Chassis wiring. Wire current capacity.

[Diabolical Artificer]

Whilst wiring up an amplifier recently with numerous circuits from 6v 300mA to 450v at 1A I've been giving a lot of thought as to what hookup wire gauge to use. As I'm always a bit skint my choice is a bit limited to 0.5mm copper stranded and various solid cored wire I've recycled.

In the past I've tended to go a bit OTT using far too thick wire but have recently based my wire gauge choice on a spec I lifted from somewhere, 2.5mm twin core 13A and 24AWG 0.5mm solid core 3.5 - 5A then derating it a bit for stranded wire by half ish depending on how many stands.

I'm winging a bit here so would like to base my wire choice on something more scientific. Is there a rule of thumb? Do you use a thicker gauge for a DC application (have never understood why thicker cable is needed for DC)? Also how close to wind can we sale as regards voltage rating, EG could we get away with using 0.75mm lighting cable for HT applications?

Andy.

[Peter.N.]

Old valve radio's used a twisted pair of not much more than 1mm and that was on 4 volt valves that drew more current than the 6v variety. You generally had fairly short runs so the loss was very small anyway.

Peter

[frankmcvey]

Hi, Andy,

What you want is a current ratings table, like this one.

Most of the hook-up wire you see on Ebay is around 22-24 AWG.

Cheers,

Frank

[Craig Sawyers]

Regular hook up wire has a voltage rating of 600V, so 450V is fairly close. You might want to double insulate runs with an external sleeve or use higher spec wire with thicker insulation that is good to 1000V, and at least gives you some headroom.

Craig

[MrBungle]

I tend to use this stuff for HV up to a kV or so. Dirt cheap as you can buy it by the metre and good quality: https://www.bitsbox.co.uk/index.php?...e47584tr2bkvo6

It's like little ropes!

I've stuck 5A through it at 12v without any sign of heating so it's suitably rated for the 3A it says it is.

[Refugee]

Look out for scrap washing machines. The heater wires are good for 10 amps.
Always screw the top back on after removal of the wiring so that it remains attached when the scrappy takes the remains away.

[broadgage]

The minimum wire size is determined by the maximum current, AC versus DC makes no difference.

It seems to be an urban myth that "DC needs thicker wire" I suspect that this derives from DC circuits in vehicles, these often need relatively thick wire, but that is because the much lower voltage leads to higher currents, NOT because it is DC.

Scrap washing machines are indeed an excellent source of recycled wire, the size and relative complexity of a washing machine means that they contain a lot more wire than any other common appliance.

Sometimes rigid single core wire is preferred, this may be stripped out of scrap fluorescent light fittings.

[Herald1360]

Bit OT, but in extremis, so to speak, ac needs "thinner" wire since the skin depth becomes important even at 50Hz. Heavy duty busbars are usually laminated using multiple spaced layers of about 6mm or 1/4" thick bars.

Streanded wire per se has same rating for same CSA- 7/0.2mm or 0.22 ish sqmm is good for 3 A or so, 16/0.2mm or 0.5ish sq mm about 6 A.

13A get away with 1.3 sqmm but 1.5 better. 2.5 sqmm good for 20A (two in "parallel" is how a 30A ring main works with some headroom for unequal length legs).

Bare wire can be run harder if volt drop isn't an issue- the ratings above are for bog standard PVC insulation. PTFE or silicon rubber can stand higher temperatures too.

Regular hook up wire has a voltage rating of 600V, so 450V is fairly close

But within the rating, it will suffice.

since the skin depth becomes important even at 50Hz

it is millimetres at 50Hz, about 10mm (as far as I recall), so if you can bend it into a chassis forget it.

[Craig Sawyers]

Originally Posted by merlinmaxwell

But within the rating, it will suffice.

Personally I would be uncomfortable using insulation rated at only 33% above the applied voltage. 450V at 1A getting loose is no joke, and even with insulation rated at 1000V I would want the chassis to be seriously earth bonded.

Craig

So why is it rated at X volts.

[G8HQP Dave]

An issue not mentioned so far in this thread is that under the chassis it can be hot so the current capability of the wire is reduced.

[Ed_Dinning]

Hi Gents, a good rule of thumb is 3A per mm^2 of copper area.

Ed

[Craig Sawyers]

Originally Posted by G8HQP Dave

An issue not mentioned so far in this thread is that under the chassis it can be hot so the current capability of the wire is reduced.

Yes - that is a good point. After ten years under the bonnet of my old 1980 Jag the PVC wire insulation was reduced to the the same flexibility as a rich tea biscuit - bend the wire and the insulation crumbled. Same with a Rover 820 I was loaned as a company car in the early '90s. I made the mistake of moving a wiring loom and a shower of PVC insulation crumbled and fell on the road leaving bare copper.

The key it turns out is that PVC is actually a rigid polymer, and is made flexible by adding plasticiser. At elevated temperature (70C and above) the plasticiser can come out, and the PVC itself degrade. Heat stabilisers are added to moderate this process, and modern PVC is probably better in this regard, but I would not like to bet on it for a high voltage high current supply in a valved amp.

All this is probably predicted by an Arrhenius process.

FWIW, for any underchassis wiring - or umbilicals connecting an external power supply to a valved amplifier - I use PTFE insulated wire, which I buy from anyone I can find on ebay that sells the stuff.

Craig

[Radio Wrangler]

The current ratings for wire are thermal

How much current can be put down an insulated wire before the insulation starts to melt or otherwise degrades depends on ambient temperature, bundling with other wires and the heat insulating properties of the insulation.

With insulation out of the way, then there is the degradation of the conductor itself. It isn't a spot temperature and ****! gone! rather time and cycling to somewhat below the melting point will have a cumulative effect. Unless you're designing fuse wire, it's the insulation which limits things in terms of temperature. Temperature rise is set by I squared R heating versus the surface area. Thicker wire drops R which is good, and also increases the surface area which is further good.

All of this looks at the wire in isolation to everything else. If your wire is getting hot, then you're wasting power and less than you intended is getting to the intended recipient. With 4v heaters, not only is the current in the wire greater than with 6.3v heaters, but also each millivolt of drop is more significant. So you need a more than proportionate increase in cross sectional area to move the same power at lower voltages.

Voltage drop also depends on length of run.

So when you've done your design, you want to see an acceptable voltage at all valve heaters, and your wire running only slightly above the air temperature in its vicinity.

You can get resistance from wire tables, and fusing currents, but you need manufacturer's data sheets for insulation limitations, and they leave you pretty much alone when it comes to bundling, though aviation cables and wires usually do have bundled ratings, though they usually assume all cables in a bundle are the same size, type and carry the same current.

David

[Diabolical Artificer]

Thanks chaps, that's the sort of info I was after. I've come across double insulated wire on PC monitors
which is good stuff and I got a wiring harness off some old switch gear that had a soldered or similar stranded core. this was great stuff as it has some flexibility but could also be bent to get neat corners. no idea if it's still made, brilliant stuff.

Thanks Frank, very usefull table, better than the ones I've found. And thanks Mr B for the link, very usefull being able to buy it by the metre.

As most of the stuff we service is high voltage low current wire doesn't need to be too thick but have good insulation. If in doubt I put heatshrink tube where I think it might be needed. I have had a situation where a volt or two was lost on heater wiring so am familiar with that issue.

Thanks again all, A.

[joebog1]

One AT type power supply will furnish enough wire for half a dozen amplifiers!
It will also furnish many colours. I have used it in amps running 1200 volts and have never had an insulation failure.

Joe