for your response. I hear that there is little concern about movement between cells so solid bar should not be an issue.

    I am a little bit confused about your statement that the aluminum flat bar would be the better choice. Is this based only on the facts that it is harder, stronger, and cheaper than copper? Since copper has a lower resistance, if they were equal in price, or in my case I already have 1/8″ x 3/4″ copper, wouldn’t the copper be better? The strength and hardness do not seem like that are very important in this application. Perhaps I am missing something here.

    My concern with using just the single 1/8″ x 3/4″ comes from the rated ampacity of such a bar. According to https://www.copper.org/applications/electrical/busbar/busbar_ampacities.html the cross sectional area is 119.4cm and a rated capacity of 215 amps. I have a 400Ah bank with the potential to pull up to >233 amps at any one time based on my 2800 watt inverter as well as the various DC house circuits, I would think that a higher ampacity should be called for.

    I seriously doubt that I will ever even come close to that level of output but I would prefer to error on the side of caution when sizing wiring and busbars.

    • Jason,

      Copper is quite soft and 3mm if a fairly thin flat bar, which needs to be clamped down very well at each terminal with a stainless steel bolt and locking washer. You will no doubt use the copper because you already have it, but a 1/4”-thick aluminium flat bar is more robust. Copper makes a better conductor for the same cross-section, but doubling the cross-section and using aluminium is no issue here. A lot of power distribution cables are aluminium for the same reasons.

      I have issues with your source above on the basis that the cross-sectional area of a 1/8” x 3/4” bar is 3mm x 19mm = 57mm2. “119.4cm” is a length and it is non-sensical in this context. Based on the resistivity R = 7μΩ/inch we calculated earlier, at I = 250A the busbars would dissipate P = R x I^2 = 0.44W/inch. If you were going to sustain this kind of current for long periods, a case could be made on the basis that you don’t want them to gradually warm up the cells over time, but it is not going to make any difference whatsoever if you occasionally peak at 250A for short periods. When it comes to a busbar actually “failing”, even a hit at a few thousands of amps wouldn’t be enough to achieve that and it would blow the protection fuse long before the bars would have a chance to heat up.

      If you made your busbars out of aluminium, you would not be restricted to a narrow width of 3/4” only and they would in fact have a much bigger cross-section and be able to handle more current for less temperature rise.

      If you stack your copper strips, you will need to sand them bright at the connection points and be very careful to achieve good conduction through the joint and it might still not last over time in a marine environment.
      I once saw a lithium bank that was exhibiting unmanageable cell balance problems just because one cell in one parallel block wasn’t contributing properly: excessive resistance in the connection, and yet it was bolted tight. It came right after sanding all the faces bright and reassembling and this is why I would choose a solid bar rather than trying to use small copper strips. 3/4” is barely wide enough to be attached with M8 bolts.

      Best regards,

      Eric


  • mike95490mike95490 Solar Expert Posts: 9,293 ✭✭✭✭✭
    Every connection is a failure point.  The fewer, the better. Layers of laminated strips would be poor practice. 
     To use aluminum properly, it must be machined properly to allow proper thread size after tin plating.  Every UL rated piece of gear using aluminum/copper interconnects, the alum is tin plated to resist corrosion,
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