- Click here for pricing on 2 gauge THHN wire. Either of the above are code compliant. How To Install A 100 Amp Subpanel Electrical Sub Panel Wiring Diagram Fresh Amp Sub Panel Wiring Diagram Install 100 Amp Su House Wiring Diy Electrical Diagram You can use 4 AWG 75 deg C or 90 Deg C copper wire in conduit for a 100 A breaker provided the breaker is labeled for use with 75 deg C wire. Wire for 100 amp subpanel . In respect to this what size wire do I need to run a 100 amp subpanel. A 100 A sub-panel requires a two-pole 100 A breaker - if you want 100 A. 2 2 THHNTHWN-2 Wire for 100 amp sub-panel. 125 amp requires a wire size 20. If installing aluminum conductors the feeder circuit needs to be 1 AWG aluminum and this results in a voltage drop of 27 or a loss of 32 volts. Using an online voltage drop calculator at 250 for 100 amp Im coming up with 40 AL. Only 8 left in stock - order soon. As I mentioned barn will be storage. Today we are running a 100 amp service wi...