Confusion regarding safe current limit for AWG #18

Question

When I look into AMAZON for buying wires, the description claim that the AWG #18 150 strands wire can bear 7.5A current.

https://www.amazon.com/dp/B0746HRVZP/ref=twister_B0746HR9HZ?_encoding=UTF8&th=1

From the engineering toolbox website, I learned that the max. current for AWG #18 is 3.5A for 43 and above core.

https://www.engineeringtoolbox.com/wire-gauges-d_419.html

So, which one is correct? Am I making any mistake?

I am a beginner and building a four-wheeled mobile robot. Can I use this wire from AMAZON (AWG #18) for passing current from the 7.5A fuse to the motor driver?

Data you see on engineeringtoolbox is most probably compiled from NEC ampacity tables e.g. Table 310.15(B)(16) through (20). Those tables are generally for cable harnesses in buildings and they are prepared for different conditions and with different assumptions. For example, according to those tables, taking the correction and de-rating factors provided by NEC into account, 150 strands of AWG18 can be used for up to 6.93 Amps under free air and up to 30°C ambient temperature. For your application, length, temperature rise, maximum allowable drop etc all limiting factors to select a size. — Rohat Kılıç, Jul 05 '23 at 20:41
@evildemonic - the length of the wire will be less than 3 inches (~7 cm). — Msr09, Jul 05 '23 at 21:10
@RohatKılıç - Can you suggest me a source or link to learn how can I choose wire size considering the length, environment, etc.? — Msr09, Jul 05 '23 at 21:12
Two words that may shed some light on the derating that the engineering toolbox site shows: **Safety Margin** — Sotto Voce, Jul 06 '23 at 02:43
Never mind that the fuse you quote is rated at 7.5A - what current do you actually expect/need to pull? — MikeB, Jul 06 '23 at 11:35

score 4 · Answer 1 · answered Jul 05 '23 at 20:18

4

What counts as ‘safe’ depends on the usage - a single wire in free air can handle a larger current than it would in a bundle of wires in a conduit. Also the manufacturer is likely to quote the most optimistic figure while an engineering toolbox is likely to be more cautious. You can make your own assessment based on the operating conditions and the impact of a failure. Another thing to consider is the voltage drop; with a 14V supply you’ll lose quite a lot of performance if there’s a 1V drop (say) in the wiring. Intuitively I’d be inclined to say that 16AWG is ok but heavier wiring would be preferable.

answered Jul 05 '23 at 20:18

Frog

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I just ran the figures here https://www.omnicalculator.com/physics/wire-resistance and the voltage drop looks like being 100mV if you have a couple of feet of wire total, or half that if you go to 16AWG – Frog Jul 05 '23 at 20:24
As far as I understand from the voltage drop calculators, the lower the AWG #, the lower the voltage drop. For example, AWG #12 will have less voltage drop than AWG #18. Isn't it? – Msr09 Jul 05 '23 at 21:35
@Msr09 that's correct – Frog Jul 06 '23 at 01:56

score 3 · Answer 2 · edited Jul 05 '23 at 23:33

When selecting cables for DC current, there are 2 mains concerns from an electrical point of view :

the voltage drop, which is directly related to the resistance (and the current), which in turn depends on the section (gauge) of the wire, its lengths and the material of the conductor. This is particularly important for long wires in a low voltage - high current setup
the ampacity of the wire (ie will the wire overheat/burn/cause fire). This depends on the resistance per distance of the wire (ie section + material), the ambiant temperature, the environment (ex : ventilation, insulation around the wire, cable bundle or not, ...). And last but not least, the insulation of the cable itself : some start melting at very low temperatures, other at far higher temperatures. Once the insulation has melted, you often get a short circuit (generating even more heat).

So to answer your question about why such a difference, I think the main reason is that the toolbox calculator computes the ampacity based on standart insulation (ie rather low melting point), whereas on the picture of the amazon wire, it is written 200°C, which is a high temperature rating.

A wire with 200°C temperature rating can carry far more heat before reaching the melting temperature of insulation than a standard wire. Still, I don't think it's a good idea to get your cable really hot (it might still destroy sensitive components nearby (for example your Lipo, which will then explode), or make some materials ignite, or you might burn your fingers).

And always take the marketing data from amazon vendors with a pinch of salt : they often overstate the quality of the product, especially on hard to test aspects.

If you wish to use that cable, I would suggest you do a simple test : you first force 1A through the cable for a few minutes, then touch it. If it is cool, then you proceed with 2A, 3A, ... until 8A.

If at 8A you can still comfortably hold it in your hand, you should be fine. If at any point it hot, then I strongly advice against using it (at best you will waist lots of energy, and worst it might melt).

Confusion regarding safe current limit for AWG #18

2 Answers2