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I read a lot of website and articles about how to choose the right wire size, this one for example, and all of them have a table with AWG size and a corresponding maximum amperage. However, they never talk about voltage.

Let's say I need a wire for 60 000 V @ 1 A, by looking at the table, I could go with a 29 AWG. If I now need a wire for 1 V @ 75 A, the same table tells me to go with a 7 AWG. This would imply that the smaller wire will withstand 60 kW and the larger cable only 75 W.

I found all of this a bit strange, why should I use a smaller wire for a higher power?

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    \$\begingroup\$ Congratulations! You found out why they use high voltage in the electric grid! \$\endgroup\$ – user253751 Sep 28 '20 at 22:36
  • \$\begingroup\$ Btw, do you know why you need a bigger wire for higher amperage? \$\endgroup\$ – user253751 Sep 28 '20 at 22:37
  • \$\begingroup\$ @user253751 I would say that as the cable is thicker, more electrons can pass through but I'm not 100 percent sure. Thanks for pointing the voltage in the grid, didn't thought of that. \$\endgroup\$ – coxese5156 Sep 28 '20 at 22:43
  • \$\begingroup\$ Do you know Ohm's Law and parallel and series resistors? \$\endgroup\$ – user253751 Sep 29 '20 at 9:34
  • \$\begingroup\$ wire gauge is about how much current can be carried (plus looses due resistance and self heating). selecting a correct wire gauge also is about mounting and packaging of multiple cables. Voltage is only a isolation thing mainly. so you might have a 1mm wire for the current but need some centimeters of isolating material around it. \$\endgroup\$ – schnedan Oct 2 '20 at 0:23
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Voltage rating is a function of insulation quality rather than wire gauge. How close two conductors are of different potential (voltage) and what the dielectric or insulator is between them is what determines whether the cable can safely carry the voltage without a breakdown of the material or an arc.

Wire gauge is determined primarily by current. For example, a wire at only one volt, but carrying hundreds of amperes would need to be quite thick.

Conversely, a wire carrying 1000V but a miniscule amount of current can be a smaller gauge but would need insulation rated for 1000V, so the insulation would be thicker than the conductor.

At multiple kV, you have to start worrying about arcing through the air. At high voltages, you have to start spacing out the conductors. (There's a reason transmission lines are as far apart from each other as they are.) Humidity will also be a factor.

I'm hoping it was just an example in your question, but if you have to ask about wire gauge and insulation, you should definitely not be working with 60kV (or even household mains voltages).

If wires were perfect conductors (superconductors with no resistance whatsoever) then wire gauge might be a moot point. The reason that you size wires according to current is because electrical current flowing through even comparatively small resistance of a wire (compared to a load) still creates heat, which in turn can start fires or cause the wire to fail open. This is also why you should use larger gauge wires for longer distances, because the small resistance of a given length of wire is additive.

Edit:

Per comments, it's worth adding that even if a wire is sized correctly for current and isn't heating up, it can still cause a voltage drop due to its resistance — just like any resistor. (A voltage drop is a decrease of voltage due to dissipation of energy.) You may have an application where a larger gauge wire is desirable to counteract this. If your application is low voltage, such as with LED strips, outdoor low voltage lighting, etc., a voltage drop can be more problematic because it is a larger percentage of the intended voltage. In short, a 1-volt drop for a 5-volt application is significant while a 1-volt drop for a 240-volt load is rarely an issue. (The voltage drop will depend on the current as well as the resistance in the wire, E=IR.)

So for this reason, you may consider wire gauge as tangentially dependent on voltage as well, when you also factor in total wire length (resistance) and the specifics of the load.

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  • \$\begingroup\$ Thanks for the complete explanation and for explaining the idea behind the wire gauge and the head produced by the electrical current. And yes, it was an exaggerated example to understand the main point. \$\endgroup\$ – coxese5156 Sep 28 '20 at 23:04
  • \$\begingroup\$ In your last sentence, you say "This is also why you should use larger gauge wires for longer distances, because the small resistance of a given length of wire is additive.", why do larger wires have a smaller resistance? \$\endgroup\$ – coxese5156 Sep 28 '20 at 23:05
  • \$\begingroup\$ @coxese5156 For a given length of wire, a larger gauge will have less resistance because there are more internal "paths" for electrons to take. For example, see this table on Wikipedia. 12 AWG (2mm diameter) has a resistance of about 5.2mΩ per meter, while 18 AWG (1mm diameter) has a resistance of 21mΩ per meter. With double the diameter, 12AWG has roughly 4x less resistance per length unit. \$\endgroup\$ – JYelton Sep 28 '20 at 23:53
  • \$\begingroup\$ To be clear: Thicker wires have lower resistance which creates less heat "because the electrons don't need to move as fast." They don't just spread the heat out more. \$\endgroup\$ – user253751 Sep 29 '20 at 9:35
  • \$\begingroup\$ @coxese5156 If you have a long thin wire at low voltage, the wire might not overheat, but you might not have any voltage left at the other end! These wire gauge charts/calculators are designed to tell you about the overheating, but they don't tell you about the voltage loss. \$\endgroup\$ – user253751 Sep 29 '20 at 9:37
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The amperage is indeed the most significant parameter. You always need a thicker wire for more amperes no matter the voltage.

For low voltage (for 12V for example) the length of the wire is also very important. The longest, the thicker your wire needs to be. It's a common mistake to assume that because your installation runs on 12V you can put thin wires. It's the opposite. There is even a formula to calculate this. And often you will need a wire larger than those used on main lines.

That being said, it would be crazy for safety reasons, to put 60 000 V on a 29 AGW cable. Or even 110V. Because you need something solid, with reliable connectors.

For home installations, 110 V or 240V, there are official requirements for the type of wire, the type of isolation and a minimum thickness (or gauge), independent on how many amperes you plan to use on these wires but valid up to a certain amount of current (current = amperes). If you want to use more current than the maximum allowed for a regular wire, you should follow the table given by the official rules.

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  • \$\begingroup\$ Thanks for your answer. "It's a common mistake to assume that because your installation runs on 12V you can put thin wires. It's the opposite." I don't totally understand this part, why should the wire be thicker? \$\endgroup\$ – coxese5156 Sep 28 '20 at 22:45
  • \$\begingroup\$ It all depends on available power, to be honest. I guess he meant, for the same total power available, lower voltages corresponds to higher currents. Imagine what happens to a thin wire when you use it to short a 12V battery. Or, have you seen those microwave transformer mods that are used to make spot welders? When you lower the voltage to 1-2 V, you'd better have a pretty thick secondary winding, or you'll end up melting it. \$\endgroup\$ – Sredni Vashtar Sep 29 '20 at 15:03
  • \$\begingroup\$ @coxese5156 To run a 50W car headlamp at 12V requires 4.17A (because 4.17 x 12 = 50). To run a 50W table lamp at 120V only requires 0.417A. The same power lamp requires 10 times the current on 12V, when compared with 120V. So you need a thicker wire. \$\endgroup\$ – Simon B Sep 29 '20 at 15:05
  • \$\begingroup\$ @coxese5156 Because the lower the voltage, the higher the power loss and voltage drop across the wire. At low voltages like 12V, voltage drops very quickly after a few feet or m of cable length and you also lose energy. If the cable is too think it can heat. Using thicker wires helps a little bit but it's still good only for short distances. Using 24V instead of 12V already makes a lot of difference, Using this calculator, you will see that you can't go very far on 12V. solar-wind.co.uk/info/… \$\endgroup\$ – Fredled Sep 30 '20 at 7:02
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There is a voltage drop due to the resistance of the wire. The resistance per meter of wire can be calculated for different wire diameters and can be expressed in terms of (ohms/meters). If you want your voltage drop across the wire to be low, the diameter of the wire must increase. So obviously the current carrying capability is not the only factor when selecting the wire.

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    \$\begingroup\$ Keep in mind that the voltage drop is dependent on current. If the wire is used for carrying large current, then there would be a "large" voltage drop. If the wire were used only to carry a potential (such as with data signals) then the voltage drop would be near zero. In summary, the diameter of the wire would not need to increase if the current were low to begin with. \$\endgroup\$ – Bort Sep 29 '20 at 15:40
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AWG is parameter of wire core. Choosing wire by AWG you are choosing maximum current which the wire can conduct. But wires also have different types of insulation. For 60 kV not any wire can fit. when you choosing wire for big current and low voltage you also need to keep in mind conductor resistance, drop voltage is important.

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Power is carried by a wire in the surrounding electric and magnetic fields, not in the sense of a pipe carrying material. For high voltages, the surrounding electric field is high (and the important characteristic is insulation, or for very high voltages, corona breakdown of air). The high voltage determines the insulation around the wire (and automobile spark wiring has to be nearly one cm diameter or it makes St. Elmo's Fire, aka 'corona discharge').

For high currents, much B field around the wire is generated. That B field causes a problem (at high frequencies) called 'skin effect' because it causes actual force on the current-carrying charges (and a solid wire ceases to have any important conductance in its center, all current being close to the outer surface). The center wire in a cable-TV wire is iron, with a little copper plating, because expensive copper in the core of the wire would not improve its conduction. The wire gage appropriate to such UHF work is going to come from a different formula (and different table) than for house wiring.

For house wiring, ohmic heating of the wire is a major consideration (it can damage insulation, cause fire, etc.) and for long wire runs, it can also cause unacceptable voltage drop. Those considerations are present in almost all tabulations of 'ampacity'. In special circumstances (in an electric stove or oven with ceramic or other high temperature insulation) where it is acceptable to have wire get red-hot, one can ignore the tabulated values that pertain to house wiring and fire safety.

The 'right' diameter of a conductive wire is... potentially a deep subject. Technically, 'AWG' applies only to copper wires, and other measures (circular mils, and millimeters) are also in use.

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