Wire Gauge Amp Question

[bostonman]

I went to buy some wire tonight and got a bit confused. First off, the packages were marked 12 STR which I thought meant twelve strands, however, after thinking about it, I realized the 12 was AWG (or at least I believe it to be).

This website shows 12 AWG is rated at 41A: https://www.powerstream.com/Wire_Size.htm

If I look at Home Depot (which I don't necessarily trust, it shows maximum current of 20A: https://www.homedepot.com/p/Southwire-500-ft-12-Black-Stranded-CU-THHN-Wire-22964158/203401696

Unfortunately the packages I saw tonight (at a hardware store) didn't list the current, but I'm wondering which chart to go by.

[prosper]

electrical code calls for 12AWG for 20A (and 14 for 15A), but, that really only applies to building wiring. Chassis wiring in a product you're designing is a whole 'nuther ball game.

So, what exactly is your use case? Wiring up receptacles and lights - go with 14AWG (solid/Romex) for 15A cct's, and 12AWG for 20A cct's

[bostonman]

I didn't think of 12/2 wiring as being code for house wiring and that would be 20A circuits. My mind was too focused on the chart and not thinking logically.

I'm looking to wire a circuit that may draw approximately 12A, so 18AWG will be more than enough.

[prosper]

If its wiring circuits in a building, the smallest you can go is 14AWG/15A, per IEC.

That said, a lot of appliances have 18AWG cords - appliances are not subject to building code and would fall under something like UL

[bostonman]

Oh no, this is for bench electronics; from a DC 12V supply and a circuit drawing up to 12 or so amps.

[prosper]

oh, yeah.

May be worth checking into voltage drop. If your power supply is applying 12V at one end of a long lead, you may not actually be getting 12V across a big load on the other end. Whether it's a problem depends on what you're doing.

18AWG is 21mohm/m. So, if you're using 2m of cable (round trip) and pushing 12A, then your cables are dropping:
E=IR=12A*42mohm=0.504V. (That's 6W, and I could see that getting a little warm.)
Your supply says 12V, but you're only actually getting 11.5volts at your load, and the rest is getting dropped in your wires.

12AWG is 5.2mohm/m:
E=IR=12A*10mohm=120mV dropped, or 1.4W

[bostonman]

Good point.

It's rare I ever consider voltage drop in a wire unless I'm thinking about a project running long distances.

I think round trip for my application should be about 1.22m. Either way, it's worth going much larger in wire diameter.

[prosper]

It's definitely a bigger factor at low voltages than at higher ones. At 120V, you still get the same 0.5V drop and 6W dissipation, but 0.5V out of 120V and 6W out of 1440W is mice nuts. (its why transmission lines are hundred plus kV)

The lower voltage rails on your PC, for example, usually have a 'sense' wire to detect and compensate for voltage drops on your 3V3 rail.

[bostonman]

I messed up.

Looking at the price of wire at Home Depot, I thought a good choice for the extra few cents would be 10AWG. Although I'm only running a short distance (about 20" each way, so 40" round trip), I thought nice heavy duty wire would be nice.

Well, I bought crimp connectors (with intention of soldering them too for extra strength since I dislike simple crimping) and discovered they don't fit on the terminal block.

One terminal block measures approximately 0.25" in width, and the other block is approximately 0.32" in width.

At this point, I either need to go with 14AWG, or, maybe someone has a better way to mount these. I considered wire only, however, the terminal blocks are too flimsy and either the wire won't fit, or will end up cracking the plastic.