How is this 22 AWG cable rated 4 Amps?

Question

I found the cable (1533576 - Actuator/Sensor-Cable, M12 Socket - Open End Connector, 2m, 4A, 250V, Phoenix Contact) here that is claimed to handle 4 A and is then written that it has a 22 AWS cross section gauge, 4x 0.34 mm².

How this could possibly be? There is the table here that says the gauge 22 can is only allowed to carry 0.92 A for "power supply". Even all four wires together cannot make it as much as 4 amps. Ok it would be 7 amps for "chassis wiring" but if another answer about the chassis wiring is right this only applies for the single wire free in the air and not in the bundle. All 4 wires in this cable are very obviously in the bundle.

I was planning to use 14 gauge wires for my 5.5 A (5V) power part. They look thick and thinner wires worked fine on my workbench during prototyping. Does this mean that my power circuit is over-engineered?

The possible answers could be:

• Forget about this cable, you need the gauge 14 to supply exactly the power to that PCB to be safety compliant (5.9 A, your SBC draws 5.5 A so table calls for this gauge).
• Something else? Is it so that I do not actually need the wiring that heavy to be safety compliant? Or some consultant can waive this requirement?

Answer 1

Ampacity ratings for cables aren't hard numbers. Different insulation, different length, and obviously the environment all affect it. For example, the material information for that cable is pretty detailed and seems to use a polyurethane rather than your typical PVC.

I also do not think the 4A rating means 4A through every conductor at the same time. Probably just two.

14AWG wire for 5.5A is overkill though. An extension cord carrying designed to carry 15A is 14AWG. And on model airplanes, for example, they carry a lot more than 15A on 14AWG wire.

Answer 2

According to Omni Calculator (chosen at random) the 2 m of 0.34 mm² annealed copper wire has a resistance of 100 mΩ. This agrees reasonably well with the technical information conductor resistance 58 Ω per kilometer.

• At 4 A that will drop 0.4 V on each of the feed and return wires.
• Power dissipated in the cable (combining feed and return) will be given by \$ P = I^2R = 4^2 \times 0.2 = 3.2 \ \text W \$ or 1.6 W per meter.

Note that the specification also quotes "Ambient temperature (operation) -25 °C ... 90 °C (Plug / socket)".

In the end cable rating tables are for a particular purpose. The Phoenix Contact cables are for use in machinery and their requirements are often size, flexibility, fitting M8 or, in this case, M12 plugs more so than low temperature.

Answer 3

From your link:

Definition: ampacity is the current carrying capability of a wire. In other words, how many amps can it transmit? The following chart is a guideline of ampacity or copper wire current carrying capacity following the Handbook of Electronic Tables and Formulas for American Wire Gauge. As you might guess, the rated ampacities are just a rule of thumb. In careful engineering the voltage drop, insulation temperature limit, thickness, thermal conductivity, and air convection and temperature should all be taken into account. The Maximum Amps for Power Transmission uses the 700 circular mils per amp rule, which is very very conservative. The Maximum Amps for Chassis Wiring is also a conservative rating, but is meant for wiring in air, and not in a bundle. For short lengths of wire, such as is used in battery packs you should trade off the resistance and load with size, weight, and flexibility.

In short, "power transmission" is not what you think as in device power wiring. It is about long distance high voltage runs, in a large bundle. Not a 2m/6ft 4 conductor pigtail like you have pictured. The manufacturer has obviously done their own test to rate their product. And the current is likely limited by the connector not the wires. Plus as it notes these numbers are conservative so they are much less than the real max safe limit.