Welding Wire Sizing for DC Rectified Load to Anodizing Tanks

[mpalatas]

My company is an aerospace metal finisher. Our primary process is anodizing, which involves taking DC current and putting it through parts dipped in acids. To do that, we have DC rectifiers that put thousands of amps at fairly low voltages into anodes (parts) immersed in tanks filled with dilute acids. Cathodes at the sides of the tank take the current back to the rectifiers. I am trying to figure out the size of cabling I need to wire from the rectifier to the tank bus work.

All of the welding cable ampacity charts I find online show welding wire with 60% duty cycle, but some tables have 600V wire rating and some different tables that don't actually say what the voltage rating is.

Bottom line is that my applications are 100VDC or less, and distances from the rectifier to the tank connection are at 50'. I have a total load capacity of 4800A on one rectifier and 2400A on the other rectifier, and I want to keep the number of wires (positives and negatives) even. By most of the tables I found, that would mean (8) total 4/0 wires to the 4800A load, and (4) total 4/0 wires to the 2400A load. Can I get an independent confirm/deny?

Thanks!

Mike in Los Angeles

 

[paulengr]

Voltage does not impact ampacity. It can be 230 kV or 2.3 kV but ampacity is ampacity, period. So you can just use the standard Neher-McGrath tables.

DC is close enough to 60 Hz AC impedance wise so even though it’s DC again the tables will work fine. It’s only in 400 Hz systems that you have to upsize.

 

[hillbilly1]

I have a customer that wants to get into anodizing, he wants to do anodized door pulls. (Commercial doors) He says it must be witchcraft, because nobody wants to give him any information on the rectifiers amperage or voltage! LOL!

 

[zbang]

Let's start with some basics-
Original voltage isn't relevant, the same current produces the same drop whether it starts at 100v or 20v.
"Ampacity" ratings of wire are based on how much heat the insulation can stand before melting or catching fire, so a teflon-insulated 12g wire can carry more current than a chocolate-insulated wire with damage, but drop (and heat loss) will be the same.
Welding cables are rated with that duty cycle so they're "allowed" to carry more, but with time to cool off. "Welding cable" isn't an NEC cable type.

We here are always asking questions ---
Does this need to play by the NEC? (Possibly, see article 668 & 669; remember that the NEC is a fire-prevention code, not a safety code or design manual)
What's the allowable voltage drop over these cables (which becomes the power loss and waste heat generated)?
What's the process duty cycle? Will the cables have time to cool off between use or is it basically continuous?
Do you need flexibility over the entire length or only part of it?

You can start with the NEC tables for THHN wire, and also look at the numbers for SC or RHH/RHW (fine-strand flexible), to get an idea of sizing.
Beware of terminal/termination temperature ratings- usually you need to use the 60deg columns since many terminals are only rated for that, some for 75.
Are there environmental factors which favor one insulation over another?

Going to the original numbers "(4) total 4/0 wires to the 2400A" (600amp/cable) - maybe for low-duty-cycle welding cable but that's not supported by the NEC. In conduit, 4/0 copper THHN at 90deg is only 260 amps (table 310.13 etc), being in free air bumps that to 460amp, but you might need to use the 75 or 60 deg columns which only push it down.

When you get to those sorts of currents, things like bus bars or bussway begins to look attractive. So does larger gauge aluminum wire.

 

[mpalatas]

Hillbilly1, pass to your anodize-frustrated buddy:

The ampacity and voltage of the rectifier (on the DC side) depends only on the square footage of parts he wants to do in the tank in a single load. For aluminum anodizing of most common alloys (6061 for aerospace anyway), the thumbrule is 30 amps per square foot of part load. So the first thing he has to do is size his tank to fit the parts, leaving about a foot on each side of the parts to accommodate the cathodes at the inside walls of the tank. Then he needs to figure out the square footage of parts he wants to be able to anodize in a single load. Then he multiplies that by 30A / SQFT - and I would add a 10% safety factor. That gives him the ampacity. As far as the voltage, the rectifier for any "Type II" (most common commercial anodize), an output of 0 to 25 - 30 VDC would be fine. Then you get a controller that allows you to adjust the DC output. And you have to have the tank at the right temp, so cooling is typically required - done by coils in the tank through which glycol is circulated. Typical Type II temps of a 15% diluted (B/V) sulfuric acid solution are in the 70 to 75 degree F range. I actually have a used 6'L x 3'W x 4'D tank for sale right now, if you want to put him in touch with me. It was used for Type III (hard) anodize, but will work fine for Type II as well.

 

[hillbilly1]

Hillbilly1, pass to your anodize-frustrated buddy:

The ampacity and voltage of the rectifier (on the DC side) depends only on the square footage of parts he wants to do in the tank in a single load. For aluminum anodizing of most common alloys (6061 for aerospace anyway), the thumbrule is 30 amps per square foot of part load. So the first thing he has to do is size his tank to fit the parts, leaving about a foot on each side of the parts to accommodate the cathodes at the inside walls of the tank. Then he needs to figure out the square footage of parts he wants to be able to anodize in a single load. Then he multiplies that by 30A / SQFT - and I would add a 10% safety factor. That gives him the ampacity. As far as the voltage, the rectifier for any "Type II" (most common commercial anodize), an output of 0 to 25 - 30 VDC would be fine. Then you get a controller that allows you to adjust the DC output. And you have to have the tank at the right temp, so cooling is typically required - done by coils in the tank through which glycol is circulated. Typical Type II temps of a 15% diluted (B/V) sulfuric acid solution are in the 70 to 75 degree F range. I actually have a used 6'L x 3'W x 4'D tank for sale right now, if you want to put him in touch with me. It was used for Type III (hard) anodize, but will work fine for Type II as well.

Thanks! He’s been buying bits and pieces here and there, and I think he already has the tank, and has been trying to figure out what size of rectifier he needed.

 

[augie47]

Are your cables in free air ? conduit ? tray ?? bundled ?

 

[winnie]

Also because the voltage is so low and the current so high, voltage drop is a significant consideration.

50 feet of 8x 4/0 carrying 4800A gives a voltage drop of about 1.5V each way. And yes, that cable is undersized by ampacity; that run is dissipating 7200W each way.

Jon

 

[zbang]

18W per foot - won't be glowing but sure will be warm; do not try this is a raceway.