How many cables fit on a busbar?

[dfelsent]

As I recall the primary issue with aluminum conductors is not the bulk conductivity of the material but surface corrosion and self passivation.
This leads to high resistance connections, heat, and fires.
When aluminum is used in houses a special compound is required on the connections.
I wouldn’t use aluminum in my home.

[Bycrick]

Cmon, Frankly. Even with infinitely deep pockets, looking at the cost/benefit ratio is something that eventually comes into the discussion. In this particular case, what benefit would NOT using SS studs provide? Why would it be worth the extra cost? Silicon bronze is more conductive than SS. But does that make much difference when the lower surface is in full contact with the bus bar, and the upper terminal surface has to go through a smaller washer, then the nut, then through the inevitability incomplete surface between the nut and the bolt? NB: I’m willing to be convinced that it would make a difference, but i couldn’t find any studies on the subject.

[Bycrick]

However I did find some data on bolt tensile strength, which is important when you want to press a terminal onto a flat surface. When comparing that, a pure copper stud would have about 1/4 the strength of SS. Silicon bronze a little less than half compared to 302/304 stainless. So the studs would be more likely to break when tightened.

So it doesn’t seem like it’s purely a $$ decision.

[Frankly]

For the 5/16 and 3/8" stud size have to be a real gorilla to bust one off. SB is not a good conductor (always a mystery to me at 95%+ copper), but have had enough SS galling experiences that $25 extra for the SB was money well spent IMO. Think through what you are facing with one of those nuts galled on a stud with a bunch 2/0 connections/ wire all crowded around.

UR right about the main conductive path, but the SB might be a little better if for some reason something got between the copper bar and the terminal.

I have seen copper all thread but never copper bolts (actually got some aluminum bolts in my house lightning protection). Not suggesting for this application.

15 years back I looked all over for something to solve what struck me as a pretty common electrical problem on modern yachts (nothing found). Today still nothing AFAIK (no reason to look anymore).

For what BS gets for their Bus Bar stuff they could get a boat buck for a good high amp dist set up.

[GordMay]

Electrical Conductivity of Materials ➥ https://www.bluesea.com/resources/10...y_of_Materials

[Bycrick]

I’ve avoided galling by using a little bit of copper antiseize on the threads. When I had to make some, I used silicon bronze because I could silver solder the screw to the bar so that it wouldn’t come loose. If I’d had a way of holding the SS in place, I’d have used that instead. It’s my opinion, without any evidence, that one could use plastic studs with little or no change in the total resistance, if it were strong enough. It would be an interesting test to put the terminal/bar in a press, and measure with and without the stud. I don’t have the energy or the equipment.

[Frankly]

As Gord and Blue Sea shows, bronze not a great electrical conductor. If you were concerned about total resitivity, brass would be a better choice but would require a light touch.


Got a high quality low resistance measuring meter. Maybe if I ever get over Michael will run the experiment. Got 10 or 15 ft of the copper bar (1 X 3/16) I picked up at the scrap yard 25 or 30 year back.


Frankly

[Bycrick]

I hope you’ve got a real, whiz-bang, low ohm meter. The bus bar is about 3/4” square or .5625 in2 or 363 mm2

According to my instant internet search, 4/0 wire 107 mm2 and 2/0 is 67mm2. So the bus bar has more than 3 times the area of 4/0 wire, and a little less than 1/3 the resistance.

The resistance of 4/0 wire .049 milliohms per foot. For 2/0 wire it’s .07793 milliohms per foot.

A number that I got from a decent source, but which I can’t support, says that each "interface" for 2/0 wire is about .0002 ohms or 0.2 milliohms.

If you can measure that accurately, I’d love to know what you’re using. Just for fun, if you had an accurate 100a current source, the 2/0 interface should drop 100 * .0002 or about 20 millivolts.

I’m really interested in what you come up with.

[Frankly]

Meter (Fluke 1520) only goes down to 0.01 Ohm resolution. Voltage drop might be a better approach. Similar to shunt setup. Relative measurements so absolute number not critical.
Probably round up brass, silicon bronze, SS, and some type of non conductive.

Don't hold your breath still working on roof ("a man has got to know his limitations" and priorities).


Frankly

[Wotname]

Originally Posted by Bycrick

I hope you’ve got a real, whiz-bang, low ohm meter. The bus bar is about 3/4” square or .5625 in2 or 363 mm2

According to my instant internet search, 4/0 wire 107 mm2 and 2/0 is 67mm2. So the bus bar has more than 3 times the area of 4/0 wire, and a little less than 1/3 the resistance.

The resistance of 4/0 wire .049 milliohms per foot. For 2/0 wire it’s .07793 milliohms per foot.

A number that I got from a decent source, but which I can’t support, says that each "interface" for 2/0 wire is about .0002 ohms or 0.2 milliohms.

If you can measure that accurately, I’d love to know what you’re using. Just for fun, if you had an accurate 100a current source, the 2/0 interface should drop 100 * .0002 or about 20 millivolts.

I’m really interested in what you come up with.

The resistance of the busbar could be worked out -

Resistivity of pure Cu at 20C - 0.0171 ohms*mm2/m
Busbar (1"x3/16") cross sectional area is 121 mm2
Busbar length 10' - 15' is say 4m

Resistance is therefore ~ 0.00057ohms.

PD at 10 amps ~ 5.7 millivolts
PD at 100 amps is ~ 57 millivolts.

Any decent DVM should be able to read those numbers.

Unless my arithmetic is wrong!

[Bycrick]

For me, the bus bar resistance isn’t very interesting. When I was teaching a seminar on DC power circuits, I was surprised to find that the connections had more influence on the total circuit losses than the wires or components. The Blue Sea bus bar appears plenty big. But our discussion of what impact, if any, the stud material has on the connection is just one more link in the chain.

So to answer my question, I came up with a test setup. About a 100amp source. Steady but not necessarily horribly accurate. A load that will draw about 100a.
Test lead is an appropriate wire with a (say) 2/0 lug with a 3/8” hole. Clamp it over a 5/16” stud on the bar using a thick metal flat washer as big as the terminal, tightetened to a specific torque. Measure the voltage drop from a marked point on the lug to a marked point on the bar.

The comparison measurement would put a small o-ring sleeve in the terminal hole to insulate that from the stud. Then a thin hard insulating washer on the top of the lug. Tighten down to the same torque. Measure the voltage drop.

I didn’t look up your Fluke meter, but all I’ve got is a 177. I haven’t done precision lab work in years and don’t need better.

I won’t hold my breath. Your roof is probably more important than my curiosity. But please do let me know the results.

[Frankly]

Bus bar resistance is probably NBD. Just looking at a connection would draw you to the wire/lug/stud as the short pole in the tent. My inverter driving the microwave could provide the 100 amp current. Just have to insert a test jig of wire/lug/stud in series with the inverter feed and measure the voltage drop consistently across different stud arrangements. Drill and tap a 3/8 hole in a short copper bar and change the stud material (hex head bolt) including an insulating washer under wire lug for each different stud material. SS with and without insulation ect.

Can't change my stud material but just turning on microwave and making a few drop measurements might be interesting. Bet the ANL fuse drop going to be first attention getter.


Frankly

[Manateeman]

The Blue Sea Stytem article confuses me. Copper at the top and tin and lead way at the bottom. My battery terminals are lead and the wire and the lugs are coated with tin so am I better off with uncoated copper wire, lugs and uncoated copper buss bars that I can make myself?
I looked at silicon bronze buss bars that I made years ago and sprayed with Boshield after assembly and they worked fine and looked clean under the lugs.
So tin can get tin pox and it’s not as good a conductor as copper. So my main battery to T fuse and from there to on/off switch and then to the main buss bar ought to be bare copper at all connections ?
Good grief, the battery flag posts are lead...way at the bottom of the list.
I’m going to need a tin cup to get money for tin removal or maybe see if I can fit a tin man suit on a manatee.
Yikes..I’m more confused than ever.
Mark and his manatees

[dfelsent]

20mv at 100 A or 57 mv at 100 A
Seems like nothing.

Let’s do a REALLY CRUDE calculation.
watts = V x A
So 2W and 5.7W respectively.
Convective heat transfer in air is around 40 W/m2K.
Assume 100 cm2. 10/10 cm being generous because of shape and heat carried by cables.
That’s .01 m2 * 40 W/m2K. Or 0.4 W/K.
5.7W/0.4W/K is about a 14 degree K (or C) temperature rise. In other words that 109A through the bus bar can reasonably be expected warm things up a bit. But not a lot. But enough that you could feel the warmth. That’s OK. Feel your battery cables when running at high current for a long time. They get warm. By design.
But you don’t want to have 10x less area, that’s for sure.
Or 10x more contact resistance.

[dfelsent]

Originally Posted by dfelsent

20mv at 100 A or 57 mv at 100 A
Seems like nothing.

Let’s do a REALLY CRUDE calculation.
watts = V x A
So 2W and 5.7W respectively.
Convective heat transfer in air is around 40 W/m2K.
Assume 100 cm2. 10/10 cm being generous because of shape and heat carried by cables.
That’s .01 m2 * 40 W/m2K. Or 0.4 W/K.
5.7W/0.4W/K is about a 14 degree K (or C) temperature rise. In other words that 109A through the bus bar can reasonably be expected warm things up a bit. But not a lot. But enough that you could feel the warmth. That’s OK. Feel your battery cables when running at high current for a long time. They get warm. By design.
But you don’t want to have 10x less area, that’s for sure.
Or 10x more contact resistance.

Btw to measure those low resistances a Kelvin (or four wire) ohm meter is the trick.