Wiring Questions

[Kmpkt]

Hey Guys,

A couple of questions regarding wiring for the low wattage (<300W) builds we often see on here. Firstly given the relatively low wattage, is there any significant problem with using 20AWG vs 18AWG wire from the DC-DC board onwards? I imagine 20AWG is necessary the wall socket to an AC-DC module or from the AC-DC module to the DC-DC board since loads are higher, but after being split is this really necessary?

One of the issues I ran into doing my recent NFC S4 build is that for very short wires the bend radius and flexibility of the 18AWG I used kinda sucked. Can someone recommend a "best brand" for maximum flexibility as the wire I most recently bought was quite stiff and was probably a significant part of the issues I had. Cheers.

 

[jeshikat]

"It depends."

SATA power for a typical SSD? Almost anything would work. Running a mid-range GPU off a single wire? Better do 16AWG.

I've been meaning to look into silicone insulated wires. I have some Pomona leads for my multimeter with it and they're stupidly flexible, there's no wire memory at all, like none.

 

[Kmpkt]

Yeah. That's what I want. The stuff I bought was standard issue wire from the in-town electronics shop and it sucks. Stiff, terrible bend radius and manoeuvrability, etc. If you figure out what the best stuff is, please let me know.

 

[jeshikat]

This stuff seems pretty interesting: http://www.polarwire.com/aufb-single.html

Super flexible, made in USA, UL-listed, available in a variety of colors, and not too expensive at $15/100ft (30m).

Only available down to 18AWG though, but if they're anything like my Pomona test leads they should literally be about as flexible as freshly cooked spaghetti noodles.

Also, they're "only" rated to 105°C, which is on the low side for silicone insulated wires since they're optimized for low temps (-55°C).

 

[Kmpkt]

Ordered an orange and black precut 18 AWG set for my S4 since I need to redo a set of cables for my DC-DC.

 

[jeshikat]

Very nice, I'll look forward to hearing what you think of it.

 

[Kmpkt]

I'm kinda playing around with doing my cables like this if the stuff is as pliable as you think.

http://www.paracordguild.com/8-strand-round-braid/

 

[jeshikat]

Did they ship the cables yet and if so, does it actually ship from Alaska?

 

[Kmpkt]

I got them last night. I'll check and see where it shipped from. What I can say is they are definitely MUCH more flexible than the PVC coated 18 AWG I used in my last attempt to make a short 24 pin cable. That being said, they are not spaghetti flexible, so any kind of braiding for aesthetic purposes definitely isn't going to happen with this cabling. That being said, I am really hopeful that I can make a ~7cm 24 pin cable that actually works. My only gripe with the wires so far is that the orange cable is kind of a peachy colour which doesn't exactly match the S4 I'm building in.

 

[jeshikat]

That being said, they are not spaghetti flexible, so any kind of braiding for aesthetic purposes definitely isn't going to happen with this cabling.

That's a bummer. Though does that mean they have enough internal stiffness that they can roughly hold their shape? Or will they still need to be zip-tied to prevent them from straying into fans and such.

 

[EdZ]

My only gripe with the wires so far is that the orange cable is kind of a peachy colour which doesn't exactly match the S4 I'm building in.

You could use a silicone dye (e.g. Smooth-on's SilcPig) to recolour the wires to the desired shade.

 

[Kmpkt]

I think they're wobbly enough that they would stray quite easily if they were on a longer lead without some sort of combing.

Another question for the more knowledgable here: reading online it seems the difference between an 18 AWG and 20 AWG cable is that the former needs to be kept under 20 amps whereas the latter can carry up to 25 amps. 20 amps at 12V is 240W. There is nowhere in most small form factor computers that requires 20 amps on a single strand anywhere except perhaps from the wall to an internal AC/DC. Is there a reason why for low wattage (say under 300W for a system) one couldn't safely use exclusively 20 AWG cables?

 

[jtd871]

Maybe not a *sustained* 20A... but peaks do happen, and the wires could get toasty.

 

[jeshikat]

reading online it seems the difference between an 18 AWG and 20 AWG cable is that the former needs to be kept under 20 amps whereas the latter can carry up to 25 amps.

Should be the other way.

There is nowhere in most small form factor computers that requires 20 amps on a single strand anywhere except perhaps from the wall to an internal AC/DC.

The terminals are the weak point for rated amperage of the whole cable harness. From the Molex Mini-Fit Jr. product specifications, page 3: http://www.molex.com/pdm_docs/ps/PS-5556-001.pdf

So IANAEE, but it looks like a 24 circuit connector with all wires powered is only rated for 5A per terminal at 20 AWG. I think the reason the rated amperage per wire goes down as the circuit count goes up is because of the extra heat generated by the higher overall wattage.

It gets weird because a 24-pin motherboard connector is not running all the wires anywhere close to full load. The special wires like Power Good barely do anything while the two 12V wires can see quite a bit of current with multiple video cards installed.

But even in the best case scenario, 18 AWG has a rated current of 9A while 20 AWG is only 7A. So while 20 AWG wire may be capable of 20A, the terminal is only rated for 7A at the most.

 

[Kmpkt]

Fair enough. I guess my question then would be how large a peak are we talking? In the case of say a GTX 1060, the card works on a steady 120W, but after looking at a few different benchmarks it would appear that the card's peaks are under about 6A from the PEG connector under heavy load (2A per wire). Looking at the 1080 and things get a bit different. 400W peaks seem to happen uncommonly over max load. If my math is correct, this would be about 325W spread between the 4 12V lines through the 8 pin PEG or 80W per wire (8A). Am I thinking of this the right way or is my math/logic faulty?

 

[Kmpkt]

Couldn't I just slap a 18 AWG terminal on a 20 AWG wire? Crimp should still be plenty tight and I could always solder.

 

[jeshikat]

Couldn't I just slap a 18 AWG terminal on a 20 AWG wire? Crimp should still be plenty tight and I could always solder.

The Molex Mini-Fit Jr terminals come in two sizes: 16 AWG and 24-18 AWG.

I think the difference in rated current is the surface area of wire that makes contact with the terminal. I have no idea how solder would affect the current rating. But also, the Molex ratings are with some safety margin, so it'd probably be fine if there are occasional spikes above the rated amperage.

 

[jeshikat]

400W peaks seem to happen uncommonly over max load. If my math is correct, this would be about 325W spread between the 4 12V lines through the 8 pin PEG or 80W per wire (8A).

There's only 3 12V wires in a 8-pin PEG connector, so it'd be 9A per wire. But if you're getting those peak numbers from Tom's Hardware's review, keep in mind that they measure at very short time increments so those momentary spikes shouldn't matter much in the context of wire gauges.

 

[Kmpkt]

Did they ship the cables yet and if so, does it actually ship from Alaska?

Straight outta Anchorage

 

[jeshikat]

Nice, that's pretty legit for a company called Polar Wire.