0
\$\begingroup\$

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?
\$\endgroup\$
  • \$\begingroup\$ Some welding machines use water-cooled conductors, in case you are interested in how flexible wires can carry high currents without melting. \$\endgroup\$ – Andrew Morton Oct 17 at 19:01
  • \$\begingroup\$ Is it supposed to be water cooled ???? \$\endgroup\$ – h22 Oct 17 at 19:12
  • \$\begingroup\$ Lol, no, I hope not. That was only something that I thought you might be interested in. \$\endgroup\$ – Andrew Morton 2 days ago
2
\$\begingroup\$

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.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ I also think so, if the cable can handle 32 A alone how it can only be rated 5.9 A with the only reason there are two in the "bundle" to deliver the power over 50 cm in my case. Still, could you give any source to ease my heart? And what that standards table is for? \$\endgroup\$ – h22 Oct 17 at 19:20
  • 1
    \$\begingroup\$ @h22 Those tables are written with specific uses in mind which aren't always stated and only known by those who the tables were intended for (for example, in house construction so will be sitting inside a wall). For example, it doesn't say exactly what it means by power transmission vs chassis wiring. Which is one reason I hate that table you linked despite its popularity. \$\endgroup\$ – DKNguyen Oct 17 at 19:22
  • \$\begingroup\$ You all keep forgetting that the ampacity rating depends on how much overheating the wire bundle can handle, which again depends on insulation material. If your wires are exposed to active air flow inside your chassis, the ampacity is higher. If your cables are tightly attached to thermally-conductive chassis's walls, the ampacity is also higher. A 22 AWG standalone wire with 75C over ambient can carry 7 A, see en.wikipedia.org/wiki/American_wire_gauge In applications with very high air flow (like drones) the ampacity is much higher. \$\endgroup\$ – Ale..chenski Oct 18 at 4:37
  • \$\begingroup\$ @Ale..chenski Not forgetting any of that, as stated by the second sentence in my answer. \$\endgroup\$ – DKNguyen Oct 18 at 18:33
  • \$\begingroup\$ Okay, okay. "Ampacity ratings for cables aren't hard numbers" +1. :-) \$\endgroup\$ – Ale..chenski Oct 18 at 22:55
0
\$\begingroup\$

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.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ It means with 5V 0.4/5= 8% drop or 0.4/12V=3.3% drop that’s at at 4A \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 17 at 18:53
  • \$\begingroup\$ Yes. But their intended application is industrial 24 V DC systems. \$\endgroup\$ – Transistor Oct 17 at 19:03
  • \$\begingroup\$ Then for long extension cables use AWG 16 so you lose 0.4/24=1.66% at 1m pair \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 17 at 19:08
  • \$\begingroup\$ Pre-fabricated cables for these connectors typically come in 2 m, 5 m and 10 m lengths. I had to make up some 30 m cables and chose a larger wire diameter and rewireable connectors. \$\endgroup\$ – Transistor Oct 17 at 19:24
  • \$\begingroup\$ Then voltage drop is x2 for a pair per meters \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 17 at 19:36
0
\$\begingroup\$

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.

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.