Wire Size For 70 Amp Breaker. 40 amp wire size. David if that cable is nm romex then it actually cannot carry 50 amps.
For a maximum of 40 amps you ll need a wire. From the main panel in the basement. Use page 70 57 9 or tables 4 and 5 if wires are all different size.
For a 40 amp you need a 8 gauge wire size and for a 20 amp you will need a 12 gauge wire size.
40 amp wire size. Note that sub panels must have the neutrals and grounds completely separated with the neutral bus isolated from not bonded to the steel enclosure. 8 awg may carry a maximum of 70 amps in free air or 50 amps as part of a 3 conductor cable. Juicehead member 4 copper or 2 aluminum per nec table 310 15 b 6.
