How much current will an RS232 cable handle?

[Andy1845c]

I am working on an LED project where I was planning (without much thought ) on using an RS232 extension cable to power it. Lastnight it kinda dawn on me the wires aren't that thick and each power pin will have approx. 500mA on it. That dosn't seem too excessive, but with 8 clusters of LEDs, the poor common will have around 4 amps on it.

I don't know what gauge the wire is, but its a standard off the self cable.

Would the 500mA be too much to ask of it? I'm pretty sure the 4 amps will be.

 

[justDIY]

500ma is too much, let alone 4 amps. those multicore cables are designed for signaling, not transferring current.

I guess the more important question is, what is the voltage? If you can live with dropping voltage on heating the cable, then you can probably force the current through. problem is, a few hundred mV can make a big difference in how much current an LED will draw. get yourself some CAT5 cable. the good stuff is 8 strands of 24ga wire, parallel them up and you can carry a decent amount of current without a huge voltage drop. Plus CAT5 is cheap as dirt.

 

[Andy1845c]

Thnaks Thats kinda what I figured after I thouht it though a bit more. I think I was blinded by how easy it is to get a nice ready made cable with molded ends and how nice the RS232 sockets would work on my project.

Voltage drop would be a very bad thing for me

I found some 18ga phone line here at work. Its only 4 strands, but I can double it up I guess.

 

[justDIY]

you could cram a much higher voltage down the wire, at a lower current, and use a switch mode regulator at each module, to transform the high voltage into high current. some switchers even accept AC input, further reducing your transmission losses.

 

[Oznog]

You can look up the current capacity for any gauge on the net. The more
advanced charts show the derating for putting the wire inside a cable- which reduces heat dissipation thus reduces the current which will produce enough I^2*R heat to raise the temp above the max temp of the insulation. Even better is the table which shows what multiple similar conductors in a cable will do. These are all a bit of an estimate since the thermal insulating value of a cable housing changes a lot with the construction.

 

[Sceadwian]

Cat3 Universal Twisted Pair wire is your friend! (10BT Ethernet cable) Each strand is 24 gauge wire and there are 4 twisted pairs (8 total) in each sleeve. If you need more wires, simply run two cables side by side, then you have 16 24 gauge wires. If you need extra current handling simply twist multiple wires together. 2 24 gauge wires are equivalent to a 21 gauge wire. 2 21 gauge wires are equivalent to a 18 gauge wire and so on, if you bind all eight wires of a Cat3 cable together you have the equivalent to a single 15 gauge wire, though the thermal capacity of the wire isn't the same as there's insulation to deal with, ampacity values usually depend on the conductor being either solid or very closely bound conductor, so de-rate appropriately. But it's immensely useful stuff, and who doesn't have a few patch cables floating around they don't need? Just watch out for Cat5 cable, which is the same as Cat3 except there are a lot more twists for each pair of wires which makes it much more difficult to work with (lots of curly wires everywhere and straightening them is a pain)

I use this site for explanations on a wire's current carrying capacity.
Ampacity and wire info

 

[Papabravo]

Rule of Thumb is 700 Circular mils per Ampere
22 ga. solid copper is about 640 CM and will carry about .9 Amperes
22 ga. stranded will actually be a bit better

This data should be available from Belden and others.

 

[Andy1845c]

Is there a standard gauge wire for RS232 cables? I have no idea what gauge the one I have is.

I think i'm just gonna use my 18ga phone/low voltage wire. I'll end up with alot bigger cable then I wanted, but atleast it will be done right. I did kinda want a plug of some kind though oh well.

Thanks for the link Sceadwian. Looks like I could learn somthing from it.

PapaBravo - thanks for the rule of thumb. I had never heard that before. It give me somthing to go off of.

 

[Hero999]

I guess the more important question is, what is the voltage?

That makes no difference, the voltage drop (therefore the power dissipation) on a certain cable at any given current will be the same regardless of the voltage delivered to the load.

If you can live with dropping voltage on heating the cable, then you can probably force the current through.

You want to increase the voltage not lower it for two reasons:

1. More power can be delivered for a given current.
2. The voltage drop will be less with respect to the overall system voltage.

This is exactly what's done in the electricity distribution system. The voltage is increased so think cables can be used to tranfer huge amounts of power. Then the voltage is stepped down at the output.

To transfer as much power as possible, use as higher voltage as you safely can, for an RS232 cable that would be about 50V, so using three pairs at 500mA each you can transer a whopping 150W.

 

[justDIY]

That makes no difference, the voltage drop (therefore the power dissipation) on a certain cable at any given current will be the same regardless of the voltage delivered to the load.

I disagree, the voltage does make a difference, which is why I asked.

If the load at the end of the cable can only tolerate a certain voltage range, then your arbitrary suggestion of 50v makes no sense. Futhermore that is why I suggested a higher voltage, and using point of load regulation.

 

[Papabravo]

Is there a standard gauge wire for RS232 cables? I have no idea what gauge the one I have is.

I think i'm just gonna use my 18ga phone/low voltage wire. I'll end up with alot bigger cable then I wanted, but atleast it will be done right. I did kinda want a plug of some kind though oh well.

Thanks for the link Sceadwian. Looks like I could learn somthing from it.

PapaBravo - thanks for the rule of thumb. I had never heard that before. It give me somthing to go off of.

There was reason for giving you the numbers for 22 ga. If you look at the cable specs you will find that many multiconductor cables use 22 ga. stranded insulated wire. Ceheck out a datasheet form a cable manufacturer. Better yet get yourself a cheap plastic caliper and measure the diameter of the wires. Then you will know what you have.

Remember 700 Circular mils per Ampere and you cant go wrong.

 

[Andy1845c]

Okay, I figured 22ga wasn't just a wild guess, thats why I asked.

 

[philba]

I would make no assumptions about the ga. of the cable. I've seen serial cables that clearly were thinner than 22 ga. There may well be a standard but random cables marked "serial" don't have to adhere to anything. I might believe if the cable was marked "RS232 Compliant" but suspect that isn't the case here. If you can measure the resistance of the cable, that might give you a clue but you'll need to be able to measure in the milliOhm range. 22 ga should measure 14-17 mOhms per foot, depending on what kind of strands.