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I have seen a wide variety of different estimates for the maximum current rating for copper wire. For example, for 24-gauge copper wire, the Wikipedia article about "American wire gauge" claims that a copper wire will fuse after 10 s if carrying a current of over 29 A. On the other hand, this page claims a maximum current of only 3.5 A for chassis wiring and 0.577 A for power transmission. What is causing the disconnect between these wildly divergent estimates of the maximum current? Moreover, which of these estimates is most useful for estimating the maximum current that is safe for use in an electromagnet (both with and without insulation)?

Thank you.

George Bentley
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  • Search here on "wire current rating" On of the hits: https://electronics.stackexchange.com/questions/292777/why-do-some-wire-gauge-ampacity-charts-state-vastly-different-current-ratings?r=SearchResults – Huisman Apr 11 '19 at 17:18
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    I wouldnt use unisolated wire to wind a coil, the windings will short. – Huisman Apr 11 '19 at 17:22
  • You probably mean magnet wire or enameled wire instead of **without insulation**. – StainlessSteelRat Apr 11 '19 at 19:07
  • If you're asking about how to figure it out for an electromagnet, specifically, then please edit your title and question. If you're asking in general, then see [this question](https://electronics.stackexchange.com/questions/292777/why-do-some-wire-gauge-ampacity-charts-state-vastly-different-current-ratings). For all that a wire is just a really long skinny piece of metal with some insulator wrapped around it, the answers get pretty involved. – TimWescott Apr 11 '19 at 19:41

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Copper melts (the wire fuses) at 1984 F (1085 C). To avoid a fire in your house or equipment, you will generally want to keep the temperature of wires very much below that, so you'll need to operate them with currents far below the fusing current.

Ampacity tables are based on some particular choice of the maximum temperature rise of the wire (as well as assumptions about airflow around the wire, insulation thickness, etc). We usually limit the temperature rise to something like 25 C to avoid temperatures that could disturb the operation of other parts of our circuit, degrade the wire's insulation, etc.

As you can see, this limit is very, very far below the fusing temperature of the wire.

Moreover, which of these estimates is most useful for estimating the maximum current that is safe for use in an electromagnet (both with and without insulation)?

Hard to say. On the one hand packing the wire close together will tend to reduce the ampacity. On the other hand the core of your electromagnet will provide some conductive thermal dissipation (heatsinking).

You could use a FEM solver to find the thermal limit, or just build one and see how hot it gets with different currents through it. After you have built 4 or 5 you will probably have a good idea what wire gauge you need to support different current requirements with a given geometry.

The Photon
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The limits for chassis wiring assume that the wires are in free air, which means that they are cooled by convection. The limit for power transmission appears to apply to the case where wires are bundled together so there is much less natural cooling. In either case the limit is based on some notion of what is a safe temperature for the insulation of the cable, considering possible degradation over a long period of time.

For an electromagnet you would need to use the "power transmission" values, but the table you cite admits that those numbers are very conservative. You might find other guidelines that are more specific and relevant to your case.

Elliot Alderson
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Each specification for current limit is based on ;

  • a known temperature rise to cause fusing >(1000'C) in a certain time

  • a max known temperature rise for rated insulation in free air 60'C rise or 85'C max for PVC or whatever insulation is rated for.

  • a max permitted power loss for power transmission efficiency in free air

    • based on Pd loss /km
  • a max current for enclosed temp rise due to poor thermal conductance and no attempt to provide heat transfer
  • Note that each of these sequential points may result in significant current de-rating depending on your designed max temp.

e.g. AWG16 using your reference links.

  • Fusing current 140 A @ 10 s, 502 A @ 1 s, 2.8 kA @ 32 ms
  • Chassis current 22 A
  • Transmission line current 3.7 A 13.17248 Ω/km means 180 W/km
  • Solenoid Coil ?? depends on duty cycle, thermal resistance & thermal time constant

Therefore if considering a solenoid the heat transfer is self-contained with very little of the original surface area for free air cooling so the max current or duty cycle of this current over time, must be derated even further.

Tony Stewart EE75
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