A.O. Smith AHPW-25, User'S Information Manual

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10
MINIMUM WIRE SIZE
Allowable Ampacities of Insulated Conductors
Single-phase heat pump water heaters are two wire circuits.
Three-phase heaters are three wire circuits. In addition to the
foregoing, a grounded conductor is required. Not more than
three conductors in raceway, cable, or earth (directly buried),
based on ambient temperature of 30°C (86°F)
TABLE 3