Cable ratings

[Trevor Rymell]

Sorry for asking these dumb questions but could I please check if the following is correct about cables?

1. For any given cable and given thickness of insulation (assuming constant ambient temp, humidity and air pressure conditions etc), it's the cross sectional area of the copper and NOT the number of strands that determine its continuous current carrying capacity. I know that in practice the thickness and type of insulation counts as does the copper composition, resistance and probably a load of other things.

So for example, a single strand of copper with a 2mm cross sectional area will carry the same current as a 40 strand with a 2mm cross sectional area. The only difference would be the mechanical durability and flexibility.

2. A given cable carries roughly twice as much AC current as DC. Is there a formula to calculate this?

3. Is there a way to calculate the AC or DC continuous current capacity if I know the number and thickness of the strands of a given cable? Manufacturers labels often given the voltage but not current capacity.

Thanks in advance

Trevor

 

[ericgibbs]

hi,
There is lots of info on the web, Google for 'cable current capacity'

example:

http://wiki.diyfaq.org.uk/index.php?title=Cables

 

[JimB]

Quick answers:

1 Your assumption is correct

2 No and No. (Not sure where you got this idea from)

3 Just look at the tables in Erics link.
Also be aware that the "current rating" of the cable may not be the limiting factor.
Sometimes in high current situations it may be necessary to use a thicker cable to reduce the volt drop to an acceptable level.

JimB

 

[Nigel Goodwin]

2 No and No. (Not sure where you got this idea from)

Probably from the fact that switches have higher AC ratings than DC ratings?, but it's not for the reason he perhaps thought?.

 

[Trevor Rymell]

JimB, Nigel

Thanks for the replies.

"2 No and No. Not sure where you got this idea from". I don't mind telling you.

I went looking for some cable to do a bit of re-wiring on a motorbike. Normally, I just use whatever looks heavy enough and leave it at that but in this case, it's for the charging circuit and I thought I'd make sure I got as close to the existing stuff as possible (37 x .2mm)

In at least 2 shops I went to, selling nothing but cables for lighting, heating, electronics, I asked if they could tell me what maximum current a particular wire could take, I was asked, "AC or DC?". When I said DC the bloke told me, "Ah. DC. then this is no good. AC is ok but not DC it would need to be heavier". I have to say this was the first time I'd heard of this.

When people are emphatic and seem so sure (they're in the business of selling the cable after all), I tend to assume they know something I don't. I thought well, AC changes direction 50/60 times a second, so maybe it doesn't need to be such a thick cable as for DC for a given voltage/current. I'm a muppet, I know.

So, does this mean then that a 15A CCC cable will take 15A whether it's AC or DC?

In the end I got a 70 x .19 which, I hope I'm safe to assume, is thicker, has a lot more copper so it should have a higher CCC than the one I'm replacing.

Thanks again and sorry for the ramble.

Trevor

 

[Nigel Goodwin]

So, does this mean then that a 15A CCC cable will take 15A whether it's AC or DC?

Yes - switching is different though, as when switching AC the arc is self-extinguishing.

 

[JimB]

In at least 2 shops I went to, selling nothing but cables for lighting, heating, electronics, I asked if they could tell me what maximum current a particular wire could take, I was asked, "AC or DC?". When I said DC the bloke told me, "Ah. DC. then this is no good. AC is ok but not DC it would need to be heavier".

Ouch, that is scarry. I wonder how much else they dont really know?
Also consider the possibility that the guys in the wire shops either worked together at some time or got this dud info from a third misguided person.

When people are emphatic and seem so sure ..... I tend to assume they know something I don't.

Often the case, someone with little knowledge becomes an "expert" because those around him have even less knowledge.

JimB

 

[Leftyretro]

Ouch, that is scarry. I wonder how much else they dont really know?
Also consider the possibility that the guys in the wire shops either worked together at some time or got this dud info from a third misguided person.


Often the case, someone with little knowledge becomes an "expert" because those around him have even less knowledge.

JimB

Also may just be selling method to get you to buy a large gauge wire then you might otherwise really need. Higher cost = higher profit margin.

Lefty

 

[Trevor Rymell]

Also may just be selling method to get you to buy a large gauge wire then you might otherwise really need. Higher cost = higher profit margin.

Lefty

It would be nice to think that they were even that helpful but no. They more likely just want to get rid of me because I'm asking too many questions, a very common behaviour here.

Trevor

 

[Papabravo]

A rule of thumb from the ARRL Radio Amateurs Handbook is 700 circular mills per Amp. You can google circular mils for yourself. I'm pretty sure that current is current. With AC current you have to keep in mind that peak current is 41% higher than RMS current.

The current carrying capacity of a wire or cable is not a hard limit where the cable goes from conducting to non-conducting. The current carrying capacity of a cable is related to temperature rise. If you can tolerate more temperature rise you can carry more current. Think about heat-sinking your cables!

One more thing to keep in mind is the difference between solid conductors and stranded conductors.

 

[Nigel Goodwin]

If you can tolerate more temperature rise you can carry more current. Think about heat-sinking your cables!

Hence the increase in capacity for cables buried in plaster!, and the decrease in conduit.

 

[Trevor Rymell]

Papabravo

Thanks. I've been doing a fair bit of reading up since yesterday and some of it makes sense. I understand about the heatsinking - keep the conductor cool and it'll carry more current. The hotter it gets the higher the resistance.

One more thing to keep in mind is the difference between solid conductors and stranded conductors.

Re your quote above, I understood from previous replies that number of strands don't affect CCC. It was the total cross sectional area of the copper that mattered. Is that not the case? The difference would be just in flexibility and mechanical durability, I thought.

Also, when calculating the cross sectional area of a multi-strand cable, is it correct to calculate the cross sectional area of one strand using Pi R Squared and then multiply that by the number of strands? It seems to me that this gives the total area of the copper but not, of course, the area of any circle occupied by the strands bunched together. Am I correct?

Trevor