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I've seen many posts on this topic, but no one ever has a clear answer on how to calculate the maximum current that can flow through a wire. There are many tables on the internet, but they are all different and it is not clear which one to use. Could anyone help me understand how to do this calculation.

For example, I have a 12AWG cable, made up of 37 individual 0.3mm wires. The manufacturer only tells me the resistance per meter of length:

  • With this information can I calculate the maximum current I can let flow in this cable?

  • If this information is not sufficient, what other information do I need to calculate the maximum current and with what formula?

  • Is there a reliable table I can rely on? I've seen that many people post and don't refer to this one: Wire size and current rating but is it trustworthy?

  • Assuming that table was reliable, can I take the value for a single 0.3 wire, i.e. 0.7A and multiply it by 37 to get the maximum current for my 12AWG wire? That is 25.9 Amps?

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  • \$\begingroup\$ There is no such thing because it depends in what jurisdiction you live and what are the local electrical codes for different use cases for any given situation where you have a wire. \$\endgroup\$
    – Justme
    Oct 18, 2023 at 20:48
  • \$\begingroup\$ the limit for those individual 37 wires in a bundle varies by the wire; internal ones have a harder time cooling than ones on the perimeter, so they do not all perform the same as a single wire alone (with 360deg of heat shed) would. \$\endgroup\$
    – dandavis
    Oct 18, 2023 at 21:22
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    \$\begingroup\$ What is the application? Wiring current carrying capacity is typically application dependent. For example, for a house in the USA, many jurisdictions follow the requirements of the National Electrical Code put out by the National Fire Protection Association. If you are wiring a combat vehicle, one would use the charts in the Department of Defense MIL-HDBK-508. Space applications use MIL-STD-975 (or whatever NASA standard replaced it). In general these standards don't agree on wiring current carrying capacity. \$\endgroup\$
    – C. Dunn
    Oct 18, 2023 at 22:34

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The wire current limit (called ampacity by the electrical trade) depends on three factors:

  • wire unit resistance (determined by material and cross section)
  • allowed thermal rise
  • cable thermal environment

The unit resistance is clear. More current = more dissipation. Smaller wire = more dissipation.

Allowed thermal rise depends on the wire's other physical properties. Typical wiring has a stated thermal limit, often printed right on the cable (e.g., 90 deg. C for NM-B 'Romex' cable.) Beyond this limit, the cable begins to be damaged, usually by melting insulation.

The reason "no one ever has a clear answer" about ampacity is that it depends heavily on the thermal environment. This affects the ability of the cable to shed heat through its sheathing and stay below its thermal limit.

So wire manufacturers (and electrical codes) will specify the wire type, the environment it is installed in (free air, conduit, multiple runs in conduit, etc.) and the ambient temperature in their ampacity tables.

High ambient temp, dense runs, or poor cooling require that the cable ampacity be derated to ensure compliance with its thermal limit. How do they determine the derating? By testing known configurations using proven methodology. I suppose these days this could also be determined by thermal simulation, but basically that's where the tables come from.

North American electrical codes rate #12 ga wire for 20A service, so your 25.9A calculation isn't unreasonable. But, to comply with code you would not exceed 20A.

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  • \$\begingroup\$ Even before considering thermal effects, one needs to estimate the acceptable voltage drop for the specific application. \$\endgroup\$
    – Eugene Sh.
    Oct 18, 2023 at 20:37
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    \$\begingroup\$ Right, but the thermal load is spread over the length of the wire. So even if a run is so long that it has excessive IR drop, this won't by itself be a thermal issue. \$\endgroup\$ Oct 18, 2023 at 20:39
  • \$\begingroup\$ The insulation rating matters too. \$\endgroup\$ Oct 18, 2023 at 20:44
  • \$\begingroup\$ @hacktastical Thanks for the reply, can I ask you where you found the number 20Amp for a 12 AWG, for example here jst.fr/doc/jst/pdf/current_rating.pdf I see 30A for a 12 AWG while here powerstream.com/Wire_Size.htm I see 9.3A again for a 12 AWG, a huge difference. \$\endgroup\$ Oct 18, 2023 at 20:46
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    \$\begingroup\$ @Matt S - yes, coiling wire increases the local density and thermal rise, just like running multiple cables together in a bundle. \$\endgroup\$ Oct 18, 2023 at 20:51

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