Bus or dist bar

I'm still working on setting this new small system up and need a little advice. I'm reading that when talking wire, bigger is better. True? Also I need to talk about a bus or power distribution bar. Does it have to be made of copper. Is Aluminum or an ally still good? Thanks... (more questions to come)...

Comments

  • BB.BB. Posts: 27,898Super Moderators, Administrators admin
    Re: Bus or dist bar

    Generally, if you can avoid Aluminum, you should...

    "Pure" (low alloy) Aluminum has a natural tendency to from a hard oxide layer on the surface (a good thing--as this seals the surface against corrosion). The bad news is that this surface oxide layer is also a very good insulator and begins to form almost immediately when the surface is cut/exposed.

    From the early days of aluminum wire, aluminum would cold flow under temperature and pressure (of a normal screw connection). For example, joint would get warm (under load), aluminum would expand and extrude a little beyond the screw/mating surfaces, cool, then form oxide between the wire and mating surface--repeat a few hundred times, eventually the connection would fail or overheat and create a possible fire hazard. Not sure a bus bar would have the same issues--but I would be concerned.

    There are now CU/AL rated outlets and components now--so you can use aluminum wire with the correct components--especially now that copper is becoming so expensive now.

    Lastly, I don't think (as I remember) that you should directly mate copper wire to an aluminum surface--the difference in electro negativity between copper and aluminum would create corrosion. Typically need stainless steel hardware between aluminum and copper connections. You might find some sort of terminal ends you can crimp onto copper wire to properly mate with Aluminum bar... A little Aluminum History (from the Aluminum Council). You can probably plate Gold or some other suitable material to the copper mating surfaces...

    In the end, I would avoid Aluminum bus bars unless you have specific information/experience in how to make reliable connections with the aluminum bar...

    Would this be a home made bus bar or do you have a commercial source for this item...

    -Bill

    PS: Generally, high alloy aluminum would have higher resistance than pure aluminum.

    And for battery systems (or any system), you really don't want to oversize the wiring by too much. Makes it difficult and expensive to wire/build. Using the wiring resistance/loss calculator (like the one on Wind-Sun--Excel spread sheet) lets you estimate voltage and power losses. Generally, the recommendation here is to keep losses to less than 3% and going below 1% loss in wire runs is usually approaching overkill...

    If you are interested in keeping wiring losses low, look towards using 24 or 48 VDC systems instead of 12 volts (inverters, charge controllers, battery banks, etc.). Heating losses go up with the square of the current (P=I^2 * R)... So, keeping the same size wire, going from 12 VDC to 24 VDC cuts the current in 1/2, and the losses to 1/4... Changing from 12 VDC to 48 VDC reduces current by 4 and losses by a factor of 16.

    Gives you the choice of using smaller wire gauge (to save money) or lower current on the same gauge wire (to save power)...

    The down side is that MPPT controller's tend to have a bit higher switching losses at higher input voltages (~1-2% or so).

    There are a lot of variables to look at when building your system--so keep in mind your actual needs (12 volts at a few amps or 48 VDC at 100's of amps--12 volts systems at 100's-1,000's of amps of current for high power demands are not really practical). It is difficult to build a small system that is hugely expandable. You are better off designing for the size system you need, but use good components as many of them can be moved to larger systems (solar panels, charge controllers, inverters)... The old batteries are, many times, not worth adding to an, otherwise new, battery bank (aged batteries will tend to shoulder less of the bank's total load).

    -BB
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Bus or dist bar

    Thanks for the info. More questions are sure to follow and it's nice that there's a place (and people) I can go to... Larry
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