Hello,
Apologies if this is the wrong section of the forum for this. I just downloaded Kicad 7 and noticed that it now has a cable size calculator (which looks more like a wire size calculator to me, but whatever). How does it calculate ampacity? Is it based off of some existing standard? Fancy thermodynamics calculations? Max voltage drop? I looked in the Kicad 7 documentation but couldn’t find anything about it.
Edit: I now see that it’s calculating the I²R heat losses. How does it go from that to an ampacity number?
I believe that this is another interesting bit of ESL? I think that “rule of thumb” was intended. But I can also imagine where a second grade teacher wants to give Johnny some mild punishment so gives him a thump on the head?
1)The standard sizes are all AWG
2)Put in 2.5mm2, which is standard mains power cable and you get “Ampacity” of 7A. This size is normally rated at ~25A and used with a 20A MCB, so something very wrong
Individual wires vs a three core cable vs a cable in a duct all make differences.
The US UL and the rest of the world tend to have different calculations
I agree that it seems pretty inaccurate, but then again just about all these ampacity calculators and tables are pretty inaccurate. Looks like the source code calculates it with the conversion of 700 square mils per amp, which this source says is “very very conservative”. A lot of these calculators are done for house wiring, which is naturally very conservative as well.
IMO I’m OK with this; I would rather Kicad be conservative about it, and people with more experience can just ignore it.
The best answer I’ve seen to “how many amps can this wire hold” is “test it and see”.
I think we might need to add a help text to explain a few things though. Or we can improve/further extend the calculations to reflect changes from most of other fields, but that is a bit complicated as far as i tried.
I do have some thoughts for a more advanced cable sizer that i will try to implement probably for v8. But no promises, please keep an eye on GitLab for a related MR in order to try and offer an opinion when its time comes.
Suggested value works fine for example for 100m 2,5mm2.
Lower value is more safe so, personally, i am good with this. But more important, you are also not allowed to size a conductor without some proper checks, and ampacity field has no impact there as far as i can tell. This calculator is really simple ATM.
