# DC bus bar ampacities



## DJBecker (Nov 3, 2010)

I don't see it as especially wiki-worthy.

The sizing is based entirely on steady-state convective cooling rates of fixed installation. None of the conditions apply in an EV, where bus bars are cooling the attached components and the environmental conditions vary widely.

Sure, the table is useful as a general guideline, but there are many other tables for general sizing. And most of the sizes listed on this table aren't likely to be encountered.


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## Tahoe Tim (Feb 20, 2010)

I thought it would help match up the proper bus bar with the common 2/0 and 4/0 wire we use. Oh well, it was a nice shortcut for me.


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## dtbaker (Jan 5, 2008)

not really what you need since the multiple thinner strips or braided straps carry way more than thick single bars...


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## EVfun (Mar 14, 2010)

Why do you feel that multiple thinner strips will carry more current? The EV battery pack doesn't have a significant AC component to it so skin effect shouldn't need to be considered.

I generally just grab this chart to figure out bus bar size or convert from metric wire size to wire gauge.


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## dtbaker (Jan 5, 2008)

EVfun said:


> Why do you feel that multiple thinner strips will carry more current? The EV battery pack doesn't have a significant AC component to it so skin effect shouldn't need to be considered.
> 
> I generally just grab this chart to figure out bus bar size or convert from metric wire size to wire gauge.


I am just going by empirical observation in that the welding cables (DC) have more and thinner strands, but maybe that is just for flex. I also note that the bus bars from TS and CALB are typically several thin strips held together with shrinkwrap... I am assuming there is a reason they do that rather than one thick bar.

I'm not an EE, so thats only my observation, no technical backup on this one.


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## peggus (Feb 18, 2008)

The cross section area, electrical conductivity, cooling surface area, insulation thermal conductivity, frequency, acceptable temperature rise and air flow, determine ampacity of a conductor.


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## Tesseract (Sep 27, 2008)

There isn't a fixed ampacity, per se, for copper busbars as it depends on way too many variables (as peggus just pointed out). 

Of course, currents in both the motor and battery loop vary wildly in EV applications, even during highway driving, which makes it even harder to nail down the required cross-sectional area for the conductors.

That said, the (_extremely_ conservative) rule from the NEC is 1.55A/mm² (1000A/in²) for pure copper busbar. This rule results in a rather low 30C rise above ambient, when a 60C or even a 90C rise might be perfectly acceptable in a given (non-NEC) application.

That said, this is a good sizing rule for the expected continuous current (e.g. - 300A on the motor side with the typical 9" advanced timing motor), as relatively long overloads of twice that will be perfectly acceptable (causing a 60C rise) while briefer overloads of 10x or even 20x that amount will be ok, too.

The ampacity of a busbar is ultimately limited by the allowable temperature rise (which can't be any higher than the melting point of copper). Consequently, a wider but thinner bus bar will have a higher ampacity than a thicker bus bar with the same cross-sectional area because the former has more surface area to shed heat.


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## EVfun (Mar 14, 2010)

Yes, but the multiple thinner straps used on the bus bars from TS and CALB are stacked directly on top of one another and covered in heat shrink, negating any increase in ampacity. 

I've built a few 120 volt EVs. I generally use 1/0 cable or 50mm^2 bus bar for the battery side and 2/0 cable for the motor loop. I've never had any problems with wire heating.


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## Tesseract (Sep 27, 2008)

dtbaker said:


> I also note that the bus bars from TS and CALB are typically several thin strips held together with shrinkwrap... I am assuming there is a reason they do that rather than one thick bar.


Yep, but the reason is because it is more flexible/compliant than a single thick copper strap, not because it has lower AC impedance or anything.


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## azdeltawye (Dec 30, 2008)

dtbaker said:


> ... I also note that the bus bars from TS and CALB are typically several thin strips held together with shrinkwrap...


This is to allow the bus bar to flex easily and not put strain on the battery terminals. Even when the battery cells are tightly bound together there is still some relative movement between cells. Especially in and automotive environment…


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## alexbeatle (Jul 28, 2020)

The bus bars made by zeroEV for Tesla batteries are 3mm thick by 30mm wide.
One moment, please...
Is this roughly equivalent to 2/0 awg wire?

Looking to diy bus bars equivalent to 2/0 wire size..


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## remy_martian (Feb 4, 2019)

I wouldn't F with them at that price and would go off and work on the stuff I can't buy.

Be honest, Alex. How long will it take you to make the jig, then drill out and bend each bar, then heatshrink them? 

You'd be further ahead with your time by flipping burgers ($15/hr now, allegedly) and then ordering those bars, imo.


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## alexbeatle (Jul 28, 2020)

remy_martian said:


> I wouldn't F with them at that price and would go off and work on the stuff I can't buy.
> 
> Be honest, Alex. How long will it take you to make the jig, then drill out and bend each bar, then heatshrink them?
> 
> You'd be further ahead with your time by flipping burgers ($15/hr now, allegedly) and then ordering those bars, imo.


I.e. 3mm by 30mm of copper bar is a good substitute for the 2/0awg cable?


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## remy_martian (Feb 4, 2019)

No.

The cable is insulated and shielded.


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## MattsAwesomeStuff (Aug 10, 2017)

alexbeatle said:


> Is this roughly equivalent to 2/0 awg wire? Looking to diy bus bars equivalent to 2/0 wire size..


Easy answer is that all that really matters is the cross-sectional area. There's a little subtlety to it, but, that'll get you 95% accurate and good enough.

Build 'em if you want to, buy 'em if you don't. No judgment.






American Wire Gauge Chart and AWG Electrical Current Load Limits table with ampacities, wire sizes, skin depth frequencies and wire breaking strength


AWG Wire size chart and ampacity table for design engineers including skin depth frequencies and tensile strength data; electrical cable size



www.powerstream.com





3mm x 30mm = 90mm^2

2/0 is 67mm^2
3/0 is 85mm^2
4/0 is 107mm^2


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