1/8th copper or 4mm stainless steel for busbar?

[Skylark]

I'm planning to join 3 x 12v battery negatives wires using a simple busbar. Of the 3 cables, the biggest is 95mm2, the other is 75mm2. My choices of readily available material is either 1/8th x 1 inch copper strip or 4mm x 25 stainless steel strip. I'm also planning to use M10 stainless steel bolts with locking washer to clamp everything together.

Which material would make the better busbar?

 

[bendyone]

Copper, tinned if possible.

 

[cherod]

Copper, tinned if possible.

ok , accepted, but is SS ok also ?

 

[Poey50]

ok , accepted, but is SS ok also ?

Copper by a factor of approx x 40. Stainless steel is a poor conductor and should not be used. It is fine to use a stainless machine screw or stud to hold conductors together as long as you aren't relying on the stainless steel to do any significant conducting.

 

[Bru]

The stainless is completely unsuitable as stated above

The copper strip you've got is 80sq.mm. Without info on the battery bank capacity and loads it's hard to say whether that's sufficient but my gut if it were me would be to double or better still treble it up. It cannot be too thick but it can be too thin!

 

[simonfraser]

the SS has a higher resistance, but we are talking a very short distance and a chunky bus bar
what would the voltage drop differential be of copper vs SS when starting the engine ?

 

[pvb]

the SS has a higher resistance, but we are talking a very short distance and a chunky bus bar
what would the voltage drop differential be of copper vs SS when starting the engine ?

Minimal, maybe 0.05v.

 

[Poey50]

One formula for copper busbar current carrying capacity = 1.2 x Busbar width mm x Thickness mm in Amps.

So 3.16 x 25.4 x 1.2 = 96.3 amps. As Bru says, might be worth using multiples but not SS under any circumstances.

 

[pvb]

One formula for copper busbar current carrying capacity = 1.2 x Busbar width mm x Thickness mm in Amps.

So 3.16 x 25.4 x 1.2 = 96.3 amps. As Bru says, might be worth using multiples but not SS under any circumstances.

I'm not sure what that formula is supposed to mean. It sounds rather arbitrary. The cross-sectional area is around 80mm2 - if it were cable you'd expect it to be able to handle much more than 96A.

 

[Bru]

Minimal, maybe 0.05v.

How can you know? It depends on the length of the bus bar, which we dont know, and the load, which we also don't know!

 

[pvb]

How can you know? It depends on the length of the bus bar, which we dont know, and the load, which we also don't know!

I assumed the studs would be about 4cm apart, and that the starter current would be about 150A. A stainless steel busbar would give a voltage drop of around 0.05v.

 

[Poey50]

I'm not sure what that formula is supposed to mean. It sounds rather arbitrary. The cross-sectional area is around 80mm2 - if it were cable you'd expect it to be able to handle much more than 96A.

You're right it does seem too low - I'll check it.

 

[cherod]

oh well , thats a pity as i have tons of the bloody stuff but no copper .

 

[Poey50]

You're right it does seem too low - I'll check it.

I used the formula from this site to work out if my copper busbars for the LFP build were up to it. But would expect 80mm2 multi strand cable to carry much more. Someone may be able to explain the difference.

What is Busbar Current Carrying Capacity Calculation 5 Types of Busbar | Electrical4u[/URL

Edit: other charts show 2-3 times more amps cable of being carried by that cross-section.

 

[Skylark]

Thanks for the replies so far. It’s for my house bank, 4x 110 AHr. I’m still trying to fit the battery monitor that I bought in February. I’ve only seen my boat twice since then ?

There are two battery negative wires, each from two batteries. One is a short run and is 75mm2, the other run is longer and the cable is 95mm2. I want to join these to a very short run of 95mm2 to the shunt.

I can’t readily find 6mm x 1 inch copper, other than one supplier with a 3m min order quantity. I need less than 100mm. Is doubling 1/8th a crime?

 

[Bru]

. Is doubling 1/8th a crime?

No crime at all

In fact, if you really want to get esoteric about it there's actually a very marginal (at these scales) advantage but it's so negligible it's irrelevant in this application

 

[Bru]

I assumed the studs would be about 4cm apart, and that the starter current would be about 150A. A stainless steel busbar would give a voltage drop of around 0.05v.

Fairy nuff

I'll trust your maths cos i hate doing sums

 

[rogerthebodger]

I used aluminum for buss bars carrying high current to my bow thruster and windlass.

Mine are 40mm wide by 20mm thick actually 2 off 10mm thick making up 20mm.

All bolted together with brass set screws to avoid the aluminium / stainless corrosion.

Copper is better but would cost me an arm and 2 legs.

 

[BabaYaga]

Thanks for the replies so far. It’s for my house bank, 4x 110 AHr. I’m still trying to fit the battery monitor that I bought in February. I’ve only seen my boat twice since then ?

There are two battery negative wires, each from two batteries. One is a short run and is 75mm2, the other run is longer and the cable is 95mm2. I want to join these to a very short run of 95mm2 to the shunt.

I can’t readily find 6mm x 1 inch copper, other than one supplier with a 3m min order quantity. I need less than 100mm. Is doubling 1/8th a crime?

If you have three terminations of this type...
Copper Tube Terminals - 70mm² Cable
(adjusted to the cable sizes in question and with similar 10,5mm hole)
could you just not simply run a 10mm stainless bolt through the three of them, secure with nut and washer and do away with the bus bar altogether?
Must admit, that's what I have done in a somewhat similar case...

 

[Skylark]

If you have three terminations of this type...
Copper Tube Terminals - 70mm² Cable
(adjusted to the cable sizes in question and with similar 10,5mm hole)
could you just not simply run a 10mm stainless bolt through the three of them, secure with nut and washer and do away with the bus bar altogether?
Must admit, that's what I have done in a somewhat similar case...

That was Plan A but I simply couldn’t get the three cables to align. I'm trying to remove two cables from the main negative switch and join them to a new cable going to the shunt. The space is tightly packed with cables and 95mm2 isn't very compliant. Adding a busbar should help. Installing the monitor is more challennging than I'd hoped