def _hbc_P(P):
    """Define the boundary between Region 2b and 2c, h=f(P)

    Parameters
    ----------
    P : float
        Pressure [MPa]

    Returns
    -------
    h : float
        Specific enthalpy [kJ/kg]

    References
    ----------
    IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the
    Thermodynamic Properties of Water and Steam August 2007,
    http://www.iapws.org/relguide/IF97-Rev.html, Eq 21

    Examples
    --------
    >>> _hbc_P(100)
    3516.004323
    """
    return 0.26526571908428e4+((P-0.45257578905948e1)/1.2809002730136e-4)**0.5