what size wire should you use for a 125 amp three phase circuit? How do you figure that for future reference?

Morrowrj

There is a good wire table avaliable here http://www.powerstream.com/Wire_Size.htm It shows current ratings of the various wire gauges

Amchiller

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 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.

AWG 0 is rated at max 150 amps

tkrussell

Assuming the wire is for a branch circuit or a feeder in a building, the previous offered charts are not applicable.

For building wire size, in the US, must follow the NEC Table 310.16, and typically use the center Column for 75Deg C shows that #1 THHN/THWN copper is rated 130 Amps.