Small Busbar connections.

[RaymondR]

It does not matter how much torque you put on the screws as long as it's enough to pull the surfaces of terminal and busbar together so that the full surface area of the crimp loop is in intimate contact with the busbar surface.

I sometimes put the screw into the busbar from the back and use a washer and nut on the thread sticking out. This helps retain the terminal whilst I put on the washer and nut and prevents stripping out the thread in the busbar if I try to over tighten.

[smac999]

The size of the screw does not really matter. All it’s doing is holding the surface area of the lug to the surface area of the buss. So the bigger the lug face. The more contact. The more amps it can take.

That said For a number 10 screw I would be using 10awg 30a max. With yellow heat shrink terminals. That’s just what I do.

Get the 5/16” studs for bigger wire. And for battery lugs

[AKA-None]

How much torque you can apply depends on the size, thread pitch, and more importantly the materials used.

Eg same screw into copper, aluminum, stainless, etc will vary

[daveNZ]

Surface area sounds like the key; obvious once it’s pointed out! Surface area and crimping contact would be much better with a 5 x 5 lug than a #10 wire ring terminal, and further I could retap for a bigger size, and mount a machine screw from the back with a washer and nut holding the lug.
I think I have the answer(s), many thanks!

[coopec43]

Originally Posted by daveNZ

Surface area sounds like the key; obvious once it’s pointed out! Surface area and crimping contact would be much better with a 5 x 5 lug than a #10 wire ring terminal, and further I could retap for a bigger size, and mount a machine screw from the back with a washer and nut holding the lug.
I think I have the answer(s), many thanks!

I won't argue with that. If you want to become pedantic.

Thumb Rule for Busbar Amp Size

"This Thumb Rule shows how much current a 1 square mm (Sq.mm) busbar can withstand.
There are two common materials for producing a busbar, they are aluminium and copper. Both aluminium and copper have their own ability to withstand currents.
A 1 Sq.mm of aluminium busbar can withstand 0.7 Amperes.
A 1 Sq.mm of copper busbar can withstand 1.2 Amperes.
Of course the examples above did not come from an international standard because we can’t find the tolerance values. Some people may still use an aluminium busbar to deliver 1 Amp. Some other people use a copper busbar to deliver 1.5 Amps.
Later on, because this primitive method became unreliable for high current in thousand amps, we needed to do proper calculation with a proper standard."

For piece of mind I always go big as it doesn't cost much more. But for very high amperage (winch, inverter etc) I would not use a bus bar. (But I have circuit breakers and isolation switches)

[daveNZ]

Initially I thought that sounded good and seemed right for a contact area of 30sq mm (rough guess for my lug) so current carrying capacity of something <40A seems ok.

But my 150A Busbar has a cross sectional area of about 40sq mm and ampacity tables (boathowto.com) for a 25 sq mm wire, modest insulation temp rating, of 140A
So both of these are 4 to 6 times area, rather than 1.2 times. Something wrong here?

[deblen]

You may find some useful advice here.
https://marinehowto.com/fusing-termi...-voltage-drop/


https://marinehowto.com/battery-melt...rowly-averted/

[coopec43]

Originally Posted by daveNZ

Initially I thought that sounded good and seemed right for a contact area of 30sq mm (rough guess for my lug) so current carrying capacity of something <40A seems ok.

But my 150A Busbar has a cross sectional area of about 40sq mm and ampacity tables (boathowto.com) for a 25 sq mm wire, modest insulation temp rating, of 140A
So both of these are 4 to 6 times area, rather than 1.2 times. Something wrong here?

I most certainly don't claim to be an expert.

I agree a AWG4 (25mm2) cable would be appropriate for 140/150A. Now how do you transfer that amperage in the cable to the busbar without using a crimp terminal?

NOTE the third one across is 70mm2


Since the current can flow from the top and bottom of the crimp terminal (70A each way) the area of the crimp terminal would have to be 70/1.2 mm = 60mm2. To get an area of 60mm2 the diameter of the crimp terminal would have to be ≈ 8mm

https://www.australwright.com.au/wp-...ght-Metals.pdf

[daveNZ]

Len, thanks for the marinehowto reminder, I’ve read them all but need to reread, especially with a specific question in mind. Love his articles (and have contributed $).

Not sure about the last comment re lug sizes? This is assuming current flows through the upper side of a lug, whereas I thought the fastener is not expected to conduct any meaningful current. This means we need an area on the underside of the lug sufficient to pass enough current.

I’m thinking sufficient torque is all about making good contact between lug and Busbar, including to some extent flattening the lug bottom surface!! Is it good practise to file it flat? I bet some would have a ‘blip’ of coating which would hold it up off the Busbar surface.

[deblen]

Perhaps some thermal paste/grease would help transfer any heat buildup from lug to busbar?
I have no personal experience with T paste in this application.
The heavy lugs shown by Coopec are better than "yellow" crimps IMHO.

2 clamp screws would be a good idea in your situation IMHO.
https://www.arrow.com/en/research-an...eat-management


I am not an expert,but have learned some some things the hard way over the years.Just passing that along-consume at your own risk / L

[Wotname]

Originally Posted by coopec43

I most certainly don't claim to be an expert.

I agree a AWG4 (25mm2) cable would be appropriate for 140/150A. Now how do you transfer that amperage in the cable to the busbar without using a crimp terminal?

NOTE the third one across is 70mm2


Since the current can flow from the top and bottom of the crimp terminal (70A each way) the area of the crimp terminal would have to be 70/1.2 mm = 60mm2. To get an area of 60mm2 the diameter of the crimp terminal would have to be ≈ 8mm

https://www.australwright.com.au/wp-...ght-Metals.pdf

No sure where you are going with this comment. The numbers stamped on the lugs have nothing to do with the cross sectional area of the lugs or the areas of the flat surfaces of the lugs. The number indicates the wire size the lug has been designed to accommodate.

For instance, the third lug (from the LHS) is designed for 70 sq mm cable. The actual diameter of 70 sq mm wire will depend on the strand count. The higher the strand count, the smaller the actual diameter. The lug will be big enough to fit low strand count cable and compress enough for high strand count cable.

[Jammer]

Originally Posted by coopec43

Since the current can flow from the top and bottom of the crimp terminal (70A each way) the area of the crimp terminal would have to be 70/1.2 mm = 60mm2. To get an area of 60mm2 the diameter of the crimp terminal would have to be ≈ 8mm

There are a couple of problems with this line of reasoning. The minor point is that if you're going to treat the terminal like a wire, you would want to look at the section with the smallest cross-sectional area through which current must flow, which is the part between where the crimp barrel ends and where the flat part of the lug or terminal starts. That's smaller than the area of the ring or tang or whatever, I believe in all cases.


But let's move on to the grand fallacy.


The terminal isn't a wire. The ampacity of a wire is based on the temperature the insulation can withstand and the heat loss properties of the wire, insulation, jacket, and enclosure system as installed. Wire that carries large currents gives off heat, quite a lot when the larger sizes of wire are considered. The insulation around the wire, the jacket, any conduit, nearby wires, and so on all contribute to the maximum temperature of the insulation. This is why there are different ABYC ampacities for engine room use (higher temperature) than in other places in the hull.


Anyway a key point with wire and overheating is that wire conducts heat. Localized high-resistance segments, up to a limited point, can have the heat conducted away by the wire. Another key point is that in a busbar application, the terminal is only partially covered by insulation and not covered by a jacket, and the busbar itself serves as a heat sink.


So the thermal situation is totally different, and if you're going to try to model it and figure out how hot it's going to get, you'll need the expensive thermal modelling plug-in for an expensive CAD system, and you'll still have to test it in the lab to see if you got the results you expected before you can believe them.


Safety in electric power circuits is based in large measure on practical field experience. When people wire boats to ABYC standards they by and large they don't have electrical fires due to design mistakes. That makes the ABYC standards, good standards, and the customary practices that go with using them, like using #10 hardware for fastening terminals for up to 10 gauge wire, are also good standards.


I'm not smart enough to figure out from first principles, on paper or in software, how much current electrical components on boats can carry. So I follow the standards, and don't try to be smarter than your average bear in this area.

[thinwater]

Originally Posted by Jammer

The screw doesn't conduct current to any meaningful degree. It's just there to clamp the wire termination to the busbar....

This.


I believe ABYC says you can only stack 3 lugs on one screw. The power is carried by the eyes, not the screw.


The thing about standards, is that they build in allowances for all sorts of unintended consequences. Wires wrapped around each other. Physical strains. You can bet that on any boat, over time, something will shift until it's not quite right, and the safety factors will save you.

[RaymondR]

Or someone could just hook up an inverter welder, set the current output and see if the connection cooks??

[coopec43]

Originally Posted by Wotname

No sure where you are going with this comment. The numbers stamped on the lugs have nothing to do with the cross sectional area of the lugs or the areas of the flat surfaces of the lugs. The number indicates the wire size the lug has been designed to accommodate.

For instance, the third lug (from the LHS) is designed for 70 sq mm cable. The actual diameter of 70 sq mm wire will depend on the strand count. The higher the strand count, the smaller the actual diameter. The lug will be big enough to fit low strand count cable and compress enough for high strand count cable.

Hi Wotname

My logic tells me that a big cable requires a big flat surface area of the lug otherwise you'll end up with a tractor on bicycle wheels