Cable current rating advice

You are potentially looking at two different scenarios here. The tables in BS7671 are based around a 40 year service life at full load. Automotive products are usually rated for 3000hrs (about 4 months continuous use). For commercial vehicles different values may be used.


When a cable carries a current the ohmic heating causes the temperature to rise. This may take some hours to reach it's maximum. The degredation of the insulation due to the maximum temperature sets the published service life.


If the cable carried a higher current for a shorter time it will not reach it's maximum temperature. The starter cables on a typical vehicle probably only carry current for 30 seconds every hour. Thermal degradation of the insulation is not going to be a problem. Volt drop is probably the key factor for selecting the size. The traction motor cables on an electric truck will only carry full rated current when climbing a gradient, that will probably be for only a few minutes followed by a decent (a decent is interesting as if regenerative braking is used the cables will still be under quite a high load) or level running at a lower current allowing the cables to cool.


Under given conditions a 16mm2 wire may be suitable for 125A or even 150A but the conditions would have to be specified.

Hi Roger, thank you for the reply. 

This wiring is for charging on a motorhome. Now days with Lithium batteries you can charge at very high currents for a longer period of time than just starting a vehicle.


I would expect there would be some sort of regulation on the amount of current a cable can withstand before it starts to degrade the insulation. I understand the VD and the cable in question is ok for this.


I wonder if we do not have a regulation then where do we draw the line,  like you say 125a or 150a what is the limit. There must be data on this somewhere for insurance purposes if nothing else

There are SAE cable standards (Society of Automotive Engineers) standards for cables in cars and so on  that may be a better design standard for this sort of application.

( SAE_ AS50881  for example, but it is not cheap..)


Actually the data you would get is the same as scroll to page 47 of 84 of the old "MIL-W-5088L - Wiring, Aerospace Vehicle" that is free..  the line for 6AWG is pretty close to 16mm2

Now depending how hard you are being shot at, and your appetite for hot cables, you can decide on the rating of the cable yourself.


I have screenshotted the graph below. It really  is that spotty in the original, sorry. Now this graph is not quite what you  need but let us slide along the 6AWG line to show the principles of how it works.


So at the bottom of the line  a 30 degree rise (40C ambient, 70C core temp) gets you 75A.


A 50C rise between air and core temp is more like 100A.  - so 20C ambient 70C core, or 30C ambient 80c core.


150A is a 110C rise, so really the cable now needs running water cooling or a different sort of insulation, as for anything other than outside  in the arctic you will be damaging PVC, and can run it like that for a few hours at most before it will need replacement. The graph carries on up, but really that is for PTFE, mineral and silicone rubber cables, which you will not have.







If it was a commercial aircraft, you'd be looking at AC.13-1B , slightly different figures,  but the rules are fiddly as you then consider whole bundle current for looms, as well as the current in any one wire. (the grouping rules in BS7671 do not handle large bundles in the same way ) There you'd be looking a something like a 50 A fuse (75A cable rating)


The very fact that BS7671 makes no distinction between sheathed and non-sheathed cases tells you that for that sort of 'zip cord' the current rating in BS7671 will be a significant under-estimate. It also does not mention clipped vertically being different to horizontally, but a moment of thought about convection tells you there has to be a difference. The ratings are just conservative.


We are not that interested in where the insulation starts to degrade, as that has us looking at lifespans of half a million years for very cool running (only out of sunlight of course).  What we need is to exceed the maximum lifetime of the installation by  a factor of perhaps 2 or 3.


In reality the PVC internals are is quite happy with short flights to perhaps 120C, but the cable loses its flexibility when repeatedly heated that hot and cooled and if you need it to remain flexible then  a maximum  of 105C is a better choice. Now, what current gets the core of the wire to that temp when the environment is at say 30C is a very variable thing, depending on airflow, bunching with other wires, mounting points and orientation.


In short, 16mm zip cord will be OK for 100 amps, most of the time, but not in poorly ventilated places or in bundles of other hot cables.

150 A almost never OK.

70-90A pretty conservative (which is still too hot to touch by the way..)

Mike.

Mike thank you for this information, very interesting. From what I am reading and looking at your chart the original IEE charts I was looking at are a good indication of safe current capacity of a cable if it was intended for long periods of use. I expect shops selling this cable at 110A rating are doing this as a starter cable like Roger said for short burst this would be fine. The issue with advertising like this, it can be taken the wrong way and can lead to serious damage to vehicles.


So if we do pull 100A through 16mm cable the cable would get up to 50 deg if I am read this correct. Yes this would be too hot to touch and may cause issues with anything near it. May even run hotter as it will be within a battery bay. Cable it rated at max 70 deg so getting near to breakdown temperature of insulation. Also I would be a bit worried about running this hot next to a Lithium battery bank. All hypothetical as you would wonder what you would be running at this current.

As Mike says you can look at current ratings in many ways and under many different conditions. We make battery cables with crosslinked insulations which allows conductor temperatures up to 150°C or more so under the right circumsatnces a 16mm2 cable in free air will carry 222A for 3000hrs. By the time this is derated due to ambient conditions and grouping this will probably realisticaly be ~150A. The information starts on page 56.

HUBER+SUHNER Transportation—Page 56

edit :

replying to Steve above, not Roger ,

Note that  100A , 50 degree rise case may be just about OK in a 70c engine bay type environment, as the core is now at 120C, but it is not good for the cable.

Of course in vehicles a lot of the loads are different durations - motors for windows and so on, are not not really the same as windscreen wipers, as the latter may be on all day ?.

At the other extreme we have the kilowatt audio soundblaster that luckily only draws lots of amps when the drums go bang, and has an average of perhaps 10% of the peak, but the enthusiasts pay for cables based on the peak current.


I'd be tempted, especially near batteries, perhaps with long charging periods, to double up (so twice the copper, half the rating) if there is any doubt. But it is not simple, and short of instrumenting the wire loom and then running it, the answer is really quite uncertain.

As an aside I have worked on things where we embed temperature indicating labels, so we can tell when stuff comes back under warranty if it has been abused. These have tended to show that bundled wiring runs cooler than the theory predicts, and on the other hand heatsinks do not always work as well as expected...

mike.

Thank you Roger another nice document, so PVC is A class and can go up to 85 deg. For most people building charging circuits on there vehicles they go by what the supply is stating. From what you have both said I would say 80 amps continues running is max this cable can take in open air but to be safe I below 70 amps would be better

Just to check if I am reading Mikes chart correct at 80amps and 30 deg ambient temp the core temp would be 62 deg is this correct

Roger, that is an interesting datasheet, & definitely one for the archive, not however plain PVC as per the OP..

Just to check if I am reading Mikes chart correct at 80amps and 30 deg ambient temp the core temp would be 62 deg is this correct


yes - you go up from 80A to the diagonal line, and read across to 32 or 33 temperature rise. Add that to you ambient air temp to get the copper core temp.

Do realise it is not quite right, as 6AWG is not quite your 16mm2, more like 14mm2, not a European size.


The cable you linked to in the original post is rated at 70C, but I assume the temperature they are quoting the ambient at which the core temp is right on the limit for the quoted rated current - so at 100A perhaps 50 degree rise from a 70C ambient takes the core to 120C or so.Which is certainly pushing it if you'd like a long lifespan, and not good for anything near the cable either.

If the insulation failed at a lower temp, then the current and temperature ratings would need derating .

M.