Copper Bus Bar Sizing for DC low-voltage high-amperage

A. Hi TJ. Please introduce yourself and your situation. I assumed you were inquiring about the open bus bar systems typically used in electroplating, anodizing, and other metal finishing shops to bring DC power to the tanks, such as illustrated in the "Anode and Cathode Rod and Bus Systems" chapter of The Electroplating Engineering Handbook -->
but after looking up your company, I don't think that's the case, and what you will read on this page may be misleading :-(

Sorry, I don't know the answers for your situation.

Luck & Regards,

A. This link should give you all the info you need.

www.stormcopper.com/design/Buss-Bar-Ampacities.htm

A. Thanks, Kurt. I think that page should be fine for AC current calculations.

For DC bussing for a rectifier, however, I think the standard number used for decades, 1000 amps/square inch for open air bussing, is an easier and safer approach.

Khor, for an electroplating bussing installation I would consider the ampacity of a 12 mm x 100 mm busbar (slightly under 1/2" x slightly under 4") to be about 1860 Amps. You need at least two, not one, 12 mm x 100 mm busbars, although most experienced installers would probably use four or five 6 mm x 100 mm bars instead. Aren't the connections coming out of the rectifier implying that four or five 6 x 100 mm bars are suggested?

Regards,

i. Ted
I have never claimed to be an electrical engineer, but have run a lot of wire in my day. I never considered that there would be a difference in ampacity for AC vs DC. I reached out to the folks at Storm Copper for validation. My question was very simple:

Is there a significant difference between AC ampacity and DC ampacity?

There response was:

Yes. DC causes greater heating of the bus. I believe de-rate AC values around 30%.

A quick study of their ampacity tables complicates the issue even more. a 1/2" x 2" buss is listed at 1000 amps. Fits your number perfectly. A 1/4" x 4" buss however is at 1250 amps. Maybe more heat transfer area to keep it cool.

However, Storm is suggesting to derate the AC by 30% for DC current meaning those two bars would be 700 and 875 amps.

! Thanks again, Kurt. I've been sizing DC bus bar the same way for 40 years, so I would dearly love to get out of that rut and hear a newer and better way. And I understand that a 1/4 x 4 bar has greater surface area and cooling, and consequently higher current carrying capacity, than a 1/2 by 2 bar.

But the fact is, 1000 Amps/sq. in has proven itself for decades. If these charts indicated higher ampacity than the simple old-fashioned method, I'd want to go along with them. To the degree that they suggest that copper can only carry 70 to 87 percent of what we know for a fact that it has safely and reliably carried in tens of thousands of plating and anodizing installations over 5 or more decades, I think it is they who need to go back to the drawing board :-)

Their figures may be for bus bars enclosed in power distribution boxes or bus ducts rather than out in the open air.

Regards,

A. Hi, Petr. As you see, we appended your inquiry to a thread which I think will answer it for you: the copper needs 1 inch of cross sectional area for every 1000 amps. Good luck.

Regards,

Q. Thank you very much for your post. I have a follow up please, what about fixtures for hanging the parts in the tank? When we do inside diameters or fixtures that require a threaded bar, we use steel to support the mass of the part(s). How can I determine the resistance of the fixture when I have no idea about its temperature? In other words, how large does the cross section of the steel fixture have to be in to facilitate proper current flow or at least so that it does not overheat and fail mechanically.

A. Hello again, Petr. Steel is an alloy, and the conductivity of alloys varies very widely, with some stainless steels being only 3 percent as conductive as copper, and some plains steels close to 15 percent, so it's hard to put a perfect number on this. But figure it is about 10 percent of copper. That means I'd go with 100 Amps/sq.in. when submerged, even if the particular steel were a little lower in conductivity.

Obviously, this is a suggestion only applicable to submerged plating jigs, and not to be extrapolated that it's okay to figure steel as 10 percent of copper in all applications :-)

Regards,

 Thank you for your help Ted.

A. Hi, Suresh. To measure the resistance of a bus bar you can use Ohm's Law. Back when I was in school it was R = E / I, today you probably hear it as R = V / A. You simply measure the voltage drop while the bar is carrying a known current.

DC currents have no inductance; inductance is the voltage induced rather by a non-steady-state current. L = E / (dI/dt)

Regards,

Q. Hello, I need to know the calculation for DC bus bars Size for Copper and Aluminium.

A. Hi, Naresh. Hi, Sujin. Please get tables from suppliers like stormcopper or consult the National Electrical Code. Aluminum bus bar seems to be rated about 50 to 60 percent as conductive as copper.

But please keep the discussion moving forward by trying your very best to phrase ongoing questions in terms of the answers that have already been offered. If you didn't understand something and want clarification, that's fine -- people will try explaining it in different words ... but few readers will even read let alone try to answer a question that was already answered and is then simply repeated  :-)

Regards,

A. Hi, Hitendra. Good question. I've used 1000 Amps / square inch (in open air) in plating and anodizing shops for many decades, so I think 1/10 of a square inch would be correct for 100 Amps continuous load. But unfortunately I have no actual experience in short peak loads, so I can't answer from experience. I think the correct answer will be slightly more than 1/10 of a square inch for a peak load for only one second, but I can't say for sure, and feel that the codes probably want you to size things based on maximum-ever loads for safety, not usual loads. Hopefully someone who knows will help us.

Regards,

A. Hi Chris.

I can't exactly answer your inquiry, but think of this: when two flat surfaces of say 1" x 1" contact each other, we might say that the contact area is one square inch, and the DC ampacity of the joint is therefore 1000 Amps. But if you were to look at that contact surface through a powerful microscope, you would see that only very small peaks and very limited areas are actually touching. We still say the ampacity is 1000 Amps; we don't calculate a reduced capacity based on the smaller contact area we see in the microscope. That is, we have a macro thumb-rule that a square inch of copper can carry 1000 Amps, and we don't apply that macro rule to a micro situation. I think that's the case with the triangular bus bars, which are intended to deliver a knife-edge (and therefore better) contact surface.

I think that we can see from Ohm's Law that the knife-edge contact is not causing a significant resistance, and we can see (with a thermal scanner if not by hand) that there is no dangerous heat buildup at the knife edge. To me, that's the end of the analysis, the ampacity of the contact is based on the cross sectional area of the bus bar or the header bar, whichever is smaller.

Note to readers: The internet is a giant one-room schoolhouse where you can "overhear" stuff and misapply it; I'm talking about copper bus bars carrying DC current in electrowinning operations; I'm not offering a general theory applicable to all electrical contact surfaces :-)

Regards,

A. Hi Sunny.

Cables are widely used in plating shops. Although it's true that the insulation may make them get somewhat warmer than an open bus bar, I've never personally seen a problem if the old thumbrule of one square inch per 1000 amps of rectifier capacity is used. Welding cable is often used because it's both flexible and designed to carry substantial currents.

Regards,

A. Hi Joe. Yes, exactly. This will work, and it should be done immediately before you risk catching the tank liner on fire. Don't forget to check the ampacity of the anode rod when you fix the cathode rod.

Regards,

A. Hi Mike. If there is something you don't understand on this page, we'll be happy to try to explain it in different words until it is completely clear. But the voltage has little to do with it (in this limited context of bussing a plating tank that operates at the usual industrial levels). The length of the run is not totally irrelevant, but for typical plating shop runs, like 10.5 m, it's not real important. My own thumb rule, which was already a well established guideline at the plating equipment manufacturer I went to work for in 1967 is simply 1 square inch (645 square millimeters) of cross sectional area in the bus bars for every thousand amps you want to carry. This gives you measurable but not excessive voltage drops, and warm but not hot bus bars. So you would need 645 * 400/1000 = 258 square millimeters, and you've only got 228.

My opinion, and that's all it is, is that you should have ordered 6 mm x 52 mm (1/4" x 2") bars. The rectifier terminal often implies what size bus bar the rectifier manufacturer feels is appropriate. But whether you should proceed or start over may be a different question, and I suspect that most people would advise you to just continue :-)

Regards,

A. Good day Mihai.

Ted has a good point about the calculations, but this is not written in stone. Your busing is 12% short of IDEAL calculations. I feel you can proceed with what you have.
I would, however, like to make a suggestion.
If you are leading each end of your cathode/anode bars from the MAIN busbar, I found that these leads from the MAIN busing should be of the same length.
You can measure the amperage along the length of the cathode bar on each rack/fixture with a tong tester/ammeter to verify the amperage is consistent.
Hope this helps.

Regards,

A. Hi Shiva. I think you may need to retain an experienced electrician, electrical engineer, or plating consultant -- someone who has worked with this stuff and is familiar with the concepts being discussed. How to size this stuff has been explained numerous times just on this one thread alone, and if you can't follow it sufficiently to do the arithmetic yourself, you don't understand enough to install bus bar without guidance. Although voltage drop is proportional to the length of the bus bar, it's not conventional to use different cross-sections of bus bar for lengths of 6 meters vs 1 meter.

Safety issues are tricky at best. Is this a manually operated installation? I am long familiar with open bus bar systems which have no insulation at all, and which have been used for many decades in plating shops -- but I've also seen movement toward people no longer accepting open bus bar systems as being safe, especially as we get to higher voltages like 24 V. Welding cable sized at 1000 Amps/square inch can be used as an alternative to open bus bar, but doesn't fully address the issue of people working with live 24 v systems. A metal tool accidentally shorting out a 15,000 Amp power supply can be a spectacular event! In any case, you obviously should minimize potential exposure to live circuits even though they are only 24 V, and people in the area should clearly understand the situation. Sorry that it's hard to be of much more help from such a distance.

Regards,

A. Hi Khaled. The resistance of the bus bar is calculated as: R = ρ x l / A
The resistivity of copper bus bar is about 1.68 x 10^-8 ohm-m, so I get as the resistance of your bus bar:
1.68 x 10^-8 ohm-m x 100 m / (1600 mm² x 1 m² / 1,000,000 mm²), so:
R = 1.05 x 10^-3 Ω

And V = I x R, so I get: V = 2000 A x 1.05 x 10^-3 Ω = 2.1 Volts

That's a lot, and it would apply to each pole if they're both 100 m long. Then again, 100 m is pretty long for a DC bus run.

The KVA waste sounds like 2000 A x 2.1 V /1000 = 4.2 KVA.

Regards,

A. Hi cousin Hojat. Sorry, but we'd just run in circles and never get anywhere if, after 30 postings on a subject, instead of asking for specific clarifications people just say "start over and tell me everything" :-)

I don't see how the voltage drop and cross section calculation you wish to do is any different than Khaled's example which I just worked for you. Sorry, I don't know the codes regarding short circuit protection (if any) in your situation, and my main experience is with low voltage busbar, not 200 V busbar. Good luck.

Regards,

A. Hi Danny. 1/4" x 3" bus bar is rated 750 Amps, not 1500 Amps, based on 1000 A/sq.in. If you are speaking of an anode rod or cathode bar on top of the tank, yes, it's capacity could be 1500 Amps if it is bussed from both ends (750 A coming in from each end).

Regards,

A. Hi Anand. Sorry, I don't have any experience with carbon brushes, so you'll have to wait for input from others on that. I've already offered my opinion on cables and the sizing criteria I use twice on this page; maybe someone will offer a second opinion.

Good luck.

Regards,

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A. Hi Drew. There should be no decline. I have seen plating shops clear coat or lacquer their bus bar runs (except for the connections of course) and it does seem to hold up well and to look better than streaky green stained copper.

Bus bar connections are actually supposed to be silvered or tinned. I visited a GE or Westinghouse bus bar plant decades ago where they hung bus bar vertically and dipped it 4"-6" deep into the plating solution to plate the ends. Bus bars are also supposed to be clamped together rather than bolted, I believe. But in my own experience, plating shops rarely use plated ends or buss bar clamps: they just drill holes, and wire brush No-Ox-Id compound onto the joint area before bolting together. Good luck.

Regards,

A. Hi Ajay. Are all components at least 1/4" x 3/4"? There should be no spot on the switch where the cross sectional area of the copper is less than 1/5 in² because the current carrying capacity of copper is 1000 A/in².

Are you sure the copper is all hard drawn electrolytically pure? The current carrying capacity is drastically reduced with even a very small amount of alloying materials. Did you wire brush the joints and apply conductive joint compound?

Regards,

A. Hi Virendra. Generally, the allowable temperature rise is fairly low and it doesn't do much good to design to a higher allowable temperature rise anyway -- because conductivity is inversely proportional to temperature. So you will see that empirical numbers are used instead. Here in the USA and in the electroplating industry, the nice round number of 1000 Amps per square inch for open bus bar was universally accepted long before I entered the industry, and that was over 60 years ago :-)

The allowable current carrying capacity of enclosed bus bar in bus ducts is a little bit lower because of lack of cooling; since bus duct is a standardized product, you'll see the numbers in product literature and electrical codes.

According to my method, your 14 mm x 6 mm bar can carry 130 Amps in open air.

Regards,